Season ending injury

[Steve Foss]

Yep, nothing like a hole in the piston to rob a fella of compression.

92 Polaris Indy Trail 488. Was hitting some really big drifts for half an hour straight and believe the left-side carb froze from eating snow through the intake. This is the left-side cylinder. You can see from the pic that there's some transfer on the cylinder wall above the exhaust port, but that should clean up nicely with muriatic acid and a Qtip. The cylinder looks OK otherwise, unless you think that what looks to me like light scoring near the exhaust port is too severe. My fingernail doesn't catch on it when I drag it across.

I have a line on a piston/rings/wrist pin kit for about $45. Looks fairly simple. More simple even then my Stihl chainsaws. Looks like I need to pull both carbs and the exhaust pipe to get the rest of the shroud off and to expose the cylinder so I can remove it.

Aside from making sure the frozen carb caused the lean condition in the first place, is there anything I should watch out for during the swap?

[leech~~]

Bummer Steve. Hopefully all the metal got blowen out of the exhaust and no ring bits went down in side the case. When you do pull the pistion take a good look down into the case to make sure no bits are down there. But it doesn't look to bad as far as throwing on a new pistion and rings with a little hone out of the cyclinder you should be good to go. Over time I have burnt 3 pistions and rebuilt them. One was running across a lake in warm weather and heavy wet snow. One the rubber boot on the carb was cracked and let in extra Ox which burnt it down and the other was a fluke that happend at the factory. When they cut one of the little holes out in the air box the assembler left the drill out in the box and a few years later since is was the perfect size as the carb it got sucked in the carb blocking it off and burning that one down! It took me forever to try and find out what happen until I clean the carb right before reassemble and noticed a nice black plastic disk stuck in the end of it!

Good luck and driver her easy for a while after the rebuild.

[fishersofmen]

Ouch. I would be surprised if no metal got down in the crank but maybe you got lucky.

[Lip_Ripper Guy]

On a fan cooled motor, the swap is relatively simple. Just make sure you use new gaskets and torque things down appropriately.

When you pull the cylinder off, make sure you stuff a clean towel in the crank case before you try to take the cir clip off the piston. Those have a tendency to go flying, and occasionally the wrist pin bearings will fall apart and end up in the crank case

FYI, my first thought with a hole in the center of the piston like that is detonation caused by too low octane (bad gas). I'd personally drain the gas, just to be safe.

[Steve Foss]

Yeah, those circlips are buggers. Have lots of experience with those from tearing down chainsaws.

Definitely planning on examining the crank closely. When I tried to start it this fall (it was thoroughly summerized), the crank was frozen. A bit of PB Blaster and judicious use of a pipe wrench on the belt shaft broke it free and it ran great all winter. Well, until . . . .

Depending on how things look, I may just do a whole rebuild with fresh gaskets and bearings this summer. I've been told that if moisture gets into the crank casing and causes a freeze up, the crank bearings often get tiny pits in them and will fail a couple hundred miles down the road.

[magicstix]

$45 that must be aftermarket. i'd price out OE and Wiesco proline.

When was the last time carbs were cleaned. Besides the rebuild, do both sides, I'd be cleaning carbs and syncing.

if it was froze this fall and needed a pipe wrench to free, there is maybe bigger problems down below with crank bearings or the cylinders got some excessive moisture in them in the off season. Where was it stored?

When i pull cylinders, I do the same stuff towels around crank area and I also lay white bath towels in front and back of motor to catch anything the fingers may OOPS. The fan cooled is a gravy job. that will probably clean right up with a couple quick passes of the hone.

[Steve Foss]

A good friend (and former Polaris mechanic) is sending two excellent used OEM pistons tomorrow, with rings and gaskets.

Thanks Chuck. I owe you big! Free guided laker trip coming up for you. Barter rocks my world, as always!

[shortfatguy]

The cyl should clean up with the acid. If you arnt in a hurry to get it back together might as well split the case and check out the crank. Make sure there are no pieces in the bearings and they all look good. I would take the carb bowls off and catch the gas in a clean container. This way you will be able to see if there is water in the gas. If there is drain the gas tank and refill with fresh gas. Dont forget to premix your first tank of fuel after the rebuild.

[Steve Foss]

The cyl should clean up with the acid. If you arnt in a hurry to get it back together might as well split the case and check out the crank. Make sure there are no pieces in the bearings and they all look good. I would take the carb bowls off and catch the gas in a clean container. This way you will be able to see if there is water in the gas. If there is drain the gas tank and refill with fresh gas. Dont forget to premix your first tank of fuel after the rebuild.

Thanks!

Cylinder is at the shop, where they are taking off the aluminum transfer and honing it ($10). I do want to get it back together for the weekend, so won't split the case. However, this sled has such low miles on it (under 2,000) that I may go ahead and get a complete bearing/gasket/seal kit and tear it all the way down this summer. If I can't get parts (piston needle bearing and cylinder base gasket) in time for getting it back together in time for the weekend, I'll hold off and do the full teardown.

Thanks for the tip on premixing the first tank.

[Steve Foss]

Hmmmm, no water in the fuel from either carb bowl. Could that mean eating snow through the intake was what put the water in the cylinder?

[leech~~]

If you can't find what the issue was I would at least try and run with a cooler plug and richen the carbs a bit. A fouled plug is a lot cheaper to replace then a piston!

[shortfatguy]

Snow in the intake can cause an issue. Not sure without seeing it but that is a sure possibility. I would not worry about a cooler plug if it ran 2000 miles on those plugs with out issue.

[shortfatguy]

I would however look into getting a better filter material on the intake if it is sucking that much snow into it.

[Steve Foss]

To be clear, when it happened, I had spent more than half an hour bucking really big drifts under pretty heavy load. Was running at 6,000 rpms to get 20 mph, and snow was blasting over the whole sled almost the whole time. I've had this sled for 3 years and have occasionally done some drift busting, but never for this long or under such loads.

Wish I had it to do over again. There was no urgent need for me to do it. Was just out having fun.

Anyway, the new piston is on. Wrist pin went in smooth as silk, and circlip seated on the second try. Got gasket material of the right thickness/conformation and cut out a new cylinder base gasket (couldn't find a base gasket for love or money around here on a sled this old). Carbs are cleaned, surfaces are prepped for gasket sealer. Cylinder is honed and cross-hatched and good to go for standard piston.

Mechanic buddy and I will be working on it tomorrow afternoon.

[Jim Uran]

Hey maybe you can pull my fat a$$ out on the lake Saturday lol

[leech~~]

Maybe you can read this post to your Mechanic buddy to see if any valid points were given? Are at least he may have a good laugh!

[Walleyehooker]

I had spent more than half an hour bucking really big drifts under pretty heavy load. Was running at 6,000 rpms to get 20 mph

I think this pretty much sums it up.

[MN Mike]

Do you know what temp range your sled is jetted for? Could be a factor in your melt down, if it's running too fat, they'll burn down in a hurry too.

I've replaced pistons before because of snow ingestion, it will leave tell tale signs similar to the picture below, did you look at both pistons? ..... sorry didn't read the whole thread.

Mike

[shortfatguy]

Do you know what temp range your sled is jetted for? Could be a factor in your melt down, if it's running too fat, they'll burn down in a hurry too.

I've replaced pistons before because of snow ingestion, it will leave tell tale signs similar to the picture below, did you look at both pistons? ..... sorry didn't read the whole thread.

Mike

Mike, Actually a lean condition (not enough fuel / too much air) is what causes a burn down. If a sled is jetted for 20 degrees and run at -20 degrees this could be a factor. Running fat ( rich ) will never cause a sled to burn down. It might be boggy or foul plugs but it wont burn down.

[shortfatguy]

The eroding (hole in top of the piston) is caused by a lean condition and detonation. The frozen carburator he suspected would be a prime cause of such damage. If snow was being sucked into the intake and down to the carb it would melt and then freeze in the carb blocking the flow of fuel to that cyl. Air would continue to flow at the same rate and the fuel to air ratio would be skewed towards the air side causing a lean condition resulting detonation as well as a high piston temp and lack of lubrication causing the failure.

[Steve Foss]

The other piston and cylinder are in very good condition, with just a little carbon buildup on the piston crown.

I found torque specs online. Cylinder nuts = 24-28 ft lbs. Head = 18-19.

Sound about right?

[shortfatguy]

Those torque numbers are correct Steve.

[Steve Foss]

Everything went together well.

Broke one ring, but had two replacements. Cylinder cleaned up very nicely, and the piston/wrist pin/clips a buddy sent were clean and good. Took a bit over an hour to get everything buttoned up. It helped to have a GM mechanic buddy along for the assist. He's interested in learning 2-strokes/snowmobiles, and lots of these jobs go better with two working.

It started and ran fine, or nearly fine. Would bog sometimes (not always) around 4,000 rpms, and would need to come back down to idle and feather the throttle to get it back up to 5,000 or 6,000.

Based on my chainsaw wrenching, that made me think I had a lean condition. When saws bog that way on rev, it's generally a sign the low end idle mix screw is too lean, and riching it out allows smooth and powerful acceleration up to the high end. You need fuel to make power.

At high idle, I sprayed carb cleaner spray around the PTO side to check for a bad seal, and hit the carb boots as well. No change to RPM in either case, so I reckon it's carb related.

But based on the tough snow conditions right now, and since there's only a few days left of laker season, I've decided to leave it parked, get a complete kit of gaskets, bearings, seals and carb kits, and do a full teardown this summer.

While a '92 is considered an old sled now, it's still only got about 2,000 miles on it, and for the most part they've been light miles. So it seems more economical (and fun) to me to keep it. The track is still in very good shape, and all other systems are operational and strong. Still looks good cosmetically, too.

I've done a lot of light/medium mechanic work on my vehicles over the years, and have done some similar work on outboards and chainsaws, so it'll be fun to get into a new type of wrenching this summer. The piston replacement was a nice confidence builder. Got a shop manual downloaded.

Would Dennis Kirk be my best bet for gaskets/seals/bearings?

[Surface Tension]

With the hole in the piston you have to assume it was caused by a lean condition.

The carbs should be cleaned without question. Easy to do as well.

The bog, again carbs and specifically a dirty high speed jet which is the most common reason for this type of failure. Improper fuel level(low)in the bowl will create the bog as well. Could be from a weak fuel pump caused by old diaphragms, air leak in the impulse line from crankcase to fuel pump from a crack or as simple as a loose connection.

[Macgyver55]

A bog can be caused by a main jet, needle jet and even in part by a plugged pilot jet. But, a plugged pilot will usually result in poor idle and/or hard starting. The three circuits all contribute fuel for a wide open throttle and if any one of them is plugged or restricted it can result in a bog. The bog will usually occur at the leanest point in the transition from one circuit to the next and mid throttle can often be the leanest part of the RPM range. Another thing to check for is the proper synchronization of the carbs. If one throttle is opening sooner than the other it is also opening further and can also cause a lean and or bog condition because one cylinder is working harder than the other.

Make sure to clean pilot, needle and main jets well, verify the needle clip is in the recommended position and synchronize them properly and you should be good to go!.