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Vapor Lock?

Tram

Well Known Member
So, I’ll try to hit the high points...

Couple months ago, went for a quick flight and had a small loss of power feeling while in the pattern. Landed, and called it a day.

Went out a couple days later and had a loss of power on the take-off climb out. Immediately landed and taxied back. Did a mag check on the ground just out of curiosity, and had a rough running mag. Trouble shot it a little bit. Thought I’d sorted it out..

Went out a couple days ago, ran it up and everything was fine, but I was unable to fly.

So, I went out today. Good run up. No bad mag checks, decided to take it up.

RV-6. O-320. Mogas. Fuel pressure gauge “stuck” at 8PSI, boost pump on or off.

Quick flight and about 15-20 minutes into the flight, the engine started running a little rough. More like a slight loss of power.

Decided to return to the airport.

Once I made the turn toward the field, tthe engine began running extremely rough. Rough enough that I was considering where I’d be landing short of the airport.

I was able to play with the mixture, boost pump, swap tanks, etc but nothing really seemed to help. I was able to kept it running so I initiated a slow climb to give myself some options, gained enough altitude to feel comfortable to continue towards the airport and started that way. Still fighting the engine the whole way.

During the descent the engine smoothed out noticeably, almost normal. Landed and taxied in without event.

Would this sound like vapor lock? Fuel pump dying? Clogged fuel line?
 
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Vapor lock?

1. OAT?
2. Altitude?
3. 150 HP or 160 HP?
4. Carburetor or fuel injection.
5. Ethanol Free Mogas?
6. What exact octane Mogas?
7. Have you always run Mogas?
8. Fire sleeve on gascolator & fuel line from gascalator to carburetor?
 
Definitely doesn't sound like vapor lock; Failing fuel pump is a suspect. Likely you have some issue with either carburation or fuel delivery, though ignition is still a possible culprit. Your engine is literally screaming at you "fix me!" Do not go on further flights until someone knowledgeable does some diagnostics and develops a troubleshooting plan. If you continue flying it, at some point your luck will run out.

an un-commanded reduction in power output is a serious symptom (especially when the cause is unknown) and sometimes difficult to diagnose. Do not take the warning lightly. If you don't know what is causing it, don't assume it won't get worse.

Larry
 
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1. OAT? Around 60* it was 71 on the deck.
2. Altitude? Started around 2,500?
3. 150 HP or 160 HP? 160.
4. Carburetor or fuel injection. Carb?d.
5. Ethanol Free Mogas? Yes. Tested before used.
6. What exact octane Mogas? Supposed to be 93.
7. Have you always run Mogas? No. I have for about the last year, but not always.
8. Fire sleeve on gascolator & fuel line from gascalator to carburetor? Only from gas player to carb. None on the gascolator.
 
Definitely doesn't sound like vapor lock. Likely you have some issue with either ignition, carburation or fuel delivery. Your engine is literally screaming at you "fix me!" Do not go on further flights until someone knowledgeable does some diagnostics and develops a troubleshooting plan. If you continue flying it, at some point your luck will run out.

an un-commanded reduction in power output is a serious symptom (especially when the cause is unknown) and sometimes difficult to diagnose. Do not take the warning lightly. If you don't know what is causing it, don't assume it won't get worse.

Larry

I agree completely. It?s not going anywhere until it?s sorted out.
 
I've run mogas in a carb 320 for 20 yrs, with one vapor lock incident. Winter blend gas on a 100+ degree day after a heat soak.

FWIW...
 
I've run mogas in a carb 320 for 20 yrs, with one vapor lock incident. Winter blend gas on a 100+ degree day after a heat soak.

FWIW...

Definitely wasn?t 100* today.. I?ve had no issues with mogas so far, that I?m aware of.
 
I've run mogas in a carb 320 for 20 yrs, with one vapor lock incident. Winter blend gas on a 100+ degree day after a heat soak.

FWIW...

And that is the classic scenario for vapor lock. incredibly rare to see vapor lock in the air, excluding the initial takeoff climb after heat soak on a very hot day.

Larry
 
Definitely wasn’t 100* today.. I’ve had no issues with mogas so far, that I’m aware of.

Some things you can start looking at without professional help.

Check the plugs for lead fouling and resistance.
Check the resistance of each plug wire.
Check the resistance of each coil (inside the mag)
If you are near the 500 hour mark, send the mags in for inspection. At some point someone will need to open them up for a look, even if not a formal IRAN inspection.
pull the screen filter in the carb to check for debris.
Pull the inlet line off the fuel pump and run the boost pump to check for a good solid flow volume. Run it all through a paper towel in a funnel and check for debris.
If the fuel pump is old or has a lot of hours, replace it. If you do this, a tear down of the pump can possibly confirm if it was a culprit. DanH has a post that shows pictures and can give you an idea how to take it apart.
Use a mechanical gauge to confirm accuracy of FP sensor/instrument. If it is accurate and can be trusted, the pegged 8 PSI reading is a good indication of a failing fuel pump. I had a pump on the 350 chevy (functionally identical to the lyc's - both made by AC) in my boat fails this way and kept flooding the carb. Kept rebuilding the carb and checking for cracks, as I had never seen a pump fail with high pressure; Always low pressure. A painfull lesson.
Perform valve wobble test. Some good threads here with info to help guide you through the process.

Just some things you can do on your own.

Larry
 
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Some things you can start looking at without professional help.

Check the plugs for lead fouling and resistance.
Check the resistance of each plug wire.
Check the resistance of each coil (inside the mag)
If you are near the 500 hour mark, send the mags in for inspection. At some point someone will need to open them up for a look, even if not a formal IRAN inspection.
pull the screen filter in the carb to check for debris.
Pull the inlet line off the fuel pump and run the boost pump to check for a good solid flow volume. Run it all through a paper towel in a funnel and check for debris.
If the fuel pump is old or has a lot of hours, replace it. If you do this, a tear down of the pump can possibly confirm if it was a culprit. DanH has a post that shows pictures and can give you an idea how to take it apart.
Use a mechanical gauge to confirm accuracy of FP sensor/instrument. If it is accurate and can be trusted, the pegged 8 PSI reading is a good indication of a failing fuel pump. I had a pump on the 350 chevy (functionally identical to the lyc's - both made by AC) in my boat fails this way and kept flooding the carb. Kept rebuilding the carb and checking for cracks, as I had never seen a pump fail with high pressure; Always low pressure. A painfull lesson.
Perform valve wobble test. Some good threads here with info to help guide you through the process.

Just some things you can do on your own.

Larry

Larry-

Thank you the list of things to start working through.

The stuck fuel pressure is getting me. Maybe it?s something as simple as the mech pump.

We are right at 450 hours on the engine, so it?s time to start having things looked at.

Again, I appreciate your help.
 
Your Marvel-Schebler carb specifies a maximum of 6 PSI fuel pressure, I think. Too much pressure can overcome the weight of the floats and give you an excessively rich mixture. Somebody help me out here but I'm pretty sure there's a spring in the mechanical fuel pump that limits the output pressure to 6 PSI or less. When your engine is running rough, maybe you could lean it out a bit and see if that fixes it.
 
Larry-

Thank you the list of things to start working through.

The stuck fuel pressure is getting me. Maybe it?s something as simple as the mech pump.

We are right at 450 hours on the engine, so it?s time to start having things looked at.

Again, I appreciate your help.

A failing fuel pump is very high on the list of potential suspects for your symptoms, especially with the high FP readings. At 450 hours, I would just replace it for peace of mind. When they fail high, the boost pump won't help, which also matches your symptoms. I would go buy a cheap mechanical pressure gauge and some tee fittings to confirm your instrument.

Larry
 
A failing fuel pump is very high on the list of potential suspects for your symptoms, especially with the high FP readings. At 450 hours, I would just replace it for peace of mind. When they fail high, the boost pump won't help, which also matches your symptoms. I would go buy a cheap mechanical pressure gauge and some tee fittings to confirm your instrument.

Larry

Yep. Had this discussion with my father today, we own the plane together, and a new pump is going to be part of the repair plan.

I’m also going to test the instrument. I replaced a bad sending unit 3 or so years ago, but the symptoms were different.
 
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Your Marvel-Schebler carb specifies a maximum of 6 PSI fuel pressure, I think. Too much pressure can overcome the weight of the floats and give you an excessively rich mixture. Somebody help me out here but I'm pretty sure there's a spring in the mechanical fuel pump that limits the output pressure to 6 PSI or less. When your engine is running rough, maybe you could lean it out a bit and see if that fixes it.

I tried leaning it yesterday, but that had very little effect on the situation.
 
Your Marvel-Schebler carb specifies a maximum of 6 PSI fuel pressure, I think. Too much pressure can overcome the weight of the floats and give you an excessively rich mixture. Somebody help me out here but I'm pretty sure there's a spring in the mechanical fuel pump that limits the output pressure to 6 PSI or less. When your engine is running rough, maybe you could lean it out a bit and see if that fixes it.

Yes, too high of a FP will exceed the floats ability to maintain the proper fuel level in the bowl, causing a overly rich condition. This would be hard to address with the mixture control, as it would need a lot of authority and likely the pressure would be fluctuating (remember, it is in a failure mode), requiring constant manipulation to get the fuel level correct. The amount of leakage past the needle/seat would vary with the actual pressure and therefore a different restriction from the mixture valve for each pressure level.

There is a flapper disc with a spring that limits fuel pressure out of the pump. However, all things mechanical are prone to failure. I can't tell you exactly what component failure causes the high pressure, though that flapper and/or spring would be high on the list, I can assure you that they CAN fail in a way that produces a PSI level higher than designed, eventhough it is not the typical failure mode.

Larry
 
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Yep. Had this discussion We my father today, we own the plane together, and a new pump is going to be part of the repair plan.

I’m also going to test the instrument. I replaced a bad sending unit 3 or so years ago, but the symptoms were different.

Be sure to give your ignition a good look as well. It is well down the list of culprits, but taken with the fact the you have also had failed mag checks means it needs a good looking at.

Larry
 
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Off topic but that sounds like fekkin paradise right about now.

Beyond paradise for me. That is literally 90* warmer than it was here 3 or 4 days ago. Though today it is now 40, so in the mood to put on shorts.

Larry
 
Bendix Magneto SB 669 & SB 670

Check out/google SB 669 & SB 670.
Replacement and maintenance of the Continental
Motors S-20, S-200, and S-1200 Series Magneto Distributor Block.
Not saying this is your problem.
About 50 hours ago we completed the 500 hour Magneto Inspection & service on our Bendix Magneto’s. Lycomimg O320 160 HP. They were running perfectly..prior to the 500 hour inspection.
We replaced the distributor block & gear with new parts; as well as other service required for a 500 hour inspection.
The mags tested perfect on the test bench at all RPM’s.
They ran perfectly for almost 50 hours.
Then the right Magneto started acting up in flight causing slight roughness. A Mag check in flight confirmed the right mag was not performing correctly.
We had just leveled off after takeoff & returned & landed at our home airport.
Could not duplicate problem during ground run up.
Pulled the Magneto & ran extensively on the test bench & could not duplicate the inflight problem.
Pulled all the spark plugs & checked resistance as well as tested on bench tester. Checked ignition leads with high tension tester. All checked perfect.
Took Magneto apart & discovered the bushing that is molded into the distributor block was loose & wobbly. According to the Experts there is a high frequency vibration that only occurs in flight; causing the bushing to debond from the block due to a manufacturing flaw. This will cause misfiring & engine roughness in flight only.
We replaced the distributor blocks & gears in both magnetos with “after market” (Kelly Aerospace) ones from Omaha Airpane supply. The Experts say “they have never had a problem with these distributor blocks”.
There are defective distributor blocks in circulation. Installed as well as in supply pipelines.
Read the SB’s Carefully.
 
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Well, just as an update to the thread, I went over to the hangar last week and pulled the prop through just to feel what compression was like, pulled the plugs and bore scoped the cylinders just to get an idea of what we are looking at, and found this in #2..

https://youtu.be/vXSCSBTHdLk
 
Vapor lock symptoms

I am glad you found the problem. I have over 1200 hours of mogas experience and have seen vapor lock a lot of times in my 0-320 in my AA-1A taildragger.

The worst case thermal load on most fuel systems is a quick turn around. (ask the folks starting up with fuel injection).

Come in drop off a friend and start up, taxi out. Add takeoff power and the motor just stumbles. Run up to around 12-1500 rpm for 2-3 minutes and get some cooler fuel in the lines and it goes away. Never saw it on 100LL, and could almost count on it with mogas in the Florida summer.

Even though it was almost routine on a quick turn, I never experienced any loss of power while in flight. I assumed it was due to higher fuel flow through the lines and much more cooling air in the cowling.

Your mileage may vary, just my personal experience.
 
whoa... ain?t good news for your wallet. Only one jug affected?
Still not sure this the source for your troubles, as others have noted there still is or was a questionable hi fuel P.

As a side note, a few of us using Mogas around here fill say the right tank with it, whilst filling the left one with Avgas used for T/O, initial climb and land. Gives some reassurance on hot days.
 
I have had fuel vapour issues with an IO360 (RV7) on Mogas, typically when fuel has heated up (100?F OAT on the ground) then on climb passing 5000' to 9000', cured by turning fuel pump on, and left on 'till fuel had cooled in OAT of 65?F for about 15 minutes. Engine can run rough on the ground if the engine is hot and
with low fuel flows due to vapour in the distributor lines I expect.
 
whoa... ain?t good news for your wallet. Only one jug affected?
Still not sure this the source for your troubles, as others have noted there still is or was a questionable hi fuel P.

As a side note, a few of us using Mogas around here fill say the right tank with it, whilst filling the left one with Avgas used for T/O, initial climb and land. Gives some reassurance on hot days.

Yeh, the wallet isn't going to enjoy it.

Only flaking on 1 cylinder. Compression is feels way down on 2 out of 4 cylinders. Bordering 3 cylinders.......
 
thinking about it those flakes will have travelled some... part of em thru the exhaust port, maybe explaining power loss thru the valve not closing fully, and part of em might be in your oil system, thanks to the scraper piston ring...

Interested in your further findings...
What was the treatment of this or those jugs?
 
Sticking valve.

I had a problem like that and it was the valves sticking in the guides.

Lycoming has an oil additive for this problem. My engine had been stored and had preserving oil in it for years. I think that caused the valves to hang up.

When I removed the valve springs, I could feel the valves sticking when I tried to move them. I used a reemer on the guides and the engine was so much better. It had idled rufff when hot, now it purred like a kitten.

Lycoming has a procedure for doing the job with-out removing the cylinders from the engine.

I hope this info may help.

Good luck.

Tom
 
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