i just picked up this atv in a trade, it had a backfiring problem but i noticed the intake /airbox boots were loose, no clamps, but it seemed to start/ idle pretty good got it home fixed the air leaks, checked filter took her for another spin, doesnt have much pep, it gets up to maybe 40 or so, but doesnt wanna spin tires, in mud, barely climbs the hills at my house, its now smokin blue, a lil, and very hard to start, but i fixed the backfiring, cant find my compression tester, pulled plug its black, dry though like coal dust, not oily, im worried it the rings, any advice? im feeling took got bout 700 into it

 

im also wondering whats involved in a new top end? i can turn a wrench and have replaced top on 2strokes, never a 4, what would be a fair price for a shop to charge? just tryin to figure what kinda cash i'd have to sink into this before summer ,got it for my son, if its not real difficult i would try it myself, if im only savin coup hundred would have a shop do it.

 

well i changed the oil / filter / plug seems to run lil better, the oil smelled strongly of gas, plan on changing it again soon, seems to start right up fine when its cold, but idles a lil rough, but when i ride it 20 minutes or so its very hard to start, doesnt seem to be smoking anymore though, thanks again for all the help& suggestions guys

 

If your oil smelled like gas then you may also need to clean the carb. Float could be sticking causing furl to runinto the cylinder and around the rings into the oil. If it is ran too long this way it can damage seals, rings and bearings. Which may cause it to smoke.

When I rebuilt the top end of my 400 about 5 years ago, dealer was going to charge me about $1500. New head, valves, pistons, rings etc. I did the work myself but I still paid almost $1000 for parts. I suspect it would be a little less for a AC 300.

Swampy

 

thanks swampy, i think it did have carb problems that were addressed, the carb looked too clean and like i was sayin all the hose clamps were missing, made me think somebody had that carb off, im gonna have a shop adjust the valves for me this week, the guy i bought it from said he had tried to adjust em by ear and couldnt get it right, i have my fingers crossed, then ill probably change the oil again just to make sure ive flushed all the gas out, rode it today bout 15 minutes, started right up, idled ran smooth& very quiet, just not much power, shut it down and it hardly wanted to start. the warmer it gets the harder it is.