I have a 01 polaris sportsman 90cc 2stroke that has been sitting awhile, and when I tried to start it I got a whole lota crank, but no start. So I checked all the switches and engine kills, and all are in good working order. Checked for spark, and I have spark. Checked for fuel (using 32:1 premix since I don't know if the oil pump is working yet) fuel is getting to the carb. I skipped over cleaning the carb since a new one is $20, and checked compression, and it had none. Ordered a top end kit replaced it and now has 58psi of compression. Still low seeing as it is possible to get faulty parts even if new I ordered another set under warranty, and same thing. I have also replaced the reed valve and gaskets for good measure. What am I missing here?

 

First off did you measure the piston to cylinder clearance before you installed everything? This is from the 2001 manual.Doesn't show clearances new,but only maximum clearance allowed:
"Piston-to-cylinder
clearance should not exceed .0047″ (0.12 mm).
Measure the piston 5/8″ (15 mm) from bottom. Then
measure inside diameter of cylinder. The difference
between these measurements should not exceed
.0047″ (0.12 mm)."
On a new cylinder and piston from Polaris I'd get between .0015-.002 clearance on the piston to cylinder. If you're close to this and have this thing apart again,put the head on a flat smooth surface and make sure no larger that a .002 feeler gauge can pass using 6 points around the head. Always replace the head gasket and base gasket if you take one down again.A teaspoon full of oil can help bring compression up a tad if you installed the piston and rings fairly dry.This can be a good indicator as to what actually compression should be. Also most people over look the crank seals. If either one is leaking bad enough,no matter what compression is,they won't run,or won't run right. Same as the leaking seal that this member found and also his post.https://atvconnection.com/forums/kid...sman-90-a.html Plus one last thing is that these little atvs can have exhaust pipes/muffler restricted or stopped up from oil residue. Make sure the pipe is clear and open. A simple blast of compressed air through one can give you an indication as to where that could be a problem area or not.

 

I did not check the piston clearance, but did check the head and it passed. I just ordered the new kit and ran with it. Kit came with piston, rings, new jug, head and base gaskets, wrist pin and clips. I have replaced the kit twice now and wet or dry I only get 58 psi, and on my harbor gauge was 50ish. I thought that was off so I brought my Snap-on gauges home from work and 58 on both kits. I am an auto mechanic for a living. Just bought 2 of these cheap for my kid. 1 to fix other for parts.

 

Did you check ring gap? When you crank it over can you hear any air leaking out? I know you replaced the gaskets but it almost sounds like the head is not true (warped)...2 strokes are too simple not to figure out a compression issue.

 

Times like this a good leak down test may uncover some problems also.In the videos they were checking for crank seals,but found leaks at the base gaskets.Soapy water and a leak down tester really helps. Also a leak around the crank case halves themselves can cause problems. This video shows a little more on the tester.

 

How many seals are there? The ebay kit show 4.

 

Only 2 seals for the crank case portion. The seals you're looking at are for a complete engine. Seems the 2 aftermarket seals are either listed with the 2 crank bearings or listed as an engine kit of 4 seals. Price isn't bad that way,around 11 bucks or so on ebay. The 2 oem seals would cost more from a dealer.

 

It's been a little while since I have been able to mess with this, but I have the seals here and trying to get the one out on the stator side of the engine. Is there an easy way to remove the oil pump drive gear?

 

Shows to have puller (0450699) but I can't remember if I needed that blamed thing on this plastic drive gear or not. Again been too long on this one. https://www.manualslib.com/manual/10...page=43#manual

 

I looked all over the internet for that tool and had 0 luck. At about 2am I gave up and used channel locks to remove it.