WAAAY too high comp. shaved heads? - Snowmobile Forum: Your #1 Snowmobile Forum

AJSparko · 10-06-2009, 02:19 PM

Here's the story. Last year kid bring my BIL his 89 Indy 500 fan to replace a piston, holed it. Checked comp, zero of course and then 180 on the good side. WTF?! Talked to the kid and he didn't care, he wanted a new piston put in, carbs cleaned and sled back ASAP. So we did just that and checked the side with the new piston, head cleaned, new gasket....180-ish. Told the kid the heads must be off something else or shaved, no way it should read that high. He didn't care, took it and blew it up 2 days later. We even checked the comp with another gauge to be sure.

Fast forward to today. Might be able to get this sled for under $100. Pretty decent sled with some suspension mods, plastic skis, new carbides, just needs a seat cover, track and some minor cosmetics...oh and a piston. But I'm trying to figure on exactly what I'm going to need and look for. Our best guess is that someone either put different heads on with a lower CC (if that's possible-not a huge Polaris guy) or shaved the heads. To have it read that high with a cleaned head, brand new OEM piston....can't think of anything else that would cause BOTH sides to read that.

As far as the track goes, I'm assuming there is a wide range of Polaris sleds I could snag a track from. I'd prefer to find a used one and keep this a budget sled since I'd be in it so cheap.

BTW- when I did see him riding it was pretty shocked at how quick it was. A LOT faster than a 540 Yamaha.

SurgeDS · 10-06-2009, 02:48 PM

If the sled is complete with just a blown engine then its worth the $100 bucks. But if theirs no track and no seat and some other parts missing, i wouldn't bother with an old sled like that even for $100 bucks. Unless you already have the parts, i would walk away from it. Too much of your time is going to go into something like this. Also too much money even if you use the beloved craigslist to find all your parts, it will still cost you more then what you could have bought and saved your time and money in the end.

I have the same situation but actually worth the trouble. I bought a 1994 INDY 440 with a Polaris 500 engine it. It has 60 psi in one cylinder and 110 in the other. I will need to replace the rings and the crankcase seals so their will be labor required. But that's besides the fact. The big thing that makes this sled worth while is the 136 inch track with about 1-1/4" lugs. So it already has track extension and tunnel extension done. The guy also put brand new fuel lines because he thought that was the issue with it. He was selling it for $650 but it didn't sell for the longest time so i ended up picking it up for $300. Now the track and extensions are worth just that. Your deal requires you to buy all that.

You can pick up a nice sled for $600 bucks specially right now on craiglist that are either ready to go or are more modern sleds but just need some minor TLC.

Just my 2 cents.

AJSparko · 10-06-2009, 03:39 PM

I see your point. The seat is there, just needs recovered which I can do for about $40 in materials and the track is usable...just would like to find one in better shape. Other stuff like a couple cracks in the hood and crappy paint job aren't a big issue since I end up repainting every sled I've owned. It's all there though, title and key.

When we worked on it, was in really good shape mechanically (clutches, hyfax, shocks, springs, bushings, cables....). Talked to my BIL about it some more and guess the kid owes him money for the some other stuff so might end up with it for free in the end. Just like to figure out that insane comp.

**RJ Gleason** · 10-06-2009, 03:47 PM

What was the condition of the piston that was replaced the first time?
This will give a clue as to why it blew out.
Was it holed/cratered, or the edge melted off?
Holed usually points to a bad crank seal. the exhaust side edge broken down usually points to either a bad carb flange or jetted too lean.

AJSparko · 10-06-2009, 04:07 PM

Top of the piston really no cratering, surprisingly clean except for the hole right smack in the center of it. Wasn't stock either, I'm guessing a Parts Unlimited. No signs of damage or wear on the edges or skirt. Basically like they replaced it, poof-holed it shortly after.

Plugs were good color and after we got it back together and warmed it up (new plugs too), putted around with it a little bit to get it up to operating temp. Checked the plugs and they look right on. We thought maybe it leaned out. Checked the carb flanges, went through each carb when we cleaned it good and checked the jetting. Everything seemed fine. It ran great, good throttle response, no backfiring whatsoever while riding it. The side that blew a hole both times is opposite the fan...the hotter side.

We did do a quick peek into the fan side cylinder with a borelight and it had more of what I'd expect to see, thin coating of dark brown with some slight wash to it. Looked more like an older piston, not recently replaced. But still with that kind of comp on both sides.

**RJ Gleason** · 10-06-2009, 07:33 PM

You will need to do some "plug chops" to find the real problem.
My guess is that either the carb needle is too low (lean in mid-range) or that the main jet is too small.
To do the chops, you run it up to a specific rpm range and without letting off the throttle, hit the kill switch and brake to a stop while holding the throttle in the position it was running at.
Check the plug color then and make adjustments as needed.

AJSparko · 10-06-2009, 11:17 PM

When we reassembled the carbs we checked the jets/needles. Factory size jet and needle was in the middle on both. That still wouldn't explain the 180 comp on both sides though. I could understand one side being that way from extreme carbon buildup, which there wasn't. And the new piston side read the same. Is there anything other than a shaved head that would cause that?

**RJ Gleason** · 10-07-2009, 09:22 AM

Just for curiosity's sake, what are the numbers on the base of the cylinder?

I'm wondering if this might be a performance motor, rather than a factory stock.

AJSparko · 10-07-2009, 11:31 AM

Have to wait until this weekend when I go check it out. Everything looked bone stock. Carbs were stock jetting, clutch was stock, pipe....

**RJ Gleason** · 10-07-2009, 11:50 AM

They built some stock performance engines back in the day, that's why I asked for the # from the base of the cylinder.
That will tell me a lot about the engine.
The # should look similar to this;
EC4*P**