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O-360 hot start

ccsmith51

ccsmith51

Well Known Member
My RV-4 had an O-360-A1F6D. When the engine was hot it was easy to start. Mixture rich, crack the throttle, and in a couple of blades it was running.

My new (to me) RV-6 with an O-360-A1A acts the opposite. If I try to start it hot like my -4, it just seems to flood the engine. It won't even hardly catch. The way to start it is with throttle cracked, mixture in ICO, spin the prop, then slowly feed in mixture until she starts.

I searched the archives and found a thread where a number of people say that this is how they achieve hot starts, too.

Can anyone tell me why this happens on a carbed engine? And why my two O-360's would be so different?

Thanks,
 
Can't help with "why" but this is how I deal with "what is". I have an RV9A with an )260-A4M and when doing a quick turnaround, I open the oil access door to allow the heat to dissipate quickly. Starting procedures are the same as normal, minus the boost pump. Dan
 
Just a guess

No 2 RV installations are exactly the same and your new to you RV 6 is probably no different.
My suspicion is that your float bowl is beginning to boil a little after shut down, causing the float to sink and flood the engine.
The way you describe the starting procedure definitely points to a flooded engine.
Your RV 4 may have had an exhaust system placed just a bit further away from the carby and caused no heat/vapor issues.

Knowing that your engine tends to flood on a hot start gives you a head start
on using proper flooded/hot start procedure before grinding the battery to death. A hot start simply requires a couple more blades before the engine catches and if I was you I open the throttle just a little more than cracked "about an inch" to achieve a quicker start.

Hope that helps.
 
ccsmith51 said:
My RV-4 had an O-360-A1F6D. When the engine was hot it was easy to start. Mixture rich, crack the throttle, and in a couple of blades it was running.

My new (to me) RV-6 with an O-360-A1A acts the opposite. If I try to start it hot like my -4, it just seems to flood the engine. It won't even hardly catch. The way to start it is with throttle cracked, mixture in ICO, spin the prop, then slowly feed in mixture until she starts.

I searched the archives and found a thread where a number of people say that this is how they achieve hot starts, too.

Can anyone tell me why this happens on a carbed engine? And why my two O-360's would be so different?

Thanks,
Click to expand...

Mine is exactly the same. After the $100 burger my start procedure is NEVER prime, mixture full rich, throttle cracked then crank. It takes quite a few revolutions before it starts and quite often it has a small backfire. Not really happy with the proceedure
 
What type of ignition do you have? I have the same carbed 0360 with light speed and one mag. No issues starting hot . I push the throttle in all the waynthen out almost full off, about 1/4 inch in, and it catches immediately
 
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Jeff,

Dual Slick 4370 series (I think that is the model number).

And to the others that have posted about starting full rich, that does not work. The engine will not even sputter... I have to go mixture full lean, throttle cracked a bit, spin the prop, then feed in the mixture slowly until it fires.

Thanks for all the responses,
 
ccsmith51 said:
My new (to me) RV-6 with an O-360-A1A acts the opposite. If I try to start it hot like my -4, it just seems to flood the engine. It won't even hardly catch. The way to start it is with throttle cracked, mixture in ICO, spin the prop, then slowly feed in mixture until she starts.
Thanks,
Click to expand...

Same thing with my engine (also an O-360-A1A). I had Lightspeed/Mag combination, recently changed to dual Lightspeed. Hot starts require mixture at ICO, then advance the mixture as the engine catches. Always starts very easily using this method.
 
0-360-A1A with two mags, primer to cylinders 3&4 only. I start the same both hot and cold with no issue:

1. Full rich
2. 1/2" throttle
3. Prime 4 sec.
4. Crank

I cant think of a time it hasn't started within 2 seconds.
 
O-360 with dual P-mags.

Hot starts are easy. Full rich and throttle all the way aft. Hit the starter and it is running by the second blade.

I don't ever remember to open my oil door
 
Lean hot start

I have o-360 duel mags. If doing a hot start, I lean all the way, crack throttle a little more then cold start. Turn over and the first sign of a sputter, slide to full rich and it starts right up.
Works 100%
I also can not start off full rich( like I do for cold starts), it will not start.
After that $100 burger, I walk out and put hand on motor. If too hot to hold hand on, then I use this hot start method.

QUOTE=N941WR;912801]O-360 with dual P-mags.

Hot starts are easy. Full rich and throttle all the way aft. Hit the starter and it is running by the second blade.

I don't ever remember to open my oil door[/QUOTE]
 
The OP wanted to know....

Can anyone tell me why this happens on a carbed engine? And why my two O-360's would be so different?
Click to expand...

Everyone is offering their best hot start procedure but no one has touched on the subject as to why. I did offer a guess but I too would like to know why
a hot start for the OP requires a hot start procedure more common when starting fuel injected engines. We know why it is required in fuel injected systems.
 
petersb said:
Mine is exactly the same. After the $100 burger my start procedure is NEVER prime, mixture full rich, throttle cracked then crank. It takes quite a few revolutions before it starts and quite often it has a small backfire. Not really happy with the proceedure
Click to expand...

Another point of interest, when turning on the electric pump, after the burger, it rattles away for some time before it picks up the fuel. No leaks in the system I can see.

I have a bypass line plummed in where the main fuel line connects to the carb, this is to relieve vapour lock. I have run this for some time before starting and still have to crank for some time, plus the sometimes backfire when starting hot.

This proceedure should cool the manual pump and relieve any vapour in the fuel system before start.

So the problem must be in the carb
 
Update: Possible carb problems...

It has been suggested by some friends with engine experience that I have a float issue with my carb, an MA4-5. They believe that it may be out of adjustment, or the float(s) may have some gas in them, causing the engine to run rich,

I also have backfiring when reducing power in the pattern, which they say could be attributed to a rich situation.

Finally, the mixture control arm is loose, having worn around the mixture post from the carb.

They suggest that I have the carb overhauled, or swap it for an overhauled unit.

I am looking at options now.
 
N427EF said:
Everyone is offering their best hot start procedure but no one has touched on the subject as to why. I did offer a guess but I too would like to know why
a hot start for the OP requires a hot start procedure more common when starting fuel injected engines. We know why it is required in fuel injected systems.
Click to expand...

Ernst,

I suspect your idea about fuel boiling out of the fuel bowl might have merit. When you advance the mixture from ICO it allows fuel to flow through the metering valve and into idle channel and/or nozzle. This would reduce the pressure in the fuel bowl, causing more fuel to boil. The net result could be a too-rich mixture.
 
ccsmith51 said:
I also have backfiring when reducing power in the pattern, which they say could be attributed to a rich situation.
.
Click to expand...

Mine also backfires in the pattern if the throttle is not closed SLOWLY. My engine expert ( very experienced, over 30 years in a top shop ) says this is normal.

I've noticed that the 360 is different to operate than the 160HP 320, it seems to have a much larger carb than you would expect for the 20 HP increase in power
 
petersb said:
Mine also backfires in the pattern if the throttle is not closed SLOWLY. My engine expert ( very experienced, over 30 years in a top shop ) says this is normal.

I've noticed that the 360 is different to operate than the 160HP 320, it seems to have a much larger carb than you would expect for the 20 HP increase in power
Click to expand...

Terminology review 101.

A backfire is when the air/fuel mixture in the intake system ignites and belches fire out of the carburetor. This is not normal nor is it a good thing.

The exhaust popping at idle in the pattern is not a backfire. The popping is caused by the unburned lean fuel/air mixture in the exhaust being ignited by the hot gasses exiting the cylinders. This popping is normal for CS propped engines with open exhaust.

Now on to hot starts.

O-360-A1A

Not all hot starts are the same. I check the fuel pressure before starting to decide whether I'll use plan B or plan C for starting. Plan A is for cold starts.

Plan B: Check fuel pressure and if below 5 psi a normal warm start ensues; Mixture to about the middle of travel, throttle cracked about 1/2 inch, left mag on, press the starter button. Starts every time, right mag now on.

Plan C: Check fuel pressure and if 5 or above a hot start ensues; Mixture to ICO, throttle cracked about 1/2 inch, left mag on, press starter button. After several blade swings the engine will start. As soon as the engine starts the right mag is switched on. Keep the engine running with the mixture only, Do NOT TOUCH the Throttle. After it starts push the mixture in to keep it running. It may try to die at this point so the mixture goes to ICO to keep it from quitting, advance the mixture when it catches. This is a mixture dance to keep it running until the fuel pressure drops below 5 psi and then the boost pump is turned on and the mixture set for taxi.

Fuel pressure is my clue. The fuel is boiling somewhere and causing a super rich mixture if the pressure is 5 or above. I have seen it as high as 7 or 8 psi and when that high it takes a longer dance with the mixture to keep it running. Once the pressure stabilizes to normal values the engine runs like it was a warm start.

I suspect that the fuel is boiling the carb bowl which floats the float to better seal the needle valve allowing the high pressure reading in the fuel line. The boiling fuel in the bowl may spill some into the carb throat coupled with the fuel vapor causes the rich mixture. Hence, the mixture at ICO for the hot start.

YMMV
 
ccsmith51 said:
I also have backfiring when reducing power in the pattern, which they say could be attributed to a rich situation.
Click to expand...

Actually you want the idle mixture to be slightly rich for year-round operation and the side effect is popping in the pattern.
 
ccsmith51 said:
It has been suggested by some friends with engine experience that I have a float issue with my carb, an MA4-5. They believe that it may be out of adjustment, or the float(s) may have some gas in them, causing the engine to run rich,

I also have backfiring when reducing power in the pattern, which they say could be attributed to a rich situation.

Finally, the mixture control arm is loose, having worn around the mixture post from the carb.

They suggest that I have the carb overhauled, or swap it for an overhauled unit.

I am looking at options now.
Click to expand...

If your mixture control arm is worn this can be an indication that this carb body has a lot of time on it. If it has a worn throttle shaft or bushings this can cause an air leak at the shaft at closed throttle settings and when trying to start it won?t pull fuel thru the idle circuit properly. Try to move the throttle shaft vertically up and down, if it moves even a small amount it?s time for an overhaul or replacement.
Have you checked the idle mixture rise? If the idle mixture adjustment is farther out than normal (more than 3 turns) this is an indication of air leaks at the carb throttle shaft or other places in the induction system.
 
I suspect that the fuel is boiling the carb bowl which floats the float to better seal the needle valve allowing the high pressure reading in the fuel line. The boiling fuel in the bowl may spill some into the carb throat coupled with the fuel vapor causes the rich mixture. Hence, the mixture at ICO for the hot start.
Click to expand...

Your statement is exactly opposite of what I believe to be true.
Boiling fuel in the carb bowl would make the float sink and flood the engine.
The float cannot be elevated by vapor.
Fuel pressure is another variable different from installation to installation.
It depends on where in the system you take a pressure reading.
Generally speaking, you'll get a low or no pressure reading if vapor is present upstream of where vapor lock occurs and high pressure if the fuel line is locked by vapor downstream in the system.
Anyone with vapor lock problems and a carburetor could greatly benefit from
installing a return line T'd out of the carb inlet fitting.

The OP stated a loose and worn mixture arm as well as some concern about
a float issue. Fix the mixture arm problem and make sure you do not have
a float issue.
Replacing the carb and not fixing the vapor lock problem will not solve the issue.
 
Vapor in the carb bowl is not an issue. The idle mixture valve is down in the bottom of the bowl where the fuel will be the coolest. As fuel is drawn up the main jet passages it is being drawn because of lower pressure in the venturi. At that point is where the vapor lock occurs. If the fuel in the bowl gets hot enough where a slight pressure drop allows vapor bubbles to form, then that's when vapor lock occurs.

The best way to handle this situation is to open the oil door if you're expecting a quick turnaround. That way cool air is being drawn into the cowl outlet as hot air is expelled which minimizes the amount of heating of the fuel thats in the bowl.

Also its important to make sure hoses are firesleeved and my favorite thing to eliminate is a firewall gascolator especially the POS blue gascolator from Van's. Those gascolators do not have a standpipe so any vapor that forms at the top is what gets drawn into the engine first.
 
Thanks to all for the great info....

I looked at the price of a new mixture arm: $130 from ACS. $790 for an overhauled unit. The overhaul seems like a better deal....

There is a bit of lateral movement of the mixture arm, I did not check for vertical movement.

The engine has about 800 hours since first major. It also has about 50 hours on four new Titan jugs!! There is nothing in the logs about any carb work, so I assume that this is the carb that came with the engine and it as 800 hours plus whatever before the major.

So, I am going to purchase an overhauled carb. I am looking at ACS, but are there any vendors for a carb that is overhauled an set up on a flow bench that might cost less than ACS?

Thanks again for all the input.
 
All fuel hoses are fire-sleeved. The gascolator is from a Cessna and will be removed next time cowl is off and I have time to mess with taking it out figuring out how to relocate and reconnect the hoses.
 
The RV I fly had one of the blue gascolators. It would vapor lock on the ground even with nothing but 100LL in it. I removed it and put inline fuel filters in the lines going to the fuel selector and plumbed them such that they are easy to remove for cleaning. Problem solved, no more vapor lock.
 
My fuel pump is mounted on the engine side of the firewall. It has a filter screen in the bottom. Should I consider relocating that to the cabin when I remove the gascolator?

It is like this one:

rrnrev.jpg
 
N427EF said:
Your statement is exactly opposite of what I believe to be true.
Boiling fuel in the carb bowl would make the float sink and flood the engine.
The float cannot be elevated by vapor.
Fuel pressure is another variable different from installation to installation.
It depends on where in the system you take a pressure reading.
Generally speaking, you'll get a low or no pressure reading if vapor is present upstream of where vapor lock occurs and high pressure if the fuel line is locked by vapor downstream in the system.
Anyone with vapor lock problems and a carburetor could greatly benefit from
installing a return line T'd out of the carb inlet fitting.
Click to expand...

At least we agree to disagree.

Boiling fuel in the carb bowl may cause the float to sink but it can also cause the float to dance in the boiling fuel creating more force on the needle and seat.
Vapor in a closed system should not cause a low pressure condition, only high pressure. Think pressure cooker. Give the vapor an out and the pressure stays at atmospheric. I have never seen a fluid line "locked" by vapor.
A return line in the system will help but will not solve the problem of boiling fuel in the carb bowl.

I do agree the OP needs to address his mixture arm problem.
 
ccsmith51 said:
My fuel pump is mounted on the engine side of the firewall. It has a filter screen in the bottom. Should I consider relocating that to the cabin when I remove the gascolator?

It is like this one:

rrnrev.jpg
Click to expand...

Relocating the pump would help. You could also try insulating it with firesleeve.
 
Overhauled carb ordered from ACS...

A bunch o' aviation units, but I think it will be a good investment.

I will report on any changes to start procedure, etc.

Thanks again for all the input.
 
zilik said:
Boiling fuel in the carb bowl may cause the float to sink but it can also cause the float to dance in the boiling fuel creating more force on the needle and seat.
Vapor in a closed system should not cause a low pressure condition, only high pressure. Think pressure cooker. Give the vapor an out and the pressure stays at atmospheric.
Click to expand...

Carb is not closed...the bowl has a vent. The only function of the float, bowl needle and seat is to keep the bowl full. It has no effect on fuel being metered. It has to be at ambient pressure for fuel to flow thru the main jet.
 
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An interesting discussion so far!

I've got a related question about what is meant by "vapor lock".

I can see where having a vapor bubble at the fuel pump would stop fuel from flowing, because the pump is designed to push liquid rather than vapor. However, I don't exactly see why vapor bubbles should cause any obstruction downstream of the pump. Vapor travels through a tube more easily than liquid. Bubbles would cause a momentary interruption in fuel metering in an injection system, but why should they have any effect at all on a carb?

Along the same lines, why would vapor bubbles enrich the fuel mixture, requiring the mixture to be at ICO for start? Seems like they should have exactly the opposite effect.

FWIW my fuel lines are fire-sleeved and my gascolator is located at the wing root, but I still find hot starts easiest with the mixture in ICO.
 
Now we are getting somewhere

Please correct me if what I write is incorrect, I am very much interested in helping the OP get to the bottom of his problem as well as gaining a better understanding of carbed systems.

Along the same lines, why would vapor bubbles enrich the fuel mixture, requiring the mixture to be at ICO for start? Seems like they should have exactly the opposite effect.
Click to expand...

The crux of this thread.

Vapor in the carb bowl is not an issue. The idle mixture valve is down in the bottom of the bowl where the fuel will be the coolest. As fuel is drawn up the main jet passages it is being drawn because of lower pressure in the venturi. At that point is where the vapor lock occurs. If the fuel in the bowl gets hot enough where a slight pressure drop allows vapor bubbles to form, then that's when vapor lock occurs.
Click to expand...

Carb is not closed...the bowl has a vent. The only function of the float, bowl needle and seat is to keep the bowl full. It has no effect on fuel being metered. It has to be at ambient pressure for fuel to flow thru the main jet.
Click to expand...

Unless the float sits low because of vapor and allows the carb bowl to be pressurized. I understand that fuel is drawn up, due to the lower pressure created in the venturi but if the bowl is pressurized it will force fuel out the jet
and flood the engine on start up necessitating an ICO mixture for starting.
The carb bowl cannot ordinarily be pressurized because of the function of the float and its shut off mechanism. To the best of my knowledge the bowl vents in
only and does not spill fuel out.
Boiling fuel during flight would create the vapor lock situation as Bob described
above. Obviously there are various option to deal with this but the best option is a fuel system installation that keeps fuel as cool as possible before it gets to the carburetor.

Fire away.
 
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The bowl vents to the outside of the venturi where the air pressure is atmospheric, so its vented into the throat of the carb. That's why it doesn't drain fuel to the outside. If it did that would be a serious fire hazard.
 
rocketbob said:
The bowl vents to the outside of the venturi where the air pressure is atmospheric, so its vented into the throat of the carb. That's why it doesn't drain fuel to the outside. If it did that would be a serious fire hazard.
Click to expand...

So, if the fuel starts to boil in the carb bowl it has two ways to get out, the nozzle and the bowl vent. Both are located inside the carb throat, so it makes perfect sense that this spillage would result in an enriched mixture. The problem is worsened by the fact that any liquid fuel reaching the venturi is going to be more completely vaporized in the warm carb throat than it would during a cold start.

Fuel vapor escaping out the bowl vent might also help explain why its possible for the engine to initially fire even with the mixture in cut-off position?
 
I love the educational aspect of VAF. This is a fascinating thread.

I don't have a hot start problem (knock on wood) but I have witnessed others with the issue and now I have a VERY limited knowledge of what's happening.

Thanks to all for their input.
 
So, if the fuel starts to boil in the carb bowl it has two ways to get out, the nozzle and the bowl vent. Both are located inside the carb throat, so it makes perfect sense that this spillage would result in an enriched mixture. The problem is worsened by the fact that any liquid fuel reaching the venturi is going to be more completely vaporized in the warm carb throat than it would during a cold start.
Click to expand...

Precisely !
 
Should have added that fuel or vapor can also flow through the idle channel - which would also contribute to a rich mixture.
Click to expand...
Maybe not vapor, since that channel is located at the bottom, but definitely another passage for fuel from a pressurized carb bowl.

http://www.vansairforce.com/community/showthread.php?t=114797

Although not common, fires do happen on hot starts in carbed engines.
Venting the carb bowl with its port on the inside of the carb throat works very well for venting and keeping the bowl pressure equalized. In the hot start scenario described in this thread and undoubtedly in the engine fire posted above the entire carburetor acted in an unplanned fashion.
Vapor in the carb bowl, low float, pressure in the bowl, fuel pushed into the carb throat through several open passages.


The bowl vents to the outside of the venturi where the air pressure is atmospheric, so its vented into the throat of the carb. That's why it doesn't drain fuel to the outside. If it did that would be a serious fire hazard.
Click to expand...

Hope to hear from the OP and others who may not agree with this.
 
N427EF said:
Maybe not vapor, since that channel is located at the bottom, but definitely another passage for fuel from a pressurized carb bowl.

http://www.vansairforce.com/community/showthread.php?t=114797

Although not common, fires do happen on hot starts in carbed engines.
Venting the carb bowl with its port on the inside of the carb throat works very well for venting and keeping the bowl pressure equalized. In the hot start scenario described in this thread and undoubtedly in the engine fire posted above the entire carburetor acted in an unplanned fashion.
Vapor in the carb bowl, low float, pressure in the bowl, fuel pushed into the carb throat through several open passages.
Click to expand...

Good point regarding the idle channel.

Regarding the concurrent engine fire thread, my impression was that this was largely due to misuse of the accelerator pump for priming?
 
Update....

Today I installed an overhauled MA-4-5. I got it from Aircraft Spruce, and they were a pleasure to work with. I also communicated directly with Marvel Schebler and they, too, were a delight to deal with.

The install took about three hours with the help of a very good friend and engine expert. No issues encountered at all. I have to say that the carb was set almost perfectly from MS. We needed to adjust the idle RPM screw about 1 turn. The idle mixture adjustment was set perfectly from the factory. Slowly go to ICO from full rich and there was about a 25 RPM increase before she quit.

MS told me that they are the only facility in the US that puts carbs on a flow bench and adjusts them to a master. I don't know exactly what that means but the carb sure seems to be right.

I flew for about 15 minutes and got the engine hot. I landed and let it sit for about 5 minutes to heat soak a bit. Then I started it. I used the procedure I used on my O-360 RV-4, that is, full rich, throttle about 1/2" in. It started after a couple of blades, totally different than before when I had to start at ICO and slowly feed in mixture until it caught.

I will be flying tomorrow to lunch. That will get the engine hotter, and it will sit longer. That will be a better test of the hot start. I'll report the findings.

Thanks again to all who helped.
 
Hot starting-carb vs fuel injection

The reason why carbureted engines hot start when rich, and fuel injected engines hot start lean is related to the location of the fuel:

Updraft carbs are below the engine, and vented to the atmosphere. There is no chance of flooding the engine while parked if carb is functioning properly.

Fuel injected Lycoming engines have a fuel distribution block and lines on top of the cylinders, and this fuel leaks into the heads after shut down. After the engine has a chance to cool, the excess fuel has evaporated away. While the engine is still hot, it needs to be started lean.

The old fashion flood start for fuel injected engines first floods, then starves the engine of fuel, knowing that somewhere in the middle it will fire

See recent thread from Dan for excellent advice on hot starting technique

Jay
RV-7A
C-FXPT
 
Alan Carroll said:
Good point regarding the idle channel.

Regarding the concurrent engine fire thread, my impression was that this was largely due to misuse of the accelerator pump for priming?
Click to expand...

Lots of inaccurate information here. I suggest you look at the carb diagrams before making statements. The idle channel is not on the bottom; It runs vertical from top to bottom. Further, you can't pressurize the fuel in your bowl any more than you can pressurize the water in your bathtub. Same concepts and physics apply. Finally, most all carburetor fires are caused by backfire. On a related note, some of those backfires are caused by an excessively rich mixture, such as that identified by another poster -over exercising the accelerator pump.

Larry
 
Update, part deux...

Yesterday, in the heat of the FL late morning, I flew about 30 minutes to see a friend. I landed the plane, parked in the sun, and didn't open the oil door.

I hung around at his hangar for about 20 minutes then hopped back in the plane. I turned on the boost pump, full rich, cracked the throttle about 1/2", and spun the prop. It coughed on the third blade and caught on the fifth. Leaned at idle and all was well.

This is the hot starting procedure I was used to on the O-360 on my RV-4.

So, it appears that my hot start issue was resolved by replacing the carb. I don't know if the problem had to do with a sloppy mixture arm and rod, something with the float, something else inside the carb, or a combination of things. In any case, it works the way I expect so I am a happy camper!!

Thanks to all for your insight and help.
 
ccsmith51 said:
I hung around at his hangar for about 20 minutes then hopped back in the plane. I turned on the boost pump, full rich, cracked the throttle about 1/2", and spun the prop. It coughed on the third blade and caught on the fifth. Leaned at idle and all was well.
Click to expand...

I do the same but from time to time I get a backfire. Told it is probably fuel pooling around the exhaust valve and igniting when the engine fires.

Did try starting with lean mixture but the engine would not start
 
lr172 said:
I suggest you look at the carb diagrams before making statements. The idle channel is not on the bottom; It runs vertical from top to bottom. Further, you can't pressurize the fuel in your bowl any more than you can pressurize the water in your bathtub. Same concepts and physics apply. Finally, most all carburetor fires are caused by backfire. On a related note, some of those backfires are caused by an excessively rich mixture, such as that identified by another poster -over exercising the accelerator pump.

Larry
Click to expand...

Actually I did look carefully at the diagrams. The the "idle supply opening" is clearly labeled, and is indeed located near the bottom of the fuel bowl. This connects to the idle channel, which runs vertically up to its delivery point near the throttle plate. I think maybe we're saying the same thing.

I'm still curious why the OP's carb required starting with mixture in cutoff.
 
For educational purposes it would be interesting to find out what exactly caused the issues described by the OP.
I don't suppose you could get a report from whomever is repairing that carburetor???

Over the years there have been many ADs issued concerning leaky floats in carburetors. A leaky float causes the same problems as boiling fuel in the carb bowl. Engine flooding.

Originally Posted by lr172
Further, you can't pressurize the fuel in your bowl any more than you can pressurize the water in your bathtub
Click to expand...

But you can make it flow over, same concepts and physics apply.
 
N427EF said:
For educational purposes it would be interesting to find out what exactly caused the issues described by the OP.
I don't suppose you could get a report from whomever is repairing that carburetor???
Click to expand...

I bought an overhauled carb from ACS, which was actually overhauled by Marvel Schebler. I sent my carb back to ACS as a core. I doubt that they will do any troubleshooting. I expect that they will strip it, clean it, and rebuild it to resell to the next customer.
 
N427EF said:
For educational purposes it would be interesting to find out what exactly caused the issues described by the OP.
I don't suppose you could get a report from whomever is repairing that carburetor???

Over the years there have been many ADs issued concerning leaky floats in carburetors. A leaky float causes the same problems as boiling fuel in the carb bowl. Engine flooding.
Click to expand...

I agree completely regarding the educational value of this discussion. Perhaps we don't have all the right answers yet but I would like to get them!

In my case I recently replaced the float (turns out the old one was fine), but still have the same issue regarding hot starts. In any case I don't think the float can account for differences during hot vs. cold starts?

I would agree with Larry's point that the bowl isn't being pressurized, but as you note this doesn't mean the fuel can't boil out.
 
I too would like to get it right and it is not about who is right here but what is correct. So, please explain if you really understand carburetors.

I would agree with Larry's point that the bowl isn't being pressurized, but as you note this doesn't mean the fuel can't boil out.
Click to expand...

That is true under normal circumstances but not true if the float sits low opening the inlet to the pressurized fuel line.
If the float is half filled with fuel it will no longer shut off fuel flow exposing the bowl to fuel pressure.

If the fuel boils in the bowl the float also sits low, it does not float on vapor.

There has got be to someone smarter than me who knows more about carburetors, so please explain, thank you.
 
N427EF said:
I too would like to get it right and it is not about who is right here but what is correct. So, please explain if you really understand carburetors.



That is true under normal circumstances but not true if the float sits low opening the inlet to the pressurized fuel line.
If the float is half filled with fuel it will no longer shut off fuel flow exposing the bowl to fuel pressure.

If the fuel boils in the bowl the float also sits low, it does not float on vapor.

There has got be to someone smarter than me who knows more about carburetors, so please explain, thank you.
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Boiling only release vapor. It can't push the fuel uphill. However, the boiling fuel will reduce the density somewhat and allow the float to drop. However, the exit of the main jet is 1/4"- 1/2" above the normal fuel level. I don't believe the float can drop enough to allow enough fuel in to raise the level high enough (i.e. 1/4"-1/2") to exit this orifice.

I think we have proven all of this discussion simply doesn't apply, as the OP replaced the carburetor and the problem disappeared when exposed to the same conditions. We all know that vapor lock causes problems and I have heard that over-filling of the bowl can be part of the problem. However, I don't believe it is the problem being portrayed here. One of us should research vapor lock. I an certain it is a well documented issue as it was a real problem for automobiles prior to fuel injection.

I do know that fuel level, as regulated by the float, is critical to fuel mixture in any carb. However, it is much less of an issue for aviation carbs, as we have direct control over the fuel flow through both the idle and main circuits. In the auto world, this is controlled only by jets and fuel bowl fill level. atmospheric pressure acting on the surface of the fuel in the bowl along with a low pressure in the venturi draw it through. It is a balancing acting with the pressure differential and the size of the channel (i.e. jet). This is why a car in Colorado will require a different jet than a car in Chicago, if they desire the same fuel delivery. In aviation we have continuously adjustable fuel delivery through an infinitely adjustable fuel channel (i.e. jet). The main jet that everyone talks about increasing only sets that max fuel flow. Everything below max flow is infinitely adjustable.

I do speculate that a significant problem in vapor lock is boiling causing the float to drop and overfilling the bowl. At full mixture setting, this imbalance between pressure and channel size (remember increased fuel bowl level reduces the amount of movement against gravity to get to the venturi) causes an overly rich mixture leading to the stumbles and stalls that are seen on take off in hot start conditions. The pressure differential is acting against a lower gravitational force and therefore delivers more fuel through the same jet size due to this fact. This is really only a problem because the engine is being operated on a fixed fuel delivery (i.e. max flow) and the raised fuel height created in imbalance causing a high fuel delivery height. I believe that I have read at least one post on this forum describing how the poster experienced this common problem on take off and resolved it by reducing the mixture.

The above would not really apply to a hot start, as the idle circuit dumps much higher in the venturi and the relative difference in fuel fill height is insignificant.

The other problem is that fuel pumps aren't effective at pumping air, only liquids. However, I still struggle to see how this is a significant issue with an upstream electric boost pump. I have no experience here and just supposition.

Larry
 
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N427EF said:
That is true under normal circumstances but not true if the float sits low opening the inlet to the pressurized fuel line.
If the float is half filled with fuel it will no longer shut off fuel flow exposing the bowl to fuel pressure.
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Again, because the bowl is vented to atmosphere, there is no pressure. Stick your garden hose in a bucket full of water. The water from the city is probably 50 PSI. Once it hits the hose, its probably 10 PSI and as soon as it hits the bucket and the water level is raised accordingly, it is 0 PSI. The fact that the hose is under water doesn't change that fact. Yes, the water will overflow, but it is not due to the pressure of the incoming water. Although the pressure in the hose was necessary to over come the pressure of the water in the bucket, once in the bucket there no pressure (pressure has equalized). The reason the water overflows is due to the volume of material overcoming the open physical space. Same thing in the carb bowl. In this case. the edge of the bucket is the main fuel delivery orifice, which sits about 1/2" above the normal fuel level. The question is, can vapor reduce the bouyance of the float enough to allow a 1/2" worth of fuel in. I do not believe that It can unless the float has absolutely no air in it and then it will also overfill in ALL situations.

By the way, I know quite a bit about carburetors. I am new to aviation carbs and try to make that clear so my input can be viewed as limited. However, I know quite a bit about carb theory and have custom tuned many carbs, from singles to four barrels. I have also custom built a fuel injection system, including the processor and all software. I am not just a guy repeating what I read on the internet once.

Larry
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Larry,

Thanks for your thoughts on this; always good to hear from someone with practical experience and knowledge on the subject. The idea that the bowl is overfilling seems plausible, particularly if its related to lower fuel density while hot. You analysis of the physics of fuel flowing from the bowl up to its delivery points makes sense.

I have to respectfully disagree though with the general idea that boiling can only release vapor and can't push fluid uphill. If you boil a full pot of water the resultant vapor bubbles can certainly push fluid uphill and out of the pot. Other similar examples are a shaken bottle of soda, or a geyser. Boiling can propel fluids quite energetically uphill. Whether this happens in carburetors is perhaps a different question, to which I don't know the answer. I would imagine it depends on temperature, fluid viscosity, and size/shape of the flow channels?
 
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