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flat spot at 3000 rpm...

  • Thread starter Thread starter fnkvn
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fnkvn

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'81 GS1000G, stock pipes and airbox. Just finished a meticulous carburetor clean and rebuild. Starts instantly, runs great below 2500, and pulls like a rocket over 4000. Stumbles at 3000. The stock (or at least what was in them) pilot jets were .170, the K&L Rebuild Kit had .200 pilots, but I stayed with what was in the carbs (bike has 29,000 miles). Bike did not run when I bought it, and no prior history is available. Where should I start? Thanks for any help.
 
When does it stumble? Wide open throttle or just going slowly at partial throttle up to 3000?
 
if I accelerate hard from off-idle on up, it is almost unnoticable. If I hold it at 3000, it is falling on it's face.
 
So you have a low throttle issue. Where/how did you set your mixture screws?
 
2 3/4" out is where all 4 were set, and that is where they are now.
 
Thank you. I'm trying to locate a tutorial on that, to no avail. Do you have a link?
 
It should be on Cliff's site someplace?

Found it, look for this n the right side of his site:

Plug Chops - Highest RPM Methods
 
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Thank you. I blew right past it, thinking it was a plug reading tutorial. I will do that tonight before I ride home.
 
If that doesn't do it, I'd try shimming the jet needle a little bit higher next. You can do it without pulling the carburetors.

First I would look for an air leak or vacuum leak, something that would explain the lean running.

Did you change the float hieght?
 
That is a carb tuning issue?

That is a carb tuning issue?

Kent's got you on the right track, that stumble is right on the transition point from idle jets to mains.

I set the air screws on my 1100G 2-1/2 out, but then later fine tuned them with a Color Tune?.

Did you sync the carbs? That's a must do, and you need a set of vacuum gages for that.

So, maybe some tiny crud in the idle and main jets. Check that with an intense magnifier.

If that don't work, you have your choice, shim the needles or put those larger mains in.
But first try to clean those OEM jets, because bigger jets or shimming the needles will drop your MPGs.

Sounds like you haven't replaced the intake pipe O rings, that's a gotta do and may be part of the problem.
 
Bill,

Thanks for the reply. When I had the carbs off, I also installed new boots and o-rings, and did a very and I do mean VERY thorough cleaning too. I did sync the carbs also. I am going to try the highest idle, and if that doesn't work - I will need to get some shims. I will post updates. Thanks again.
 
Update - I have tried the "highest idle" method, and the air mixture screws are all (roughly) out by 2 turns. It hasn't made any difference at all. When I re-installed the carb bank after the rebuild, this condition did not exist. It appeared about 400 miles after the rebuild.

I will try shimming the needles, but where is the best place to get the shims? And, any idea why it took a while to surface?

edit - i searched and found that Radio Shack is the place to get small washers. I'm headed there now :)
 
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Shimming (well, de-shimming?) worked! The stock plastic shims were .097" thick, and I lowered them to .062" by stacking two washers instead of the stock shim. Much, much better now. Had a little trouble sneaking in the slide to remove the circlip, but past that it was easy. I may try losing one more washer, but it is just about perfect the way it is.

Thanks to all for the replies and advice.
 
fnkvn said:
Shimming (well, de-shimming?) worked! The stock plastic shims were .097" thick, and I lowered them to .062" by stacking two washers instead of the stock shim. Much, much better now. Had a little trouble sneaking in the slide to remove the circlip, but past that it was easy. I may try losing one more washer, but it is just about perfect the way it is.

Thanks to all for the replies and advice.
Click to expand...

Note that you actually raised the needle, not lowered it. But that is what you needed to do anyway, so it's all good. :cool:
 
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