Loping/stumbling at idle

[superbenk]

Been trying to track this one down for awhile. I get a slight stumble or loping at idle or low RPM (ie, under 1000 RPM). The carb was rebuilt last year & I've had some success tweaking the idle mix screws. I've also checked timing several times as well as dwell.

This video kind of shows it with a slight engine shudder if you watch closely: http://www.youtube.com/watch?v=8scxmL-nHlQ

I also checked & adjusted valve lash though I'm not sure if that plays into this. Where else should I be looking?

 

[sw33ttooth]

Re: Loping/stumbling at idle

guessing that is some sort of an inline 6? if it has esa might want to check and adjust it, i didnt hear anything unusual on your video watched it 3 times over.

 

[superbenk]

Re: Loping/stumbling at idle

guessing that is some sort of an inline 6? if it has esa might want to check and adjust it, i didnt hear anything unusual on your video watched it 3 times over.

It's an inline 4 cyl (OMC 140). It's a hydro-mechanical shift, so no ESA. I admit the video isn't very helpful because the shudder is so subtle there. If I drop the RPM down to ~500 RPM it becomes significantly more noticeable as it rattles the whole boat & the engine sounds like it's about to die. I'll try to grab a video of that later.

If I bump the idle up to ~850 RPM it's much less noticeable, but still there. Seems like the momentum of the engine at 850 RPM overcomes the subtle stumble so you don't hear/feel the rattling/shuddering of the engine.

I'm wondering if this would be due to bad idle needles or something else? Most of the ignition system has been replaced (coil, plugs, wires, rotor, cap, points, condensor).