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1998 yz250 surging done a ton of tests

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hey guys

motor was rebuilt 28 hours ago

the motor ran great for 20 of those hours


it surges from idle to 1/4 throttle.


if I put a crazy fat PJ 65 then it almost goes away.

the bike runs great with the choke on


 done a leak down test and it does not show anything


checked crank seals


swapped carbs with another great running bike so its not a plugged PJ or anything


done compression check. 190 psi


no leaks of coolant or tranny oil


I have sprayed the entire motor area with starter fluid and no changes. 


took the bike to a shop and they did the same tests.


the shop wants to spilt the cases to look round inside..


with the bike running better with a crazy fat PJ 65 .... and or with the choke on ,,, it would scream air leak or lean.


anyone ever had this before??????????????




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my bike did this when i had too high of compression ratio head and hot weather. no matter what jet it would run hot. lower CR fixed the problem. now it runs like a champ even on the hottest days. if you ran a leakdown and it showed to be fine don't split the cases. if you have no air leaks and your carb is good its probably something other than air fuel ratio casing the issue. look at the reeds and make sure they are sealing. make sure your timing is not too far advanced. 

Edited by MCRIPPPer

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I had an issue like this earlier in the air. Same tests run etc etc.  Turned out to be bad timing for me anyway.  I messed with jetting until I cried.  Checked my timing.  It was retarded 4 degrees.  Moved it back to 1 degree just for detonation issues and to tame power hit slightly.  Runs like a dream.

Oh btw  I did a carb slide mod, where to notch the slide right at the Pilot Jet   Holy hell   Bike has never been this easy to jet.

I’m running a 2001 YZ250.  Keep in mind with these settings, I’m 0-1000ft elevation, it’s 40-60 degrees right now.  I lean it out for summer time heat .






JD Jetting Blue needle 3rd clip

VP112 80/20 with non ethanol pump

Klotz SuperT 32:1

FMF Fatty and Short combo

Boyesen Dual Stage Reeds 

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I had a surge once.  I would do a wide open pull and when I shut the throttle off she would surge like crazy.  Turned out to be the jd needle.  I went from red to blue fixed the problem.  Since then I have gone back to stock needle N3EW position 3 and the surging is completely gone.  

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