I finally called in the local bike man after spending some time on my Polaris 1994 400 cc 2 stroke

he said spark was great and no fuel problems so it must be a "scored cylinder", and was talking about $300 for rebore and new piston [and I do the work myself]

so I took the cylinder off and yes there was scoring but seems to me main issue was both rings were stuck firmly in the groove at one end [ie for about an inch or 2] and flush with the piston

this may be a silly question but would the hung up rings most probably explain why no start

secondly, as I have now got the rings working fine, would it be smart to simply put it back together without new piston etc

 

did you do a cmpression check before you pulled it apart. stuck rings would be a cause of why theres no compression (IE. no start) for sure. theres a reason the rings would have stuck and chances are they are no good now as well. if theres is scoring of the cylinder wall then chances are its time for a bore out w/ new piston and rings anyways. you need to check everything when this apart to find out why it happened,lack of oil,gas, too much air, air leak some of the differnt reasons that could cause this. imo.

 

I would for sure replace the rings, and hone the crap out of to see if you can get the scoring out, if not then I'd rebore it, and be done with it

 

just my .02, but if you have it apart, just spend the little bit and fix it right. why spend all the time and effort to put a bandaid on a problem that you can easily fix and know that it will last for years, instead of saving a hundred bucks and have to repair it again later. i have an older trailboss with low compression(100lbs.) starts really hard, once started, idles rough (if it even idles). i am going to tear it down in a week or two. waiting to get a dirtbike off the lift. have severe back issues and bending over working on the ground kills my back. my local shop bores cylinders for 55 and parts are very competitive to online. 300 sounds a little high if you are doing the labor and all he is doing is the machine work.

 

thanks for replies

the main issue is the bike has been used for 4 years now on a beach in the tropics and even with hosing down fishoil etc, rust is a huge problem

I am spending some time now [time I have as I am retired, but also have bad back] on stripping the frame etc [what is left of it] and properly rust proofing, but I think if I get 3 more years out of it I will be very lucky. Plus I don't really have much money to spend on it

also I just putter down the beach at 30 mph max to do fishing so dont need any great engine performance or suspension, I just need it to start and putter, so that's why I was thinking of just trying a band aid on rings [and no problem to do a proper job later if it does not work]

I think the oil pump may have been underperforming and caused the problem as there was slight "hammering" of the piston where the rings were hung up, and slight burrs were holding rings in [all of which I have now fixed].

also my bike man said it would be better to simply use pre mix than trust the oil pump - any ideas on that? [I figure I would need to clean plug more often]

 

on the frame, if it wouldn't cost too much, I'd have it powder coated
on the rings I's say hone it, and a $30 or $40 set of rings and be done for a while
as far as the oil pump, they're super reliable, but some people just feel better with pre-mix, me being one of those people

 

so if cylinder is simply honed [not bored] then no need for oversize piston?

 

<div class="FTQUOTE"><begin quote>so if cylinder is simply honed [not bored] then no need for oversize piston? </end quote></div>
As a rule of thumb, if you can catch your finger nail on the scratches "after" the hone job it's needs a fresh bore job.

You might want to use a magnifying glass to see if the rings are stuck in the lands because of carbon or if the ring lands are all beat to hell from detonation or a lean condition.

 

well I used 1200 grit wet and dry to the cylinder and my fingernail does not catch so that's why I am thinking I might be OK just freeing rings

the rings were stuck because of both, ie slight hammering [on intake side] had burred sides of lands plus there was lots of carbon build up. I used a fine tapered file to fix both and the rings now move very freely.

by the way, do I use a ring compressor to get piston back in, once piston is back on crank. I had a hard enough job just doing this on the bench, so it would be most difficult trying to lower the cylinder on to the piston and trying to compress rings working upside down

 

it would be easier w/ a ring compressor but you could do it w/out one if you had too. as far as the oil pump goes when they work theres nothing better(imo) but i mentioned earlier, you better check the bearing to see if there worn out or hanging up as well. if there is that much piston slap to cause knocking, im guessing maybe the cyl. walls are worn to point of a bore job(just a guess) so you really should either measure it or take it to be miked to see if you can just get away w/ honing it. imo.