I recently put a post up looking for an engine for my machine. What i would like to do is give ya'll what it is doing and hear some input from anyone.

The machine had the top end rebuilt less than 2 years ago and i have put about 10 to 15 hours on it since. The machine was running fine, no problems. a few days ago me and my son rode it and when we got done i parked it in the barn. I went to ride it the other day and started up just fine, backed it out and it went dead (figured still cold, less than 2 min of running). Shut the door to the barn and started it back up, reved the engine slightly. then the loudest backfire i have every heard (rung my ears). tried to restart, sounded like there was no compression. for giggles I changed the plug and placed a new one in ( old plug was black\carbon buildup). tried starting again, after turing over 15 sec. it finaly started. ran about 3 sec. then another backfire ( not quite as loud ) and went dead. checked the engine, and found what looks like a small crack in the jug (not 100% on that yet). I stopped at this point until i can get everyting off and look around the case\jug\engine area.

could stright gas cause this problem? (oil injection failure)

that is my first guess? anyone have a opinion?

Thanks

 

Have you checked the other simple things like is the throttle lever very loose! These older ones would backfire if throttle had too much slack and when revved up and throttle came down fast and contact points in throttle housing touched, also has it been sitting up any length of time? Have you removed the oil pump cover plate and checked the oil pump for mud/dirt or just frozen up? Plus around the carb and air box area(air boxes had a problem on cracking and sucking in air?? Lot of things to check,but its not a simple engine. Plus they were bad on reverse limiter modules going out,may have something to do with it? Keep us updated. OPT

 

OPT, I was driving the thing just 2 days prior to this happening. have not checked the oil injection cable linkage. i removed the air box so it is direct to carb ( open end ) going to put a k&n on it ( go cart type ). I am also thinking about draining the gas tank and put a 40:1 mix to eliminate a no mix condition to check that. What do ya think about that? Could a non-mix or straight gas cause a hellashious backfire?

 

NO,but This could have happened since box was off\(being lean??) I've had em back fire so bad that the head pipe on the exhaust pipe has blown off stretching the springs and the cause was the loose throttle setting! Plus its possible a rev limiter is about to go out? They cause problems,but mainly dying when you put in reverse,limiting in forward,etc. You might also pull the black wire from the module under the front cover and give it a try? Popping back in most cases I've found is electrical related not fuel. I've had some of the old ones coming down from a fast run and letting off throttle then have had em back fire bad,but usually traced it to the throttle slack, they will load up on gas and when internal contacts are touched would cause an explosion in the cylinder!. OPT

 

OPT, gotcha. i will check that first thing before I tear it down. start simple...i will check the throttle at the carb and the etc switch...

 

After some research, i was wondering (have not checked this yet) could i have 1- a hole in the piston? 2- severe blow by and the spark is igniting the gas in the case? 3- i have a polaris? LOL!

number 3 was for laughs!

 

Easiest way to check top end condition is a compression check! OPT

 

very true, thats another thing on the list!

Thanks OPT

 

OK, today i had a chance to look over a few things. Throttle linkage is all good and set to the specs in the book. I did a comp. check and had 85lbs of compression. From what i have heard and seen 90 is the number to rebuild.
I pulled the head off and I seen nothing obvious, i could move the piston side to side and twist it in the cylinder bore, dont know if this is normal or not? pulled the jug and checked it as well, no scaring in the walls- smooth and shiney. piston has very minor carbon build up on the top and very little on the intake side but other than that no scaring on it as well. but with 85 on the comp. i am going to do a topend rebuild. never done one myself.


1 - do I need a new piston or just rings?
2 - do i need to have the jug bored?
3 - new piston, rings and bore?

If anyone has done one ( which I am sure someone has ) please let me know if i am on the right track?????

Thanks,

Fire3096

 

New piston,rings and bore needed! 85psi WAY too low! You shouldn't be able to move piston at all in the cylinder! Plus check for ANY up and down movement on the rod on the crank shaft ,plus roll the rod around and see if you can feel or hear any lower bearing roughness or noise! If uncertain take the engine to someone qualified,BETTER than just having a bore job and piston,because if bottom end (rod bearing,crank bearings) are bad,it WILL take out a new top end again! Lot of people have tried to get away with just the top end rebuild,but didn't have the bottom end checked!! Have the cylinder properly measured as to what oversize piston is needed! Plus while you're at check the counter balancer fluid to make sure it isn't contaminated by a leaking water pump seal,PLUS if the engine has to be split,replace the water pump seals and bearing. The 400 is a good engine,but costly to completely rebuild everything (unless you are mechanically experienced) when the need arises! OPT