Engine stumble off idle and need some human help. ChatGPT is leading me astray !

[Darren S]

Hello group. I needs some help from the carb experts on here.

The details: Lycoming O-360, overhauled carb purchased several months ago from ACS. Carb is a Marvel Schebler MA 4-5 (model number is 10-5193). Engine has 100 hours. The original carb had a fuel leak, hence the purchase of an overhauled one.

The problem: upon accelerating after idle warm up, there is a noticable stumble and then recovery
: on short final, idling at about 1000 RPM, when I advance the throttle relatively quickly .... stumble then recovery
: while flying, if I go to idle and the RPM is around 1000, if I advance the throttle slowly, at around 1500 RPM, a noticable stumble and then recovery.

I've adjusted the idle mixture screw many times. It makes no difference.

There is no external accelerator pump adjustment on this model of carb.

When advancing the throttle arm there is a "healthy" squirt of fuel .

To my uneducated mind it seem to be needing more fuel when I rapidly advance the throttle.

SUGGESTIONS PLEASE !

Thank

 

[lr172]

Hello group. I needs some help from the carb experts on here.

The details: Lycoming O-360, overhauled carb purchased several months ago from ACS. Carb is a Marvel Schebler MA 4-5 (model number is 10-5193). Engine has 100 hours. The original carb had a fuel leak, hence the purchase of an overhauled one.

The problem: upon accelerating after idle warm up, there is a noticable stumble and then recovery
: on short final, idling at about 1000 RPM, when I advance the throttle relatively quickly .... stumble then recovery
: while flying, if I go to idle and the RPM is around 1000, if I advance the throttle slowly, at around 1500 RPM, a noticable stumble and then recovery.

I've adjusted the idle mixture screw many times. It makes no difference.

There is no external accelerator pump adjustment on this model of carb.

When advancing the throttle arm there is a "healthy" squirt of fuel .

To my uneducated mind it seem to be needing more fuel when I rapidly advance the throttle.

SUGGESTIONS PLEASE !

Thank

An off idle stumble on acceleration can be one or several things. Always start with idle mixture first. It may or may not be part of the problem, but it is an important foundation for further work and does have an impact on off idle acceleration. I recommend the lowest MAP approach and have posted instructions before. Next you need an educated guess on whether the accel pump shot is too much or too little. Do some research here. Generally too little shot causes a "bog," referred to as a lean bog. It is noticeably different to the trained ear than too much. The bog is smooth and stable, like a fall off followed by a run up. Too much is similar but rougher, more like choking/coughing., often with a puff of black or grey smoke from the exhaust.

There are several parts in the accel pump area that could be a problem, but need to first figure out if there is too much or too little shot. While haven't worked on that carb recently, there is almost always an external linkage to a lever arm controlling the accel pump and usually adjustable via diff holes in the arm, though the extra holes could be at the throttle plate. Do more research here.

If you advance the throttle from 1000 slowly, does it still stumble? what MAP are you getting at idle?

 

[N9018D]

Wow. I just replaced my Marvel Schebler carb last week and I'm in the same predicament. Mine stumbles around 1200-1400 rpm. Barely noticeable unless you're really paying attention. I thought it may be from the throttle cable resistance as the carb throttle lever moves from aft to forward. Can't wait for the experts to chime in....

 

[lr172]

Wow. I just replaced my Marvel Schebler carb last week and I'm in the same predicament. Mine stumbles around 1200-1400 rpm. Barely noticeable unless your really paying attention. I thought it may be from the throttle cable resistance as the carb throttle lever moves from aft to forward. Can't wait for the experts to chime in....

start with idle mixture. they usually come extremely fat / rich and this can create symptoms like yours.

 

[Quotafiller]

Is that the correct carb ? Was there a reason the 5193 chosen over the 4164 ?

 

[Full Throttle]

The Marvel-Schebler MA-4-5 is the standard and correct carburetor series for the Lycoming O-360 engine family. However, you cannot bolt just any MA-4-5 carburetor onto your engine. The generic designation "MA-4-5" refers only to the body size and float type. To ensure safety and proper engine performance, you must match the exact Marvel-Schebler part number to your specific Lycoming O-360 engine suffix (e.g., -A1A, -A4M). Different part numbers feature unique internal jetting, venturi sizes, and economizer settings tailored to that exact engine model

 

[Darren S]

An off idle stumble on acceleration can be one or several things. Always start with idle mixture first. It may or may not be part of the problem, but it is an important foundation for further work and does have an impact on off idle acceleration. I recommend the lowest MAP approach and have posted instructions before. Next you need an educated guess on whether the accel pump shot is too much or too little. Do some research here. Generally too little shot causes a "bog," referred to as a lean bog. It is noticeably different to the trained ear than too much. The bog is smooth and stable, like a fall off followed by a run up. Too much is similar but rougher, more like choking/coughing., often with a puff of black or grey smoke from the exhaust.

There are several parts in the accel pump area that could be a problem, but need to first figure out if there is too much or too little shot. While haven't worked on that carb recently, there is almost always an external linkage to a lever arm controlling the accel pump and usually adjustable via diff holes in the arm, though the extra holes could be at the throttle plate. Do more research here.

If you advance the throttle from 1000 slowly, does it still stumble? what MAP are you getting at idle?

I don’t have a MAP gauge so I don’t know what it is. Yes. If I advance slowly it still bogs. On the ground and in the air.

 

[Darren S]

start with idle mixture. they usually come extremely fat / rich and this can create symptoms like yours.

I’ve messed with the idle mixture many, many times but I haven’t tried it leaner than 1.5 turns out. ChatGPT was saying to start there and back it out (go rich). I will try leaner for sure if nothing comes up from this thread.

 

[Darren S]

The Marvel-Schebler MA-4-5 is the standard and correct carburetor series for the Lycoming O-360 engine family. However, you cannot bolt just any MA-4-5 carburetor onto your engine. The generic designation "MA-4-5" refers only to the body size and float type. To ensure safety and proper engine performance, you must match the exact Marvel-Schebler part number to your specific Lycoming O-360 engine suffix (e.g., -A1A, -A4M). Different part numbers feature unique internal jetting, venturi sizes, and economizer settings tailored to that exact engine model

I have the A4M engine. I just bought what model number I pulled off.

 

[Darren S]

The Marvel-Schebler MA-4-5 is the standard and correct carburetor series for the Lycoming O-360 engine family. However, you cannot bolt just any MA-4-5 carburetor onto your engine. The generic designation "MA-4-5" refers only to the body size and float type. To ensure safety and proper engine performance, you must match the exact Marvel-Schebler part number to your specific Lycoming O-360 engine suffix (e.g., -A1A, -A4M). Different part numbers feature unique internal jetting, venturi sizes, and economizer settings tailored to that exact engine model

I tried calling Tempest but they are closed today. I’ll try them on Monday. Hoping to get some solid advice before then. I have the A4M engine

 

[SteveL]

Had this problem several years back, Make sure your air filter is clean and unrestricted. Solved my issue. Give it a try.

 

[Norman CYYJ]

Hello group. I needs some help from the carb experts on here.

The details: Lycoming O-360, overhauled carb purchased several months ago from ACS. Carb is a Marvel Schebler MA 4-5 (model number is 10-5193). Engine has 100 hours. The original carb had a fuel leak, hence the purchase of an overhauled one.

The problem: upon accelerating after idle warm up, there is a noticable stumble and then recovery
: on short final, idling at about 1000 RPM, when I advance the throttle relatively quickly .... stumble then recovery
: while flying, if I go to idle and the RPM is around 1000, if I advance the throttle slowly, at around 1500 RPM, a noticable stumble and then recovery.

I've adjusted the idle mixture screw many times. It makes no difference.

There is no external accelerator pump adjustment on this model of carb.

When advancing the throttle arm there is a "healthy" squirt of fuel .

To my uneducated mind it seem to be needing more fuel when I rapidly advance the throttle.

SUGGESTIONS PLEASE !

Thank

Stay away from AI when it comes to things that could affect your life. It will only tell you things that it has found on the net and repeat to you a LOT of miss information and if you don't know, it could be disastrous to you.

 

[Aden Rich]

The Marvel-Schebler MA-4-5 is the standard and correct carburetor series for the Lycoming O-360 engine family. However, you cannot bolt just any MA-4-5 carburetor onto your engine. The generic designation "MA-4-5" refers only to the body size and float type. To ensure safety and proper engine performance, you must match the exact Marvel-Schebler part number to your specific Lycoming O-360 engine suffix (e.g., -A1A, -A4M). Different part numbers feature unique internal jetting, venturi sizes, and economizer settings tailored to that exact engine model

Even that may not be enough to get your carb matched to the engine. We must remember you can bolt on a -A4K or A4M or what ever other engine model you have on your data plate. But the intake you have, along with the mags or EI you have, paired with the exhaust, created a "unique" engine configurations that you have to tune to get them to run right. A prime example of that is when we got a O-320 engine off a Cessna 172 and put it on a RV-4. Everything should work but when it was all ready to fire up the engine and test it, the engine did not run right and we have to get the jetting/low idle mixture configured a little bit different for it all to work. RV's tend to breath better than most spam cans.

 

[Aden Rich]

Stay away from AI when it comes to things that could affect your life. It will only tell you things that it has found on the net and repeat to you a LOT of miss information and if you don't know, it could be disastrous to you.

Totally agree! Call professionals or people with experience in these items. AI can really drag you down a road......

 

[rwarre]

Totally agree! Call professionals or people with experience in these items. AI can really drag you down a road......

just went through a confusing experience with ai. Trying to fix a wiring issue. Ai tried to have me change wires that were ok. I corrected the Ai chat and it apologized for the mistakes.
I had some engine stumbles with my last airplane that was carbureted and after finding some debris in the tiny holes that are inside the carb, I was able to get the issue solved. I did have an expert who was knowledgeable about carbs.

 

[mburch]

Stay away from AI when it comes to things that could affect your life. It will only tell you things that it has found on the net and repeat to you a LOT of miss information and if you don't know, it could be disastrous to you.

This can't be amplified enough. Once someone understands what LLMs are actually doing, hopefully they'll ignore these random-word generators.

 

[rmarshall234]

Stay away from AI when it comes to things that could affect your life. It will only tell you things that it has found on the net and repeat to you a LOT of miss information and if you don't know, it could be disastrous to you.

So this is what we’ve come to …. It used to be one would turn the machine over to a qualified mechanic and now it’s…Chat GP, online Forum, back to AI, another forum attempt and eventually, the problem gets solved, If, the owner hasn’t run out of patience and/or $$ by throwing both at the problem. Not a dig at the OP, just a comment on our race to the bottom and why the accident rate sure as hell seems to me to be running an upwards trajectory.

 

[riobison]

I had a similar experience with an O360 in my 4. It would only happen when the density altitude was high and this would catch your attention in the circuit.

My solution during the summer was to lean it out in the circuit and that solved my problem.