Hi All
I am having a problem with my mc21. It has had a full rebuild now it wont idle , I start it up with choke it is OK turn the choke off and after about a minute it will stall. Before it droped the big end bearing it was running fine.
Just before it stalls it will go splutterey then stall . If i rev it sounds good but when goes back to idle will go down to 800rpm then go back up to 1200rpm .
I have cleaned the carbureters out no change
I am running hrc jet kit ,Hrc delimiter ,no airbox yet,changed crank, new piston ,rings
This problem has got me stumped has anyone had the same problem or any suggestions of what the problem may be
Thanks All

I'm not a big expert on two strokes...

but it sounds like either 2 things (imo)

1: It could be running lean because you dont have your airbox fitted (check your jetting, needles if your modle of caries have that and check your mixture screws for idle (again if your model has those)

or 2: you might just be simple enough that you might need to re adjust your idle again after your rebuild.
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1991 CBR250RRL mc22 modified.

also... being a twin carb bike... are they connected?

could also mean you need to balance them.

give 4 spark plug points a clean. 2 on each of the leads. Make sure you got a nice tight fit aswell
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yeh i have done all of that changed coils new plugs swaped carbs. back to stock jetting. starting to think maybe electrical right hand exast comes out cold left warm at idle when reved both come out warm.

Have you checked your air screw is set correctly? No hoses or pipes kinked? Done a compression test?

Just done a rebuild so compresion good
I think i need a leak down test .

I have read somewhere here before that you need to balance the two carbs to get them both running fine at idle . If not , the one cylinder is doing all the work as the other one is out of tune . Stock jetting is no good for open carbs IMONeal

Maybe a leak in the bottom end after the rebuild. Several two strokes I have rebuilt have had a 'hunting' idle that was impossible to control. I tried to 'dampen' down the idle with a richer setting on the air screw. I also fully rebuilt two different sets of carbs twice. After a complete rebuild for the crank seals I suddenly also had a perfect idle.

OK an update : I pulled motor back out pulled the barrels of and poured some petrol into one side of the crank and went straight into the other side . So i hope i have pinched the o-ring or missed sealing properly. Or could the centre crank seal leak that bad ?

i don't think its possible for the center seal to let a liquid through that fast. Its a new crank isn't it? It sounds more like a problem with the outer O-ring to me too.
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After years of moaning about immigrants now i am one...