MS Carb stumbles on throttle advance

[werxcv01]

My RV-8 with an O-360 Lycoming and fixed pitch Sensenich prop has a Marvel Schebler MA-4-5 carburetor (P/N 10-4164-1). The engine will stumble a bit if the throttle is not eased forward at the beginning of the takeoff roll, which I always do. I remember reading that this is because the carb shifts from a low power jet to a higher power one. Is there any truth to this and if so is there any way to correct it?

 

[Scott Hersha]

All of my carbed Lycomings have had a slight stumble at around 12-1300 RPM (approximately) on acceleration during takeoff. It doesn’t matter how warm the engine is. Advancing the throttle slower will eliminate this. Let it get above about 1500 RPM and then just open the throttle all the way - no stumble.

 

[Dan 57]

the cure is probably to install a bigger main jet.
Most if not all carbed O-360 equipped with Van’ air filter box suffer from some lack of fuel during power application, either at take-off, or go-around.

Upgrading to a bigger jet is the solution, try the search machine for Mooney Carburetor modification. Bought mine from ACS at the time…

 

[Kyle Boatright]

I've seen this when the carb or carb bowl wasn't mounted tightly and there was the opportunity for a vacuum leak in or around the carb itself.

 

[lr172]

My RV-8 with an O-360 Lycoming and fixed pitch Sensenich prop has a Marvel Schebler MA-4-5 carburetor (P/N 10-4164-1). The engine will stumble a bit if the throttle is not eased forward at the beginning of the takeoff roll, which I always do. I remember reading that this is because the carb shifts from a low power jet to a higher power one. Is there any truth to this and if so is there any way to correct it?

Yes, there is a transition from the idle circuit to the main circuit (happens around 1200 RPM), but that is not your issue. With rapid throttle increase, the engine needs more fuel than normal to accomplish this. For that reason, your carburetor has an accelerator pump to provide this extra fuel. That pump has an adjustable arm with different leverage points to adjust "the accel shot." Yours is either sending too much or too little transition fuel. Without hearing the stumble, it is impossible for me to say which it one it is. There are also leathers and check balls in there that can cause issues, so not necessarily adjusted wrong.

 

[Cth6]

Yes, there is a transition from the idle circuit to the main circuit (happens around 1200 RPM), but that is not your issue. With rapid throttle increase, the engine needs more fuel than normal to accomplish this. For that reason, your carburetor has an accelerator pump to provide this extra fuel. That pump has an adjustable arm with different leverage points to adjust "the accel shot." Yours is either sending too much or too little transition fuel. Without hearing the stumble, it is impossible for me to say which it one it is. There are also leathers and check balls in there that can cause issues, so not necessarily adjusted wrong.

I agree with Larry. I experienced a similar issue and after detailed inspection the accelerator pump seals were starting to go. After rebuild, everything worked as expected.

 

[rmarshall234]

Yes, there is a transition from the idle circuit to the main circuit (happens around 1200 RPM), but that is not your issue. With rapid throttle increase, the engine needs more fuel than normal to accomplish this. For that reason, your carburetor has an accelerator pump to provide this extra fuel. That pump has an adjustable arm with different leverage points to adjust "the accel shot." Yours is either sending too much or too little transition fuel. Without hearing the stumble, it is impossible for me to say which it one it is. There are also leathers and check balls in there that can cause issues, so not necessarily adjusted wrong.

^^^^
This is the first thing to investigate