Engine running rough on throttle down after cruise

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hankrausch

Pre-takeoff checklist
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This has just popped up on the last 6 flights:

Engine feels like it is missing on throttle-down from cruise (2450 rpm) to descent (2300-2100 rpm).

engine: Lycoming O-235 L2C; plane is Piper tomahawk with fixed pitch propeller

I have cleaned the plugs several times, they are new and I cannot find any problems with them. The HT wires are new.

I will have a mechanic take a look as soon as possible but as I am currently 600 miles from home I am asking the group if this is a dangerous condition? Could I lose the engine, I have lost an engine before due to a stuck valve, could this be the same? it runs smoothly at cruise rpm and also seems to run OK at near idle rpm (700-900 rpm) but it runs rough right after throttle down. Many thanks for any ideas!
 
Not nearly enough information to even guess. But if you think it might be a valve issue, the Service Bulletin 388C valve guide "wobble" check would be a good places to start. Also, talk about your leaning procedures/techniques.
 
Thank! Will check that SB--to be honest am running rich of peak, I will try to lean a bit more aggressively on the next hop--could it be as simple as not leaning enough?
 
Intake leak? Sounds like a problem I had when the carb was slightly loose on the sump. At high MP, it would suck right up against the sump, but with a reduction in MP, enough of a gap appeared to cause a stumble.
 
kyleb said:
Intake leak? Sounds like a problem I had when the carb was slightly loose on the sump. At high MP, it would suck right up against the sump, but with a reduction in MP, enough of a gap appeared to cause a stumble.
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In take leaks will be more prominent at high power not idle.

I'd hate to guess what the OP is feeling/hearing.
 
hankrausch said:
Thank! Will check that SB--to be honest am running rich of peak, I will try to lean a bit more aggressively on the next hop--could it be as simple as not leaning enough?
Click to expand...
Yes, it could. When you reduce throttle the engine goes richer, and at altitude with low power and not much leaning, it could be rich enough to stumble. Dig into the Lycoming papers on leaning in their Key Reprints -- google it.
 
hankrausch said:
This has just popped up on the last 6 flights:

Engine feels like it is missing on throttle-down from cruise (2450 rpm) to descent (2300-2100 rpm).

engine: Lycoming O-235 L2C; plane is Piper tomahawk with fixed pitch propeller

I have cleaned the plugs several times, they are new and I cannot find any problems with them. The HT wires are new.

I will have a mechanic take a look as soon as possible but as I am currently 600 miles from home I am asking the group if this is a dangerous condition? Could I lose the engine, I have lost an engine before due to a stuck valve, could this be the same? it runs smoothly at cruise rpm and also seems to run OK at near idle rpm (700-900 rpm) but it runs rough right after throttle down. Many thanks for any ideas!
Click to expand...

The way you describe the symptom it would seem that the engine is having a problem transiting from cruise circuit to idle circuit.

do you have a newer carb?
 
Ron Levy said:
Yes, it could. When you reduce throttle the engine goes richer.
Click to expand...

Why would it?

when the throttle valve closes the vacuum produced in the venturi stops and no fuel will be drawn from the float bowl.

that is why we have an idle circuit.
 
Thank you for all the input as I said I am getting it to the shop when back to home field & will look at the leaning at altitude that does sound likely. I did swap out a magneto a few hundred hours ago, if i had a mag problem I figure that would show up on a runup? The differential on the runup is 75 rpm, in spec (just) per the POH. I don't know how old the other mag is ( records not now with me in transit).
 
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Look for an exhaust gasket leak. (yeah, I know it is counter intuitive, but I have seen it more than once)
You will usually also get some 'popping' if you pull the throttle back smartly, instead of a partial throttle reduction. Try that in flight.
 
If it pops as you are reducing throttle or immediately after it is a lean condition. If you are leaned at power reduction richen up a bit before moving the thrittle. If you are full rich then try carb heat to richen. If that solves it you are a bit too lean and there isn;t much that can legally be done to fix it. A different carbuiretor or a richer one if approved would be the only possibilities. Charlie Melot Zephyr Engines
 
I had one pop and backfire on throttle down occasionally ... intermittent mag problem that finally failed completely.
 
gkainz said:
I had one pop and backfire on throttle down occasionally ... intermittent mag problem that finally failed completely.
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Had a little roughness on the Frankenkota after landing on the flight immediately before the flight on which the mag failed. It was fine on run-up prior to that last flight. The EGTs were a little wonky too. If I ever see EGTs like that again I'll know what to do. Unfortunately I was clueless at the time as to why they had so much variation.

Anyone know of good training resource for engine analyzer data interpretation?
 
I have seen EGTs rise on throttling back in some aircraft, indicating a leaning condition. If the OP has this issue as well as a weak spark, roughness could easily result. The plugs might look ok but that means nothing. They need testing under pressure, and the mags might need looking at.

90% of engine troubles are electrical. I was taught that in high school 44 years ago, and have learned the hard way that it's true. We'll chase fuel and carbs and injectors and so on, then in the end discover some electrical glitch. Weak spark is responsible for many problems.

Dan
 
Thanks for all the help after I have the mechanic look at it I will post what he said.
 
Halfway home and pulled the plugs (again) and think I have found the culprit, the bottom plug on #1 cylinder was completely wet with oil, the mechanic here at the field says that it could be a stuck ring and that he has had success in freeing stuck rings by putting an old plug in the bottom, pouring Marvel Mystery Oil in the top and cycling the engine a few rotations using the propeller. I will get a compression check down on return home, if the comp check shows the ring is stuck, does anyone have any experience/comments with his suggestion?
 
Hello this is a followup just to let the community know what it was, I just got it back from the shop and it was an intake leak, they replaced a gasket and the engine just hums now. I think someone else mentioned something like this in the thread. Hope this helps someone else in the future!
 
hankrausch said:
Hello this is a followup just to let the community know what it was, I just got it back from the shop and it was an intake leak, they replaced a gasket and the engine just hums now. I think someone else mentioned something like this in the thread. Hope this helps someone else in the future!
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I have pretty much eliminated an intake leak. I just ordered a different carb, though (5 to 7 business days). My plane has the -32 carb, which has an "economizer jet" that causes it to run somewhat leaner at the midrange. The engine is definately running lean, overall. I just installed an engine monitor and it shows cylinders 3 & 4 to have significantly higher EGTs than 1 & 2, which indicates poor fuel distribution. There is a newer carb approved for my engine, the 5817 and it has an atomizer jet, which is supposed to really help with fuel distribution. We shall see.
 
This forum is amazing.

I'm having the exact same problem with the same engine and airplane. Now I know where to start looking for the problem!
 
hankrausch said:
Hello this is a followup just to let the community know what it was, I just got it back from the shop and it was an intake leak, they replaced a gasket and the engine just hums now. I think someone else mentioned something like this in the thread. Hope this helps someone else in the future!
Click to expand...

Blown intake gaskets sometimes happen when someone turns the mag switch to OFF during the runup, and back on again. The mufller and exhaust risers fill with raw fuel/air mixture that goes Bang when the mags are turned back on, and if there's one cylinder with the exhaust valve almost closed and the intake just opening, it sets off the mixture in the intake, too. The concussion blows the intake gasket.

Dan
 
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