O-200 champ puzzle

A buddy of mine has a champ with an 0-200 when the rpm’s get to 1500 it stumbles and runs bad. If the carb heat is pulled it runs good. They put a new carburetor on it and it does the same thing. They did all the obvious like check the intake tubes , fuel flow . It sounds like it came on gradually. Any of you have an idea? thanks


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have they checked the fuel flow? crap in the tanks or gascolator pluggin up or slowing the flow?

akavidflyer said:

have they checked the fuel flow? crap in the tanks or gascolator pluggin up or slowing the flow?

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Yes they did

look for a valve issue. I know it sounds odd but I have seen something similar that was a valve going bad.

Valve or intake issues. Pulling the carb heat makes it run rich. A intake leak could be making it lean out as power is increased. Also check all the primer lines for holes or broken ones. If it has slick mags check the internal timing.
DENNY

Hi all. Doug, thanks for getting my question out there. I'm the owner of the Champ, no claim to any mechanical talents and completely befuddled by the problem. History is that the plane has 250 hours since major and truly ran like a sewing machine until recently. I know my planes pretty well and I thought I could sense some background rough while flying several months ago. Then, it became hard starting. Has Slick mags, so timed them internally and to the engine. Started great but roughness became more noticeable over the next two flights. Third flight, I pushed the throttle forward and engine stumbled very noticeable. Back to the hangar (for nearly two months now). Idle is good with no "morning sickness." It just started stumbling at about 1500 rpm and can't make it past that. It does the exact same thing left mag, right mag, and both. BUT, pull the carb heat on and it will run up to static rpm with very little hesitation.

First thought was intake leak, so checked and triple checked all hoses, tubes, clamps, elbows, gaskets, etc. No change, so changed out all the plugs with a tested set. Test run...no change. Changed the air filter....no change. Removed carb to check accelerator pump (which worked fine), tore apart and cleaned....no change. Tested all the plug wires. All test good. Compression check and borescope all the cylinders and valves. All valves appear to move freely. Nothing obviously wrong to an untrained eye there. Installed new plugs...no change. Started down the rabbit hole and put on a new carb...no change. Cleaned gascolator and checked fuel flow at the gascolator and at the carb....no change. Capped off the primer lines and checked every inch for cracks...no change. Chalked all the exhaust to check for cold cylinder. No evidence of a cold cylinder and confirmed this with infrared heat gun. Removed exhaust to check for collapsed internals in mufflers. No evidence of anything wrong there. In sum, it does start very good, idles fine, and runs up with carb heat on. Same exact behaviors on left, right and both mags. The plugs do seem to be pretty sooty for the short ground runs I've done. I don't want to believe its a valve issue, but don't know where else to look.

I've hit a wall here with my limited skills, so any insights are welcome. Thanks in advance.

Mike

How much rpm increase do you get when going to idle cutoff on shutdown??

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About 50 rpm increase on shutdown

Sounds very similar to a problem we just solved on an o200 by backing out the idle mixture screw a bit for a bit more rpm rise.
Doesn't make much sense but maybe give it a try.

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Carb heat enriches the mixture, so it is probably running lean.
Stan

chadronmike said:

Changed the air filter....no change.

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Not that this is the answer, did you try it without any air filter? This will tell you if it's a temperature change issue or an air flow issue. Sooty (rich mixture) plugs makes it sound as though there is an airflow restriction.

Around 1500+- some is where the main discharge nozzle starts to feed more off the main jet. Idle circuit to main nozzle feed. The nozzle has gone through a few iterations with design changes to the air mix hole design. There are couple of carbs for the O-200, plus series of Service Bulletins that may pertain. Contact (https://msacarbs.com/) and have a talk or text about your issue.

If you suspect valve problems check for sticking and function with the valve covers off. [EDIT: I see where you borescoped the cylinders] Run a borescope in the plug hole and look at the valve facing for uniform color. To check for induction leaks some spray brake cleaner on the joints/gaskets while running...others make an adapter hose and copper tubing nozzle attached to a small propane torch. Blow the unlit gas near joints. If leaking, you can get an rpm rise as the fluids/gas get sucked in. Not an A&P so think before doing.

Gary

Airbox door restricting flow in the cold position? Saw this on a champ recently. Not an 0-200 but the air door came loose and was sucked up. Happened in the air. Forced landing. No damage. Ran fine on the ground up to 1500 when it got enough vacuum to suck the door up.

Long shot, but you’ve checked everything else. Easy to check.

Rich

Does it have a primer? Is the primer leaking?

behindpropellers said:

Does it have a primer? Is the primer leaking?

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Leaking primer would make it run rich, pulling carb heat would make it even richer yet.

Per the MSA troubleshooting manual: "The engine starts to transition off from the idle system to the main jet at 1000 rpm and is fully on the main jet at 1400 rpm."

Additionally, from the MSA Troubleshooting manual: Hesitation on Acceleration from 1200 - 1500 RPM range: Rich - look for Air bleed blockage in Idle Tube. Solution: Remove idle tube from bowl and clean air bleed. Clean whole idle circuit with compressed air and solvent.

Sounds like it's time to open up and clean the internals in the Carb.

-Cub Builder

chadronmike said:

.... Started down the rabbit hole and put on a new carb...no change.

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Cub Builder said:

Per the MSA troubleshooting manual: "The engine starts to transition off from the idle system to the main jet at 1000 rpm and is fully on the main jet at 1400 rpm."

Additionally, from the MSA Troubleshooting manual: Hesitation on Acceleration from 1200 - 1500 RPM range: Rich - look for Air bleed blockage in Idle Tube. Solution: Remove idle tube from bowl and clean air bleed. Clean whole idle circuit with compressed air and solvent.

Sounds like it's time to open up and clean the internals in the Carb.

-Cub Builder

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These two comments suggest it is not the carb. That is unless "new carb" does not mean "new carb" and instead just means "another carb". If in fact it is a new carb or an overhauled carb, it would suggest something other than carb.

skywagon8a said:

These two comments suggest it is not the carb. That is unless "new carb" does not mean "new carb" and instead just means "another carb". If in fact it is a new carb or an overhauled carb, it would suggest something other than carb.

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Thanks to everyone for the ideas. Please keep them coming.

Just to clarify, I did cap off the primer lines in case of a primer leak. There was no change. I did remove and thoroughly clean the old carb. There was no change. The carb I then installed was a brand new carb and there was no change at all. I sure hate to tear into a brand new carb.

I haven't run it without the filter yet as it is snowing here today. That said, it behaved the same with the old filter and the new filter. This morning, I did replace the fuel line in case it was collapsing internally on run up. I'm not optimistic that this will change things, but I'll know in a day or two. Planning on doing an induction leak test today with pressure.

Again, thanks for the brainstorming and ideas. Much appreciated.

Mike

After you eliminate the filter or no filter test if it still happens, remove the airbox and let it hang below completely eliminating anything associated with the carb heat valve/airbox.
Why would the plugs be sooty if there was an induction air leak leaning the mixture? Are all the plugs sooty or just 1 or 2?
It seems strange that it gradually came on until it was just bad. That indicates something was in the process of failing, until it did. Supercrow's idea makes some sense.
I'm just grasping at straws based upon what you've said you have already done.

Doug Budd said:

A buddy of mine has a champ with an 0-200 when the rpm’s get to 1500 it stumbles and runs bad. If the carb heat is pulled it runs good. They put a new carburetor on it and it does the same thing. They did all the obvious like check the intake tubes , fuel flow . It sounds like it came on gradually. Any of you have an idea? thanks


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Click to expand...

This time of year I need to adjust my carb about a 1/4 to 1/2 turn more rich. C90-8F, but the O-200 has the same challenge usually.

Same here on small Conti's - 1/2 turn out below zero. I also like 75-100 rise in winter at cutoff on those cold running engines.

chadronmike....have you contacted M/S carb yet and discussed your carb model/SN/condition and ops issue? If not I would.

Gary

BC12D-4-85; ....have you contacted [URL="https://msacarbs.com" said:

M/S carb[/URL] yet and discussed your carb model/SN/condition and ops issue? If not I would.

Gary

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I appreciate the suggestion, but test ran my new carb on a friend's 0-200 Aeronca Chief..and it works perfectly. Thus, pretty sure it's not the carb...

The only things between the carb and engine are a base gasket, the induction spider and primer tap, four induction tubes and their rubber sleeves, then the metal elbow and its gasket that bolts to the cylinder. If the carb is ok on another engine and therefore deemed airworthy (correlation may not always be causation), and you're convinced the induction system is leak free, then you are left with an increasing problem with your engine. I'd make sure the valves are correct and not sticking when warmed up - not cold. And also confirm some of the valve lifters are not bleeding down by checking valve lift when also warmed up.

The tricky deal is the apparent correlation between carb heat and smoother running. Unburned fuel can affect valve to guide operation, but that's an unusual effect. Another might be the carb heat is affecting some induction system clearance issue (including the carb) by heating a component that then expands to seal better.

Gary

I had this happen when I was burning some bad gas. Drained and replaced and all better

Glenn

skywagon8a said:

After you eliminate the filter or no filter test if it still happens, remove the airbox and let it hang below completely eliminating anything associated with the carb heat valve/airbox.
Why would the plugs be sooty if there was an induction air leak leaning the mixture? Are all the plugs sooty or just 1 or 2?
It seems strange that it gradually came on until it was just bad. That indicates something was in the process of failing, until it did. Supercrow's idea makes some sense.
I'm just grasping at straws based upon what you've said you have already done.

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All the plugs are equally sooty. Been thinking about your question and cubdriver's comment about bad gas. I have replaced and/or ensured free flow of all fuel lines from the tank to the carb. In the process, I had to drain all the fuel. It all looks nice and blue but of course, some spilled on the floor and it stayed there a long time. It also has a vague JetA odor. Would 100LL contaminated with JetA cause the sooty plugs? It seems like it would cause the symptoms I'm having. Going to put everything back together today and put new fuel in. I'll sure kick myself if I went through all this process and the problem was bad fuel.:???:

It's called troubleshooting.

Web

If after all this it's bad fuel? Where would you get bad fuel and not know?

Gary

chadronmike said:

All the plugs are equally sooty. Been thinking about your question and cubdriver's comment about bad gas. I have replaced and/or ensured free flow of all fuel lines from the tank to the carb. In the process, I had to drain all the fuel. It all looks nice and blue but of course, some spilled on the floor and it stayed there a long time. It also has a vague JetA odor. Would 100LL contaminated with JetA cause the sooty plugs? It seems like it would cause the symptoms I'm having. Going to put everything back together today and put new fuel in. I'll sure kick myself if I went through all this process and the problem was bad fuel.:???:

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If you somehow happened to get a batch of Jet A .... you should consider yourself lucky that you are here on the ground doing trouble shooting instead of underground growing grass. There have been many instances of using Jet A in recip engines with disastrous results.

BC12D-4-85 said:

If after all this it's bad fuel? Where would you get bad fuel and not know?

Gary

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If fuel is, in fact, my problem, I don't know the answer to that question. I only fly the Champ locally and fuel at self serve pumps at the local airports. No barrel or gas can fueling at all...and I most assuredly did not accidentally put JetA in the bird. I have not been able to burn the tank of fuel that was in there since this problem presented itself, so that 10 gallons is going into the tractor. I still have my reservations on bad fuel, but it seems that it would explain the slow then sudden onset, the rough running and sooty plugs.

Auto fuel that's aged can leave a residue and stink like Jet fuel when mixed with 100LL. My experience from fueling with aged fuel stored at remote camp. My test is to put some of the auto fuel or mixed fuel on a plate and let it evaporate. Crap fuel will leave behind more of a signal.

Gary