partial power failure after annual inspection

Lawson Laslo

Lawson Laslo

Pre-takeoff checklist
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Jan 18, 2019
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403
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Sundance airport Oklahoma
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N2005H
Hello, The annual inspection was completed last week on the O-200 powered ercoupe, it took just under a month due to having to order parts etc... anyway after the annual inspection we took it for a flight and everything ran fine until we were practicing slow flight with carb heat on since it was a moist morning and when i went to add power the engine stumbled and wouldn't go above 2,000 rpm so we went back and landed. Second flight we just taken off (no carb heat) and were on crosswind and the engine went down to about 2000 rpm running very rough I throttled off and making tailwind landing and when i went to add throttle it ran better again. My mechanic never adjusted anything on the carburetor during annual. Still not sure what the issue was, today i went and did some full power ground runs with the following results
First run i went to full power with full rich mixture, it ran fine (2600 rpm) till i turned carb heat on and the RPM went down to about 1800-2000 rpm at times 1500rpm and was extremely rough. I put carb heat off and it still ran rough and wouldn't go over 2200 rpm. I then lean the mixture out and the engine then ran like normal
Second time I Started with mixture leaned out and then went full power and was steady at 2,600rpm which is normal i then applied carb heat and only had a 50-100rpm drop and engine was still smooth.

Ive never had this issue before. I am thinking that maybe after time the idle air mixture vibrated and made the mixture to rich? the weird thing is that in May before the annual it was running perfect and now in July its not running properly, maybe its because the high heat 90+degrees and density altitudes over 4000 feet (ground level at HSD is 1,200')

let me know if you think its the idle air mixture screw or maybe something else.
Thanks
 
This will be the first summer you've owned it right? It does seem like DA could be the cause...but I'm guessing from a computer screen many miles away.

Is that little video in your post before or after you leaned the Ercoupe? ;)
 
This will be the second summer we have had it but dont think it was as high DA last year, last summer we really had no issue and never leaned for takeoff, But one thing to note is that on runup my carb heat has always had about a 250rpm drop but today when i had leaned it for runup and did the test it only had about a 50rpm drop
 
I remember flying an O-200 powered 150 on an extremely hot (high DA) day, and carb heat nearly killed the engine during runup. We had to turn down river in order to clear the hills past LUK even with the thing leaned.
I wouldn't be surprised if the thing wasn't just running extremely rich.
 
Lawson Laslo said:
the carburetor is a Marvel-Schebler MA-3SPA p/n:10-4894-1
Click to expand...
That is a 0-200- carb, but is that is required for a O-200 powered ercoupe.
Lawson Laslo said:
Hello, The annual inspection was completed last week on the O-200 powered ercoupe, it took just under a month due to having to order parts etc... anyway after the annual inspection we took it for a flight and everything ran fine until we were practicing slow flight with carb heat on since it was a moist morning and when i went to add power the engine stumbled and wouldn't go above 2,000 rpm so we went back and landed. Second flight we just taken off (no carb heat) and were on crosswind and the engine went down to about 2000 rpm running very rough I throttled off and making tailwind landing and when i went to add throttle it ran better again. My mechanic never adjusted anything on the carburetor during annual. Still not sure what the issue was, today i went and did some full power ground runs with the following results
First run i went to full power with full rich mixture, it ran fine (2600 rpm) till i turned carb heat on and the RPM went down to about 1800-2000 rpm at times 1500rpm and was extremely rough. I put carb heat off and it still ran rough and wouldn't go over 2200 rpm. I then lean the mixture out and the engine then ran like normal
Second time I Started with mixture leaned out and then went full power and was steady at 2,600rpm which is normal i then applied carb heat and only had a 50-100rpm drop and engine was still smooth.

Ive never had this issue before. I am thinking that maybe after time the idle air mixture vibrated and made the mixture to rich? the weird thing is that in May before the annual it was running perfect and now in July its not running properly, maybe its because the high heat 90+degrees and density altitudes over 4000 feet (ground level at HSD is 1,200')

let me know if you think its the idle air mixture screw or maybe something else.
Thanks
Click to expand...
carb ice?
 
So what parts did you actually change (or f- with) during annual, other than taking things apart for inspection and putting back together?
 
455 Bravo Uniform said:
So what parts did you actually change (or f- with) during annual, other than taking things apart for inspection and putting back together?
Click to expand...
only thing that was done to engine was new air filter, cleaned and replaced spark plugs, tightened the screw on the carburetor since they were loose( didn't touch the idle air mixture screw or anything over than the mounting screws etc), replaced valve cover gaskets and replaced intake boots, also after the first power loss we had the mags overhauled since they were already overdue.
 
You may not have messed with the mixture, but you did mess with at least a couple things that could be causing the trouble (the intake boots and carburetor mounting screws, possibly air filter). Check them over and check the linkages attached to the carburetor as well.

Based on your description one of my suspicions would be that the air box and carb heat cable need to be checked for proper travel.
 
2B8983D8-A2D3-41F9-9CBC-598F04F0442D.jpeg 2B8983D8-A2D3-41F9-9CBC-598F04F0442D.jpeg
mondtster said:
You may not have messed with the mixture, but you did mess with at least a couple things that could be causing the trouble (the intake boots and carburetor mounting screws, possibly air filter). Check them over and check the linkages attached to the carburetor as well.

Based on your description one of my suspicions would be that the air box and carb heat cable need to be checked for proper travel.
Click to expand...
Carb heat is functional and flapper is working properly
Going to airport today and will test to see what rpm rise I get when I go to idle cutoff in reference to this article
 
Lawson Laslo said:
Carb heat is functional and flapper is working properly
Going to airport today and will test to see what rpm rise I get when I go to idle cutoff in reference to this article
Click to expand...

Did you actually look in the air box to make sure the door makes the full travel after messing with the carburetor mounting?
 
Were the float bowl screws so loose that the air pressure in the bowl was near cowling back side air press instead of ram air? There is a small port in the carb air intake that vents ram air to the top of the float bowl. Anything that messes with that bowl pressure will affect the mixture. My guess is that it would flood out though when resealed but not sure.
 
Looks like carb hasn’t been overhauled since the 70’s
Do you think it might be worn out and a rebuilt one would solve the issue?
 
You mention new air filter. What kind? In the past, we had 2 different airplanes here that had issues with power/rpm, I think on takeoff. Engines would stumble or sort of bog down with throttle application. Changing air filters from Brackett to simple paper seemed to solve the issue. Might be a basic place to start.
 
Lawson Laslo said:
Do you think it might be worn out and a rebuilt one would solve the issue?
Click to expand...

That is called "Throwing parts at a problem". Usually the most expensive way to fix a problem.

Your best bet is a good diagnosis confirmed by testing. You are asking us to do the diagnosis. I would talk to the A&P who knows these engines best, in your local area.

-Skip
 
If
Lawson Laslo said:
I’ve been able to recreate the same power loss 20+ times on the ground and I have a carb heat gauge
Click to expand...
you have a carb temp gauge you obviously can tell when it is/isn’t in the range. His point was that carb ice can and does occur on the ground, too. My CFI had one of the 172’s ice up while holding short for an extended period.
 
Is it possible that someone richened the carb to compensate for sucking air because they didn’t look hard enough, and now by tightening the mounting screws it’s fat?
 
Omalley1537 said:
Is it possible that someone richened the carb to compensate for sucking air because they didn’t look hard enough, and now by tightening the mounting screws it’s fat?
Click to expand...
:yeahthat:

I have seen that 250 drop when the carb heat is applied on engines that are running to rich.
An EGT gauge would likely confirm. Is there a better way to verify the correct mixture settings?
 
The idle adjustment is the only mixture adjustment that could be messed up, and it can't make this sort of issue. It only controls the tiny idle fuel flow.

A leaking primer can let excess fuel into the engine, but it's worst at low power where manifold vacumm sucks fuel through it.

A broken muffler baffle can intermittently block the exhaust outlet.

Go back to my post, #14. The mechanic might have found the carb spider "loose" and tightened it. If that carb can't move a little, the engine shakes it viciously and it slops fuel out of the bowl vent into the carb throat and floods things. Certain RPMs resonate with the carb/airbox weight and cause it. And I have actually fixed a rough-running, RPM-losing O-200 exactly like this. Took me two weeks to find it, and I only found it when I found that little paragraph in the overhaul manual. Learn from my mistakes, and get the mechanic to check it out. There's no need to spend weeks like I did learning something the hard way if this is indeed an issue.
 
I had similar issue on a Lyc O-540.

rebuilt the carb and my problem went away.
 
I'd check free movement and full range of the mixture and throttle arms. Simple stuff first.
 
This seems like a mixture issue. It ran well with the mixture leaned, which means it got better with less fuel. Maybe something obstructing the intake air? can an air filter go in backwards? Mixture linkage slipped?
 
bflynn said:
This seems like a mixture issue. It ran well with the mixture leaned, which means it got better with less fuel. Maybe something obstructing the intake air? can an air filter go in backwards? Mixture linkage slipped?
Click to expand...
The air filter doesn't care about flow direction. Any slippage of the mixture cable would result in restricted movement of the mixture control, since that carb has hard stops for the mixture lever, and the lever is indexed to the valve shaft. It can't go any richer than full rich. If the valve's internal socket came loose and slipped it would go leaner, not overrich.

One possibility is a sinking carb float, but that would be steady overfuelling, not intermittent, and fuel would be dribbling out after shutdown.
 
Skip Miller said:
That is called "Throwing parts at a problem". Usually the most expensive way to fix a problem.

Your best bet is a good diagnosis confirmed by testing. You are asking us to do the diagnosis. I would talk to the A&P who knows these engines best, in your local area.

-Skip
Click to expand...
Throwing parts at it fixed it:lol:
I believe it was a bad float.
 
What kind of mags? I also had a 2,000 rpm situation after t.o. Turned out to be one of the Bendix mags on my Beech.
 
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