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1997 YZ250 Idle Choppy or Pulsing

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Friends bike, just did new top end and pressure test was good (5psi for 5mins). Was a bit choppy during break in period/rides so now working on cleaning up the mixture from idle through full throttle. Starts first kick, but when trying to get the bike to a low idle for testing it chops.. well pulses is more appropriate. If slowly accelerated on the stand or under load it still pulses to about 1/8 throttle then starts pulling. It can be cleaned out but once it returns to idel the pulsing re-appears. Does not waver in idle speed, or increase/decrease on its own. Top rpm is a also little burbly but trying to cure idle first. Sprayed carb area/boots while running and and no change (seal seems to be good) and turning mixture screw does not seem to affect the chop though I can get it to die if turned too far in. Float level is within spec at 6.0mm.

This feels like the bike is getting repeated spurts of gas and just as it starts to clean out another spurt comes but it will stay running. Part throtlle (1/8-1/4 maybe) it is just a faster version of the chop. Under full accel the bike is peppy and there are no bog spots. Raised the clip on the needle one and thought it was better at low rpm but still an issue - though it did help the top end some. Will refrain from more changes until I have some further wisdom input.


Model: 1997 YZ250 Yamaha

Conditions: Riding in OK sun at 80+ most of time, some times to the sand hills but always hot.

Mods: No airbox lid, Uni free flow filter with spray on oil (very clean), stock carb, very clean, non-stock jets..

-Main: stock=170 current=165

-Needle: N4DG - middle position (3rd clip) which is stock

-Pilot: stock=48 mine=48

-Pro-Circuit Works pipe (I think the pipe has a small leak - looks like a repair was attempted but still spays timing cover with minor spooge during a ride)

-Pro circuit factory sound tail pipe

Issue: Choppy or pulsing low speed operation. Happen warm, won't idle down that low when cold without dying.

Thank you all.

Edited by blufie
updated info...
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Too weird an issue or lack of supplied info?

How about this...

Is a pulsing/choppy 0-1/5 throttle likely due to rich or lean condition? I suspect that rich is the answer and was going to start tuning the idle circuit. Plan is to drop from 48 pilot to 45 and assess the change. Sound like a reasonable plan?

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I would like to apologize for posting on something that I have since found other posts on that have given me a series of things to work on.

What i DID learn is that the "Search This Forum" quick search does NOT really do a great job of handling multiple keywords and I needed to use the advanced search to more fully cull out some posts to read. Strange that I just noticed this after almost 3 years using this site. I kept thinking that "surely this is not a new issues and someone has covered it on a 1997 bike?!?". I was correct.

I will be dialing in the pilot circuit with Eddies sticky proceedure after new reeds and a 100% leak free exhaust are obtained. Then go from there. I feel pretty confident I can solve this and will report back.

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