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250cc Popping under load

  • Thread starter Thread starter Woodsy
  • Start date Start date
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Woodsy

Guest
I have Mikuni BS30SS carbs on my 250 twin and am having problems with the bike popping when doing roughly 5000rpm in 4th and 5th gears, in 4th it will clear and run okay but in 5th it pops the whole time no matter what the throttle position is, i have to go to 6th for it to stop.

Opening the choke only makes the problem worse
No matter what position the pilot screws are in it still does it
Plugs are a little sooty with a brown tip but wipe clean
Checked the gaps
Inlet rubbers have new o-rings
I've sprayed the carb with water and carb cleaner and the revs dont change
I've set the float levels & cleaned the carbs
Air cleaner is fine
Fuel tap is fine tested that today

Any Ideas?..
 
Last edited:
Anyone?.... Otherwise i'm going to replace everything in site lol
 
Someone will definitely ask, so it might as well be me.
Did you do the full carb tear down and soak?
 
Yep played on the carbs alot lately and have soaked them and blown air through them and poked them with 1 bristle from a wire brush shoved into the end of a matchstick, put abit of sealant on the intake o rings, made sure i had the inlet rubbers around the right way and did it all again today to double check everything. I'm beginning to think its not fuel related maybe its spark related?
 
All I know is popping indicates some kind of lean condition somewhere....whether that's in the carbs, boots, airbox or exhaust, is anybodies guess!

But it can also be caused by a massively rich condition.(my 750 does it)

My guess would be that your carb(s) are malfunctioning at some point around 5k rpm's(or whatever RPM's you said it was.) Or you have a fuel/air supply problem at that flow rate of 5k rpm's.

Maybe you overlooked something?
Can you simulate it in Neutral by revving?
 
No cant replicate it with just revving. I've run it on prime, plugged the vac line etc and still get it.
 
Are the carbs synced properly? one isnt opening a little less than the other?
or is one of the slides sticking at that point?
Just throwing some ideas out there.
Its got me buggered why it only does it in fifth?
 
Where is your throttle position when this happens? If it is a lean/rich condition at high rpms, I would think that it's a problem with the needle position or main jet size.

If it is strictly a matter of RPMs, maybe your timing is off at the advance position, causing the bike to misfire, accumulate fuel, then backfire when the excess fuel ignites. Maybe a timing check at 2500 RPM will reveal the issue?
 
going to do a plug chop tomorrow, was flat out today so couldnt do it.
 
Check the advance/retard unit is not sticking and re-route the breather/vent tubes to the correct location as per the manual.
 
I dont think i have an advance unit etc on it? nothing in the timing is adjustable.
 
Josh, what year is your 250, put it in your signature.

If it's an '81 GS250 then it does have a mechanical advance/retard unit. Check your timing with a timing light, check to see if advancer is actually advancing.
 
His is an '83 Don, definitely electronic advance...

Edit: Just noticed you updated your sig. finally Josh ya cheeky bugger :p
 
can someone tell me what the magnets are under the pickups for the ignition under the right hand cover? One is cracked as has parts of the magnet missing?
 
Hmmm I believe that is part of the triggering for the pickup coils, and that perhaps could be contributing or causing your issues...

I could be wrong, but I think that's it...
 
This thing is just smashed up, could i replace it with any type of magnet as long as its a snug fit?..
 
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