engine roughness at 1200 rpm

[JimWoo50]

Need some help with a persistent engine roughness in my O-320b3b with a MS 1078-32 carb. Anytime-on the ground or in the air,hot or cold, rich or lean,-
I pass through the 1200-1400rpm range the engine stumbles. At first I thought it was vapor lock because its been hot so I hooked up heat shields on exhaust piipes and a blast tube pointing at the mechanical fuel pump. The engine still stumbles in the aforementioned range and once I am out of the range it runs fine. I pulled the plugs and they looked gray.
I have read posts about drilling the main jet on the carb. Does this sound like the type of roughness that would be caused by too small a opening on the jet? Causing me to run lean at low power? I asked the A and P who assisted me in building the engine and he was of the opinion that drilling the main jet was a extreme measure; he stated that he had seen instances where guys had passed pipe cleaners through the jet with good results. But he could not say if my symptoms could be cured by opening the jet.
The engine now has 33 hours on it and it runs up fine every time with minimum disparity in the mag drops and uses only a minimum of oil. Now if I could just get it to run smoothly at 1200 rpm! TIA.

[DanBaier]

What you're describing is pretty common with Lycomings - it's when the idle circuit cuts out and the main jet takes over. Annoying though it is, there's nothing seriously wrong with the engine.

I don't think I'd be drilling the main jet for this. If, when you're flying (not on the ground) you can lean the mixture and get 100 degrees of EGT increase, you are probably okay with the main jet.

Was the carb rebuilt when the engine was done?

Dan

[Low Pass]

Originally Posted by DanBaier

What you're describing is pretty common with Lycomings - it's when the idle circuit cuts out and the main jet takes over. Annoying though it is, there's nothing seriously wrong with the engine.

I don't think I'd be drilling the main jet for this. If, when you're flying (not on the ground) you can lean the mixture and get 100 degrees of EGT increase, you are probably okay with the main jet.

Was the carb rebuilt when the engine was done?

Dan

I sure can't offer anything beyond what Dan wrote. Sounds like good info. But I can say that each of the 4 or 5 MA-4SPA carbs I've had experience with on O-320s were all "acted" slightly different. Kind of like women. The parts are all pretty similar, but assembled - they each provide a slightly different final product.

[jrs14855]

This was just discussed in some detail with Mahlon explaining why this happens.
see "unable to lean for max RPM,why?"

[JimWoo50]

Flat spot at 1200rpm when increasing power as described by Mahlon in his post is exactly what I have. Is there a remedy or should I just get used to it? Thanks for your help.

[lockeed]

Mine does the same thing. O-360. No worries.