Run on and over rev

I need some assistance folks !

1972 WR250 runs sweet and starts sweet, after a bit (not long, say 15mins if that) it over revs (a fair bit) at idle / between gears and altering the carb idle is not doing anything to calm it down. Tonight tho, after this and I had killed it, I moved the bike to the back of the house and started it with a view to try to idle it, but it revved out full scream throttle closed - I pulled the plug cap off and it continued to scream until the fuel was out from turning the tap off. It took a minute or 2 to use the fuel. I can’t figure this one, thoughts folks as it changes the colour of your pants .....!...?

Cheers Craig

Blocked spark arrestor and or air leak

I think you need to look at your crank seals. My Bultaco used to do something similar.

I agree. Your a brave Man to grab the plug wire in this condition! Your misses will not like the laundry!

Classic air leak symptoms

I see trouble ahead .......

Are you running the OEM Bing or have a Mikuni conversion?

It’s Mikuni carb....

Just found the 2nd plug was well not tight but not loose, the threads all the way up were oiled up so poss air leak here . But I’d have thought not enough to keep it reving through?

May want to consider performing a Leak Down Test - Gold Standard for finding source of air leaks. A tester can be made for around $20 (USD). A web search for 'diy 2 stroke leak down tester' yields many examples.

Attached is a 'crib sheet' focused on the 72 - 74 Big Clutch Motors that may help. It is in order of the easiest things to check/fix to the more involved.

Cheers Vinskord

I have the exact same bike with mikuni that did the same thing, the real reason was the little boot on top of the carb causing an air leak around the cable. I did fatten up the jetting some also, never did it again. Edit: The brass fitting on top of the throttle slide cap where the little boot is I mentioned was loose in the cap too.

Cheers Dpete I’ll look at that too.

Craig

Do you think pressurizing the clutch cover area would be able to lead to confirm the crank seal is not sealing any more? I have one that has started behaving in a similar manner after run hard and does not start quite like in years gone by. Stall it out with the gears and clutch if the kill switch or plug wire does not work. I do not even bother with the kill switch if this occurs.

I’ll need to get onto this but I’ve done my back in somehow so hobbling at present, I’m stuck at 45 degrees !!

Due to my sore back (!) I’ve stripped the spark arrestor as per an early reply in the thread - as minimal bending by me is needed !!. I’m not sure it’s correct as I can’t see how it would work. It’s a J&R Answer AP650, anybody have a drawing of what the internal make up should be ?

An update folks, a bit late tho - full strip and crank seals done, air leak found inlet manifold, cylinder head had a few possible face marks but seemed to rest and test ok. All now running sweet.

Yes well at least husky bike has metal under the rubber intake....when they went to make bergs all too got was rubber and it split under the hose clamp without you knowing causing same problem.....