Stumble on Accelleration = ??

[TWG1572]

Many years ago, someone told me a rule of thumb. It was for small engines and went something like.... "If it stumbles as you are pushing the accellerator up it's too lean/rich (I don't remember which), but if it accellerates smoothly and then stumbles it's too lean/rich"

I just don't remember which scenario was too lean, and which was too rich. Or maybe it's not true at all. That's certainly possible too. It's just stuck with me through the years and I'm annoyed I can't remember it. I guess I could just play with a carb and try it out, but was hoping someone would know...

 

[wayne50]

I would say lean for the first part and rich for the second.

 

[actionstan]

Originally Posted By: wayne50
I would say lean for the first part and rich for the second.



+2 makes sense, if you accelerate and it stumbles it needs more gas, if its not getting enough (running lean) it will stumble. If your not accelerating/idling and stumbling, could be running rich, and have to much gas causing mis-fires/stumbles.

That being said... this cant be 100% of the time rule.... I could also see the "vice-versa" happening in both situations.

 

[djb]

An easy way to remember this is that larger carbs have accelerator pumps. They squirt in extra fuel if you open the throttle quickly, but not when the throttle is held steady (or opened slowly).

Stumbling on acceleration more often indicates bad spark plug cables, or perhaps too large of a plug gap.

 

[SteveSRT8]

A good carb design allows you to tune the AP for both duration and volume. We used to change nozzles, arms, and cams/linkages to get our old double pumper Holleys set up correctly.

Quite challenging to get it really correct...

 

[TWG1572]

Originally Posted By: djb
An easy way to remember this is that larger carbs have accelerator pumps. They squirt in extra fuel if you open the throttle quickly, but not when the throttle is held steady (or opened slowly).

Stumbling on acceleration more often indicates bad spark plug cables, or perhaps too large of a plug gap.



Interesting. The reason I asked is that my briggs twin developed a pause/stumble this summer that only happens when I accellerate the throttle. I'll push it up and it will begin accellerating smoothly, pause, then pick right up where it left off and finish accellerating.

I hadn't really thought much of it, but it has seemed to be more pronounced this winter on a cold engine. It will actually kill out in the same spot it usually stumbles in when the engine's cold. I'm still not ready to tear into it and figure out what's going on, but maybe next spring/summer.