i have recently bought a scrambler 400 and did the top end on it... when turned over by electric started it just rotates till it explodes in on big backfire....when i pull start it it runs but has a bunch of lil backfires and it ran wild on me and i couldnt get it 2 shut down... plus it is throwin gas out the float over flow... any help would b very much appreciated..

 

Sounds like the carb needs to be rebuilt/needle and seat replaced/float level set if gas is flowing out the overflow tube. "rebuilt"? Was it bored out, new piston and rings or did you just install a new std piston and rings? What is the compression reading? I'd check these two things out first. If you solve the fuel problem and it still has problems starting,backfiring,etc ,it could have an electrical problem.Plus did you bleed the head from any trapped air in the cooling system? If not they can get hot quickly and start dieseling . Keep us updated.

 

i did have it bored...the first time i put it together i didnt. it ran fine but it was leaking gas so i took off the carb and cleaned it and adjusted the float...when i put it back on i started it and it ran good and wasnt leaking gas but 3 min in to running it died and wouldnt restart..then it just started building up gas and exploding on me seems thers plenty of spark..im not absolutly sure about the cdi box.....i dont have a compression gauge that works no me so i just held my finger over the plug hole and it felt week so i had it bored. when i got it all back together when i used the electric start it was doing the same thing w the big backfire...then just before giving up i decided to pull start it and it fired rite up but had the lil backfires and was leaking gas out the carb again

 

Rebuild the carb don't just clean it! Borrow a compression gauge and check it out. If the compression is above 130(should be at least this on a new piston/bore job) then proceed to bleed the 10 mm screw on the top of the head to remove any trapped air from the cooling system. Plus simple things like is piston installed correctly? Ring gaps at the intake side and not at the exhaust? Have had people do this before. Even crank seals which most people over look can be a problem if they leak. It's a process of elimination of possible things that keep it from running.If you eliminate all of these and it's still popping back it could be an electrical problem.