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Powervalve jerking on deceleration issue

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I know two strokes are a bit jerky here and there anyway but when i let off the revs on my 02 250 EXC from high speed/high revs to pulling my clutch in and coasting the revs stay high then the powervalve starts slamming open and closed like it doesn't know what to do with itself?

 

If i do this using engine braking the bike jerks forwards and backwards like mad.

 

Does anybody have an idea what's causing this?

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Sounds like pipe bang.
Common issue.
Could be detonating, bad issue. How's your jetting?

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9 hours ago, lhc450 said:

Sounds like pipe bang.
Common issue.
Could be detonating, bad issue. How's your jetting?

I agree. It was pretty noticeable on my 300 when I got it. I guess stock jetting and tuning leaves a lot to be desired. I worked with my jetting, different needles and tuning to minimize it. I also learned to control my throttle is such a way as to minimize it.

>>>>>>>>>when i let off the revs on my 02 250 EXC from high speed/high revs to pulling my clutch in and coasting the revs stay high then the powervalve starts slamming open and closed like it doesn't know what to do with itself<<<<<<<<<

Then don't do that.

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My 2013 250 had a little pipe bang action. Jetting fixed some then a FMF turbine core took care of the rest and cleaned up the low end gurgle.


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1 hour ago, lhc450 said:

My 2013 250 had a little pipe bang action. Jetting fixed some then a FMF turbine core took care of the rest and cleaned up the low end gurgle.

 

Oops, Forgot to say I swapped the stock exhaust for an FMF Q turbine core spark arrestor too.

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Sounds like pipe bang.
Common issue.
Could be detonating, bad issue. How's your jetting?


Iv got the jetting set up as the manual says it should be for my environment but I'm using a JD needle..

It's useable but could probably be leaned out as i think it runs a little richer than it needs? Could it possibly have something to do with excess fuel/oil igniting on deceleration?

I also have a FMF gnarly full system fitted and run 50:1 oil ratio if of that would make a difference

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Pipe bang is thought to be caused from the exhaust gases being too rich, so yeah, I'd work on tuning.

How do you tune the air screw? I use the "whack the throttle" method. I start with the screw turned out further than I think it should be and gradually turn it back in towards the rich side 1/4 turn at a time until I get close to no hesitation and clean response when quickly snapping the throttle to near WFO and back. Then fine tune in 1/8 turn adjustments. Record where that setting is at from the screws seated position. Doing this gets best throttle response off idle "on the lean side". If you go too far in with the screw you start getting on the rich side again where the pipe bang is happening.

Also, check to see if your pipe neck is bent or dented, that would restrict exhaust flow and it's also thought to compound the pipe bang phenom. Keep a supply of exhaust orings on hand too.

Edited by Trailryder42
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Pipe bang is thought to be caused from the exhaust gases being too rich, so yeah, I'd work on tuning.
How do you tune the air screw? I use the "whack the throttle" method. I start with the screw turned out further than I think it should be and gradually turn it back in towards the rich side 1/4 turn at a time until I get close to no hesitation and clean response when quickly snapping the throttle to near WFO and back. Then fine tune in 1/8 turn adjustments. Record where that setting is at from the screws seated position. Doing this gets best throttle response off idle "on the lean side". If you go too far in with the screw you start getting on the rich side again where the pipe bang is happening.
Also, check to see if your pipe neck is bent or dented, that would restrict exhaust flow. Keep a supply of exhaust orings on hand too.


Thanks for the advice I'll give it a try.

I usually set my screw as the manual says then slowly tweak it and go for test rides to work and back.

My exhaust is brand new so that shouldn't be a problem

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Did this just start recently or has it been an ongoing issue.

If it was running fine and now has this problem I'd look at something mechanical has changed.

A 2002 bike should have these issues sorted by now unless you just recently bought it and don't know the history or something else let loose.

@Trailrydet42 is spot on with steps to work on tuning if that's the culprit.

 

 

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Did this just start recently or has it been an ongoing issue.
If it was running fine and now has this problem I'd look at something mechanical has changed.
A 2002 bike should have these issues sorted by now unless you just recently bought it and don't know the history or something else let loose.
@Trailrydet42 is spot on with steps to work on tuning if that's the culprit.
 
 


I have fully rebuild the bike from the frame up and the engine and set all the jetting back to stock you see!

It has done this since it's been rebuilt so i think its just a jetting issue i will lean it out a bit and see how it goes.
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