PITA @ top end revs

breno · May 25, 2011

Hi all.
I have a mikuni 18mm carby with a zorst I picked up off a little dirt bike.
Seems to run fine the whole way through with a fairly good pick up only once it reaches the top end it will start running rough.
To clear it out I just back off from WOT for a moment then give it another hit and it will run clean for another sec or 2 then back to running rough and I find myself having to repeat that if I want to maintain high revving.
ALthough on the odd occasion it will run fine up the top and it seems like I can cruise with high revs only at 1/4 to 1/2 throttle.
Its running a .20 pilot jet and a .60 main jet. The plug looks a nice coffee colour and i'm running 25:1 on valvoline non-synthetic.
Any thoughts?
Thanks Breno

bairdco · May 25, 2011

it could be starving for fuel at WOT. that could explain why when you let off and get back on it, it starts working again.

could be a few different solutions, petcock and fuel lines may not be flowing enough to fill the bowl (i had this problem with the stock china kit petcocks.)

floats might be set too low.

or it could be jetting related. might need a larger fuel jet.

i'm thinking starvation, since your plug is dry and tan. if it was wet and black, then it could be too much fuel.

one other thing, it could be the air filter. i set up a 16mm mikuni on my bike and it did the exact same thing. when i built it, i didn't have the air filter. i figured it was jetting, and while looking for jets at an MC shop, i found a cool foam filter. as soon as i put it on, all the problems went away and i never had to re-jet it.

breno · May 26, 2011

thanks bairdco I'll give these things a try tomorrow. I have the red foam uni-filter too looks sik.

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PITA @ top end revs

breno

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bairdco

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breno

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