All 2st low compression, top end ?

[pollolittle]

DISCLAIMER: I realize this is a Husky forum and a bike forum. But since I have always gotten good info, here I am again, unrelated to Husky.

Working on 2005 P0l@r1s Sportsman 90cc 2 stroke for my boy. It was rolling down the trail and kaput. No worky and no cranky. Put a compression tester on it and it showed around 35 psi, twice. I couldn't believe it, I had to give it a go again.

So, the questions are:

1. Do I do the whole topend, which I am presuming means piston, rings, clips, quick cylinder deglazing.

2. or Just re-ring it, with a cylinder de-glaze.

3. Or do I go for a different piston that gives a little more CC, without changing the crank.

It seemed to have plenty of power and would scoot on down the trail quick enough to scare momma. I was thinking of just keeping the piston stock and just re-ring, but this is my first tiime into this area. Never done just a re-ring.

It has a few aftermarket go fast goodies already on it and we only trail ride.

 

[justintendo]

how are you peforming the test? are you cycling it until the gauge quits climbing? with the throttle wide open? just making sure....doug is correct as far as the rest...will have to inspect for damage...also depends on if its iron bore or plated

 

[oregonsage]

This was just posted on our local OMRA forum.
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I was recently helping a friend with their 300 KTM and went to kick it over and it had almost no compression. After throwing a compression gauge on it, the highest it got was 65psi - horrible! This was my old 300 that the friend bought and I remember telling him to install a new set of rings. He didn't and now it really showed. This made me create this posting as there may be others out there that also are not aware of 2-stroke, top-end maintenance. Some may lambaste the 2-strokes for having to perform top-end maintenance, but it's cheap and quick!

Bottom line: 2-stroke mills need new rings installed occasionally. Symptoms are hard starting and low power. The more they rev (especially tiddler motors), the sooner they'll need new rings and perhaps a piston. It's easy/cheap maintenance. You can dissected a mill in less than an hour easily. Generally a compression gauge is the easiest method for checking top-end status. Simply install the gauge in the sparking-plug hole, open the throttle completely and start kicking. The gauge needle will stop after about 10 kicks and that's your compression reading. Generally 160psi to 190psi is good when new, and around 20% less or say 120psi is on the low end of the scale and means it's time to replace something. Write it down in your manual when new, then check it occasionally with the same gauge from then on.

There are many sites that describe how to do a compression check, such as these:
http://www.dansmc.com/compression_test.htm

http://www.your-adrenaline-fix.com/comp ... -test.html

Don't put it off, do it now. You might be pleasantly surprised how much more power a new top end will provide!

 

[pollolittle]

Checking compression as stated above, hold throttle open till gauge quits climbing, only 5psi difference between the two. Still at 30-35psi. Cylinder looks good, no deep gouges, nothing that I could scratch. The rings are way worn, they have gotten thinner especially on one side. The piston looks like it has made a few trips up and down the bore. The piston has some wear on the top edge of the crown like it was rubbing. Just looks to be in rough condition.

 

[MotoMarc36]

My experience with trail-ridden ATV's (always have piles of them at the shop) is that they get poor airfilter maintenance, have inadequate airfilters to begin with, and they ingest tons of dust/dirt. We're CONSTANTLY getting units in for "Warranty" that have dirt coating the throttlebody and intake. Trashes the valves and topend on 4t, trashes the topend and crank on 2t's. Fortunately the crank is fairly resilient and may still have service left.

It's pretty rare where I'll put a new piston in an old bore, even rarer that I'll re-use a piston and just re-ring. As Lanky stated, if the bore checks out, you can just give it a light hone and re-piston/ring it. If you're unsure or can't get it measured for free, just do a 1st over and have it bored, be done with it.

Compression spec for that unit at 0-1000 ft is 90-110psi. It is very important that your compression gauge adapter hose has the same reach as the sparkplug, if it is shorter your reading will be low.

 

[pollolittle]

Thanks again. Good info. So, it sounds to me that you would buy the $100 cylinder, new piston, new rings, new wrist pin and bearings. For around $250 a totally new top end.

 

[MotoMarc36]

Thanks again. Good info. So, it sounds to me that you would buy the $100 cylinder, new piston, new rings, new wrist pin and bearings. For around $250 a totally new top end.

Yep. Or, Polaris sells both a .25mm and a .5mm oversize piston that can be bored into your cylinder. CC gain is negligible, but boring typically runs about $50, you would save the difference on the new cylinder.... but that is a pretty cheap cylinder! Don't forget, always fit a new top pin bearing and new piston circlips.

 

[pollolittle]

Now I'm confused. Not sure what the piston to cylinder clearance should be. Its a snapshot from the service manual and it states in the writeup differently than picture caption. I didn't buy the new cylinder. The new piston felt the same in each bore of old and new cylinder and the salesman didn't break out the feeler gauges. I haven't honed It yet. This might be entertainment in this next statement. The parts guy tells me not to put the piston in from the top with the dome going down because I would ruin the piston and the cylinder. I wonder what's gonna happen when 90 psi is exposed to it and then what is the cylinder pressure after ignition. Sometimes I just wonder.