O-360 hot start

Larry,
thanks for the detailed reply and I am sure that you know more about carburetor than I'll ever know.
Most of us understand the concept of carburetors under normal operating conditions.
The entire thread is dealing with an unusual condition possibly involving a defective carburetor.
Unfortunately we'll never know since the OP has replaced the carb and the hot starting symptoms have disappeared.
The carb may have only needed a new float as per the following SB issued by Marvel Schebler.

http://msacarbs.com/pdf/MSA-13.pdf

In case you don't want to read the whole bulletin:

In the case of the advanced polymer floats, there is also a possibility of leaks through the welded seam. This allows a portion of the float to fill with fuel and thereby reduce the buoyancy of the float, which could lead to
flooding or poor idle performance.

Click to expand...

The bulletin does not go into detail as to why it is flooding but I have explained those reasons in detail. Vapor in the float bowl causes the same kind of problems.
And for those who lean the mixture on take off at sea level, I think you are masking a problem with the carburetor or a hot fuel issue. A properly functioning carb does not have to be leaned at seal level to keep the engine from dying.

I fully understand that the float bowl is vented but the vent hole is
about .005 " (correct me please) you supply fuel at a pressure of 4 to 5 psi to an "almost" closed system (except for .005" vent hole idle mixture channel and
mixture channel) and a 3/8" fuel line capabable of delivering 45 GPH,you will pressurize the bowl unless the float closes the inlet valve causing the flooding at idle or hot start or even cold starts.
What say you Larry?
I hate to argue by myself

Like I said, I hate to argue by myself and I did find this quote on the internet
It, along with info from the Marvel schebler bulletin confirms my initial theory on why a carbureted engine can flood under hot conditions.

Flooding

This is a problem that is usually (but not always) the carburetor's fault. The carburetor may flood if dirt enters the needle valve and prevents it from closing. With no way to shut off the flow of fuel, the bowl overflows and spills fuel into the carburetor throat or out the bowl vents. A flooded engine may not start because the plugs are wet with fuel.

WARNING: Flooding can be a very dangerous situation because it creates a serious fire hazard if fuel spills out of the carburetor onto a hot engine.

A carburetor can also flood if the float inside the fuel bowl is set too high or develops a leak and sinks (this applies to hollow brass or plastic floats primarily). If all that is needed is a new float, there is no real need to replace the entire carburetor. Floats are not part of a rebuild kit, so if new gaskets are also needed, a rebuild kit will have to be purchased, too.

Flooding can also be caused by excessive fuel pressure forcing fuel past the needle valve. Flooding may also be caused by excessive heat in some instances. A heat riser valve on a V6 or V8 engine that sticks shut may create a hot spot under the intake manifold that causes the fuel in the carburetor bowl to boil over and flood the engine.

Click to expand...

The analysis describes exactly the 2 scenarios I mentioned earlier, vapor problems or leaky float.

N427EF said:

Larry,
thanks for the detailed reply and I am sure that you know more about carburetor than I'll ever know.
Most of us understand the concept of carburetors under normal operating conditions.
The entire thread is dealing with an unusual condition possibly involving a defective carburetor.
Unfortunately we'll never know since the OP has replaced the carb and the hot starting symptoms have disappeared.
The carb may have only needed a new float as per the following SB issued by Marvel Schebler.

http://msacarbs.com/pdf/MSA-13.pdf

In case you don't want to read the whole bulletin:



The bulletin does not go into detail as to why it is flooding but I have explained those reasons in detail. Vapor in the float bowl causes the same kind of problems.
And for those who lean the mixture on take off at sea level, I think you are masking a problem with the carburetor or a hot fuel issue. A properly functioning carb does not have to be leaned at seal level to keep the engine from dying.

I fully understand that the float bowl is vented but the vent hole is
about .005 " (correct me please) you supply fuel at a pressure of 4 to 5 psi to an "almost" closed system (except for .005" vent hole idle mixture channel and
mixture channel) and a 3/8" fuel line capabable of delivering 45 GPH,you will pressurize the bowl unless the float closes the inlet valve causing the flooding at idle or hot start or even cold starts.
What say you Larry?
I hate to argue by myself

Click to expand...

The vent does not have a metered channel and is probably closer to an 1/8". You simply cannot pressurize it, by design. Further, while the fuel feed is 3/8", the clearance between the needle and seat, especially when mostly closed as it would be with the float near the closed position, is quite small. However, orifice size mostly impacts flow rate and not pressure.

As I stated earlier, allowing the fuel level in the bowl to rise above the designed limit will cause a rich condition, period. However, it is due to the pressure dynamics that I outlined in my last post and has nothing to do with the inlet pressure, which will always equalize to atmospheric pressure due to the vent.

I agree that a high fuel level will richen the mixture, especially the main feed mixture. I still contend that the impact on idle circuit will be much less. However, I still don't believe that a less bouyant float, excluding a fully saturated float (i.e. 0 buoyancy), could cause fuel to flow through the main jet. the reduced denisty of boiling fuel would not cause the float to have 0 buoyancy.

Larry

QUOTE: "Flooding can also be caused by excessive fuel pressure forcing fuel past the needle valve. Flooding may also be caused by excessive heat in some instances. A heat riser valve on a V6 or V8 engine that sticks shut may create a hot spot under the intake manifold that causes the fuel in the carburetor bowl to boil over and flood the engine."


the first sentence is discussing how fuel pressure exceeding the designed level will force fuel past the needle/seat. The float buoyancy and leverage / fulcrum of the arm is designed for a certain inlet pressure. If you exceed that design pressure, your fuel level in the bowl will rise beyond the designed limit. It does not introduce pressure into the bowl.

The second sentence relates to exactly what we are discussing here. When the fuel gets hot and or boils, the density is reduced and the float is less effective, allowing the fuel level in the bowl to rise, causing a rich condition.

These are serious problems in a fixed mixture system, like a car. There is no good way around them other than getting cooler fuel to the carb. In our world, we have a red knob that adjusts mixture. If you understand what's happening, you can temporarily adjust the mixture to make the engine run. Just be aware that as the fuel cools, you will move to a lean state and will need to re-adjust.

To the poster that calls this a defective carburetor, I challenge you to find a carb that can deal with boiling fuel by design. Running rich due to boiling fuel is not a design defect nor is it a sign of a broken or mal-adjusted carb. The only resolution is to keep the fuel cool. This obviously excludes rich conditions caused by a defective or mis-adjusted float. Clearly there was another problem in the OP's carb that magnified the boiling fuel problem or possibly something unrelated.

Alan Carroll said:

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?

Click to expand...

Disagreement is good and is often a good path to learning.

First, shaken soda and geysers both operate on a closed system principal and don't apply here because the carb is an open system. Boiling water is a volume and agitation issue. The volume of the water in the pot grows because of the constant presence of air bubbles caused by the boiling. Assuming the pot could hold the extra volume, the overflow is caused by the movement of water at the surface as the air bubble pop In this case, not much volume of water overflows. If you put a lid on it, you pressurize the environment because more air is generated via boiling than can escape through the air outlets.

If you but a small glass tube in the pot extending up and observe, I think you would find that the water level does not rise above the original water. This tube would correlate to the fuel feed channel.

N427EF said:

Larry,
thanks for the detailed reply and I am sure that you know more about carburetor than I'll ever know.
Most of us understand the concept of carburetors under normal operating conditions.
The entire thread is dealing with an unusual condition possibly involving a defective carburetor.
Unfortunately we'll never know since the OP has replaced the carb and the hot starting symptoms have disappeared.
The carb may have only needed a new float as per the following SB issued by Marvel Schebler.

http://msacarbs.com/pdf/MSA-13.pdf

In case you don't want to read the whole bulletin:



The bulletin does not go into detail as to why it is flooding but I have explained those reasons in detail. Vapor in the float bowl causes the same kind of problems.
And for those who lean the mixture on take off at sea level, I think you are masking a problem with the carburetor or a hot fuel issue. A properly functioning carb does not have to be leaned at seal level to keep the engine from dying.

I fully understand that the float bowl is vented but the vent hole is
about .005 " (correct me please) you supply fuel at a pressure of 4 to 5 psi to an "almost" closed system (except for .005" vent hole idle mixture channel and
mixture channel) and a 3/8" fuel line capabable of delivering 45 GPH,you will pressurize the bowl unless the float closes the inlet valve causing the flooding at idle or hot start or even cold starts.
What say you Larry?
I hate to argue by myself

Click to expand...

The vent does not have a metered channel and is probably closer to an 1/8". You simply cannot pressurize it, by design. Further, while the fuel feed is 3/8", the clearance between the needle and seat, especially when mostly closed as it would be with the float near the closed position, is quite small. However, orifice size mostly impacts flow rate and not pressure.

As I stated earlier, allowing the fuel level in the bowl to rise above the designed limit will cause a rich condition, period. However, it is due to the pressure dynamics that I outlined in my last post and has nothing to do with the inlet pressure, which will always equalize to atmospheric pressure due to the vent.

I agree that a high fuel level will richen the mixture, especially the main feed mixture. I still contend that the impact on idle circuit will be much less. However, I still don't believe that a less bouyant float, excluding a fully saturated float (i.e. 0 buoyancy), could cause fuel to flow through the main jet. the reduced denisty of boiling fuel would not cause the float to have 0 buoyancy.

Larry

Update, part trois...

OK, after replacing the carb I thought the problem was solved. Alas, I guess I didn't let the engine get hot enough. After flying and letting it heat soak, the hot start problem is still there....

Today I spoke with Jeff at MS and explained the issue and that I had installed an overhauled carb from them, yet the problem persisted.

He suggested I have a mag issue. He said he has seen this before when the coils start going bad and the mag gets hot. The spark gets weak and can't fire the mixture.

He asked when the mags, Slicks, were last serviced. They were new when the engine was overhauled about 800 hours ago. There is a log entry 190 hours ago saying that the harnesses were replaced and the mags checked and they were OK. Now I am not sure what that means...

I'm guessing that he did not have them actually removed and checked at around 500 hours.

Today I also talked to Shaun at my mag shop and he said that it could be worn points, a weak condensor, a weak impulse coupling, a worn cam. He quoted $300 - $400 to overhaul each mag if they needed everything.

Jeff suggested that when the engine has cooled overnight I pull the cowling and heat the mags with a heat gun to get them good and hot. Then see if I have the starting issue.

The heat gun seems like a reasonable test, but I am thinking that if the mags have not been serviced then I need to bite the bullet and pull them and have them taken care of.

So, that is where I am at now. I'm leaving tomorrow for NC for the weekend to see the colors so will address this on Tuesday and keep y'all informed with what I find.

Thanks again for all the help given by everyone.

Sorry to hear

I do hope you find the source of your hot start problem.
For now you have bought a new or overhauled carburetor.
By your own account you probably didn't need one and it can't hurt to take a close look at your mags. At 800 hours they should have had at least an overhaul at 500 hours.
I suspect however that in the end you will still need to fix your hot fuel problem. That would be your least expensive approach.
Please keep us posted on what solution you find to cure your hot start problem.

Thanks, Ernst.

I don't feel too bad about the overhauled carb. The mixture arm was worn out and the cost to purchase just that part was about 20% of the total overhaul cost. There was nothing on the overhaul sheet about the carb, so I assumed it was the original, not been serviced, so erred on the side of caution and got an overhauled one.

The mag(s) may or may not be the culprit, the only way to tell is to pull them and have them checked. And since there is nothing in the logs about them being serviced it makes sense to have that done while they are off the plane.

As far as hot fuel, all hoses are fire sleeved and at the bottom rear of the engine compartment. The only thing not insulated is the firewall mounted Facet fuel pump.

I'll report in a couple of weeks when I get the mags done and back on and have a chance to fly it enough to get it really hot and then let it sit in the sun.

fuel pump

Chris, is that Facet fuel pump on the engine side of firewall or cockpit side?

Engine side, bottom right on the firewall. Firesleeved from firewall fitting to pump, then firesleeved from pump to carb.

I've thought about running a blast tube to it?

Another update....

I took both Slicks to Shaun at Eagle Magneto, my mag guy. It appears that they had not been serviced in the 14 years and 800 hours since the engine overhaul.

Between the two mags he replaced two point sets and cams, one pinion gear, three bearings, and all seals. Fortunately, after a thorough exam the coils and impulse coupler tested good.

He said the points were pretty rugged and the cams were getting worn. He said that they really needed the servicing.

I then brought them back to the plane and had my friend Walt install them and time them. He has the laser and box and all the tools needed to do it right.

I then went for a test flight. The engine started fine, as it always does when it is cold. I flew for 0.6 and flew pretty hard. Aileron rolls, loops, barrel rolls, cloverleafs, etc. Power changes and loading the engine to get all the temps up.

Then I landed, taxied back to my hangar, and parked outside in the hot FL sun. I let it heat soak for 45 minutes, just as if we had flown the 30 minutes to XFL and had a great blackened Mahi-Mahi sandwich with red beans and rice at HiJackers.

Then I went to start it in the normal way. Boost pump until I hear it load up then off. Mixture rich. Throttle cracked. Hit the ignition and it started right away. Just the way I would expect from an O-360.

So, is my problem solved? Who knows. I thought it was when I replaced the carb.

Only time will tell, I guess, and if the hot start problem resurfaces I'll be back here asking for help again!!!

In any case, I now have an engine with 800 SMOH on the bottom, 60 on new cylinders and rings, 15 on an overhauled carb, and 2 on serviced mags. I should be golden for a long time!!