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John Vance

Power loss in flight

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Yesterday, I experienced a reduction in engine power during flight and and am hoping for some insight. After doing an oil / filter change, I flew to a neighboring airport a short distance away.  During engine warmup for this flight, the engine was smooth at 2200 RPM but slightly rough at 2400.  After warmup, I taxied to end of the runway at idle power and did the runup there, everything normal. Takeoff RPM was normal, and I proceded with the flight. When reducing power for descent to pattern altitude, I noticed some slight roughness. I did one touch & go, and during climb-out, power was sub-par and climb was slow. I circled the airport for awhile at WOT while climbing & evaluating.  The engine was running smooth, but max power rpm in level flight was around 5200.  Several minutes of carb heat did not improve things.  Pulling it reduced rpm slightly, but response was not the pronounced rpm shift that you normally see at WOT.  I checked left/right mags, and both worked normally with the expected reduction in power with respect to the 'both' position.  I did not try the choke.  Fuel flow was steady at around 5.2 to 5.3 gph at WOT, should be about 6.1 or so.  Fuel pressure also steady 4.6 psi. I decided that it wasn't going to quit, so I headed home.  After landing, idle performance was normal.  I stopped during taxi-in and ran a WOT rpm check. Result was normal for this airplane, about 4900 rpm, so at this point the problem seemed to have disappeared.   I suspect dirt in a carb bowl, but I have not had time to start poking around yet.  Any thoughts welcome.

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It's non-ethanol 93 octane auto gas.  I've been using fuel from this station for years, and haven't bought any from a different source since last October.  But the main reason I don't suspect fuel quality is the on/off nature of the problem.  

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I would suspect carb icing: what was OAT and dew point? I have personally experienced recently a severe carb icing while flying here in Europe at 6000 feet, even with carb "heat" on. Since then, I have installed a water heating carb system to remove any risk during marginal conditions.

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OAT at cruise altitude was 53 F, but I don't know the DP.  Ice was my first thought.  Usually when you have carb ice, carb heat initially makes the engine run rough while the ice is melting, but I didn't experience that. Point taken, though - I'll take a look at the carb heat system. 

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Hi John.  This sounds different than your situation last year?  Not a rollback in rpm's but just low on power temporarily?  Are you back in Indiana?

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Also, are you sure choke was fully off?  I had an issue once where the choke was partially on and it got rough at low power settings.

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1 hour ago, Runtoeat said:

Hi John.  This sounds different than your situation last year?  Not a rollback in rpm's but just low on power temporarily?  Are you back in Indiana?

Yes, this is different.  I want to cover all the bases, but my working theory is a chunk in one of the carb bowls that gets sucked into the main jet, then falls out during idle. 

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30 minutes ago, FlyingMonkey said:

Also, are you sure choke was fully off?  I had an issue once where the choke was partially on and it got rough at low power settings.

I'm pretty sure it was off, but I can still check it when I go back to the hangar. I would not have touched it after landing. 

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1 hour ago, Runtoeat said:

Hi John.  This sounds different than your situation last year?  Not a rollback in rpm's but just low on power temporarily?  Are you back in Indiana?

I'm in NC now, not in indiana. 

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The choke has no effect over about 3500-4000 and especially at the higher rpms. Try it. Fly up at your cruise rpm and apply the choke. Nothing will happen.

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8 minutes ago, Roger Lee said:

The choke has no effect over about 3500-4000 and especially at the higher rpms. Try it. Fly up at your cruise rpm and apply the choke. Nothing will happen.

That was my understanding. I did visually check to make sure the choke was off, and I might have pushed it too, but don't recall. 

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Happened to me twice to me. Both times in early spring. The first time I did much trouble shooting, cleaned the crabs, replaced the throttle cables, replaced the fuel line, cleaned the fuel distribution manifold. As a last resort I emptied the fuel tank (91 octane no ethanol auto fuel) and filled up on 100LL. Problem went away. The following year the same thing happened about the same time of year. I dumped the auto gas and put in 100LL. Problem went away. Possibly the problem was the local fuel distributor changing from winter to summer blend auto fuel?? Now I run on half 100LL and half 91 octane car gas all the time. No more problems.

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9 hours ago, bitten192 said:

Happened to me twice to me. Both times in early spring. The first time I did much trouble shooting, cleaned the crabs, replaced the throttle cables, replaced the fuel line, cleaned the fuel distribution manifold. As a last resort I emptied the fuel tank (91 octane no ethanol auto fuel) and filled up on 100LL. Problem went away. The following year the same thing happened about the same time of year. I dumped the auto gas and put in 100LL. Problem went away. Possibly the problem was the local fuel distributor changing from winter to summer blend auto fuel?? Now I run on half 100LL and half 91 octane car gas all the time. No more problems.

Could you describe how your engine was behaving?  Was it intermittent or consistent?  Did problems occur just after refueling, or ??

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Had this happen to me last summer, including the extremely perplexing and frustrating return to normality on the ground. 

It was gunk in the carb bowl. It returned to normality because the idle speed released the gunk from the jet. 

occams razor: the simplest explanation is usually the correct one. 

I also proposed in my mind it was a bad batch of gas as well as some other “exotic” possibilities. In the end it was just some gunk in the bowl.

keep in mind also that poor fuel quality is going to cause issues at all RPMs, and low octane is going to cause problems with predetonation, not impose an “RPM governor” at the highest rpm range as you’re experiencing. 

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Follow up question: 

when was the last time you’ve had your carbs rebuilt and all new gaskets put in?

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2 hours ago, CTMI said:

Follow up question: 

when was the last time you’ve had your carbs rebuilt and all new gaskets put in?

It's been just over 2 years. I have not been suspecting fuel quality for the reasons you mentioned. I will be dropping the carb bowls hopefully within the next few days, and maybe the answer will be lying there on the bottom. 

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Hi John.  Mentioned your problem to Dave today.  Dave wondered if you gave carb heat time to work, should this have been icing?  Our carb heat is marginal and needs to be given time to do it's job.  I'm thinking junk in carb bowl too but thought I'd ask about the carb heat.

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9 minutes ago, Runtoeat said:

Hi John.  Mentioned your problem to Dave today.  Dave wondered if you gave carb heat time to work, should this have been icing?  Our carb heat is marginal and needs to be given time to do it's job.  I'm thinking junk in carb bowl too but thought I'd ask about the carb heat.

On a couple of occasions, I left it on for several minutes, waiting for some roughness indicating that it was melting the ice.  I never got that, just a very slightly lower RPM. I suppose it's possible that it was ice, but performance seemed very stable and did not continue to decline during the 15 minute trip home. 

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51 minutes ago, WmInce said:

During these events, regardless of OAT, was the humidity high?

Yes, it was cloudy and there had been some light showers around a couple of hours prior.  I didn't encounter any. 

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