Flat spot, from Idle After Rebuild and Tune-up

[leonardhcross]

Hello naked friends. There is tons of discussion about the "flat spot" or "stumble" upon take-off from idle. I am just adding this as a visual. I'll keep working at it.https://www.youtube.com/watch?v=tW1bp7f ... pt2kQ4Lfu3

[robin1731]

That isn't running right. May only be firing on 3 cylinders. May need a carb sync. May be something else. But it is not right.

[leonardhcross]

That isn't running right. May only be firing on 3 cylinders. May need a carb sync. May be something else. But it is not right.

I sure don't have your experienced ear so I'll continue fiddling. Out on the road, the bike has wonderful pull and power. I can't imagine that it could pull like that if it was only firing on three cylinders. So, I will go over everything and see if I can smooth her out. Thanks for the input.

[Whiskerfish]

Yep just watched the video. Agree with Rob. Try and do a idle drop procedure. That will tell you which jugs are dead and which are firing at idle. viewtopic.php?t=43626#p453009

[CYBORG]

That bike need help. Lots of problems I think

[gltriker]

Yep just watched the video. Agree with Rob. Try and do a idle drop procedure. That will tell you which jugs are dead and which are firing at idle. viewtopic.php?t=43626#p453009

hi Len,
After having listened to your video, multiple times...here's my take on the off-idle hesitation.
First thing, the coolant temperature gauge indicates the engine isn't fully warmed up yet.

At 19 seconds into the video you twist (blip?) the throttle and the stumble* whack! sound sounds like a lean fuel off -idle backfire into the throat of a carburetor. Yes, whiskerfish's advisement to check each of the 4 carburetor's pilot air/fuel screw adjustment is well advised... after the engine is fully warmed up.

A visual witness of that backfire phenomenon would possibly present as a blue flash into the plenum from the throat of the affected carburetor. Continued repeats of that backfire phenomenon will eventually fracture the 'button' on the top of that carburetor's vacuum piston ( aka slide) cap. Check all 4 vacuum piston cap buttons' structural integrity.

Even If you are confident the carburetors' engine idle speed synchronization is still correct, I'd proactively utilize the synchronization gauges hook-up, again, to double check if that is still true. Instrumentation witness...
The synchronization procedure will identify if in fact there is a lean fuel off-idle backfire event, immediately identified by an abrupt pressure spike in the affected cylinder's carburetor intake manifold synchronizing adaptor.
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the stumble* whack! sound

[leonardhcross]

Thanks for the detailed analysis Cliff. I will do the idle drop and re-balance shortly. I appreciate the time that you put into this analysis. I do remain confused about one point.........Why does the bike pull and run so beautifully down the highway if, as Cyborg says, the bike has "lots of problems"?

[robin1731]

They can be way out of sync and still pull ok at higher rpm. Need to start from scratch on the carb tuning. Let the engine get up to temp before you can really get everything correct.

[Shadowjack]

Having gotten hooked on automotive "revival" videos lately, I'm amazed at how many presumably knowledgeable people attempt to give a cold engine a lot of throttle with a non-functioning choke, and are surprised that it spits back through the intake.