i just purchased a 97 400 scrambler 4x4 for 300. i knew it wasnt running when i bought it figuring how bad can it be for the price. the machine is in pretty nice condition plus i have a running 95 scrambler 400 i can use for test parts. it will start and run for 30 seconds when cold but after it fires for the 30 seconds it will not restart untill you let it sit for 40 to 50 minutes. compression is at 135 cold with throttle open and engine cold. seems a little low but should be good enough to run. leak down test is under 15% leak down at 80psi. after it fires for the 30 seconds, when you try to restart it it will backfire through the intake. the carb has been compleatly disaaembled cleaned, soak in a gallon of carb cleaner with the same results. when i try starting it after the intial 30 second run, i check for spark with a spark test and have a good looking spark. i try to spray fuel through the carb yet it will still not run. i pulled the reeds and the tips were curled bad so i flipped the pedals around(which really didnt make them sit flush with the cage any better) and still same issue. if the reeds do not seat to the cage well could that cause backfiring and a no start and be a little low on compression? what about somthing moving to change timing (fly wheel, pick up?). what else could i look for

 

Don't overlook crank seals leaking like most people do. Plus if it runs for while and dies,check compression right after it does. If it's lower than the cold reading might as well install a new top end. Plus on the 400 there are quite a few other things that are involved such as water pump bearing and seals,counter balancer problems,rod kit and bearings if needed that can double or triple your 300 buck investment in a hurry. Having a spare engine helps on some parts if needed.