Snowmobile Forum banner

90 Compression on broke in rebuild

2003 Views 42 Replies 8 Participants Last post by  kccats
Hi. I need some help. I have a 2002 Polaris trail RMK 550. The manual says spec compression is 120 psi. Its fan cooled. I did a rebuild, new crank, new pistons, new rings. I gapped the rings and the pistons were the right size. The cylinders were honed. I reassembled used oil and broke it in but I am only getting 90 psi compression. I have used 3 different testers and tried them on another older sled that read 110 psi. The sled seems to run ok, start ok, except when it is warm and I stop for a few minutes and just let the sled idle and then when I push the throttle to go it bogs down and dies. I have to let it sit a few minutes and then pull it several times before it will start. Is this because I have added oil to the fuel for the break in or is it something else? Also should I have a good shop machine down the head cover or should I have them machine down the cylinder housing? Is it better to do the head cover or the cylinder housing? Am I stupid to do either one? Thanks for any help.
1 - 20 of 43 Posts
Did you clean up the cylinders themselves? If not that could be a reason why you are showing such low PSI. Cylinders need to be cleaned up via honing if possible, re-plated or oversized to remove any damage that may have been in there. If you just left them alone while doing all that other stuff then you are not sealing correctly.
They were honed by a professional and then I washed them with soap and water and then I applied a light oil film layer.
I didn't think those Liberty engines could be honed. Thought they needed to be re-plated or bored out. Dan would know for sure. You say professional but who what company? When doing rebuilds if possible you send the pistons, rings and cylinders to a place like Indy Specialties or US Chrome so they can all be matched up.

Also still this thread going as well on this subject. Do a leak down test as they said. Do what KC said as well. Too thick of gaskets. Could be anything.

You can hone nikasil but all it does is remove any glazing on the cylinders. Unless using diamond hones, the hone won't cut the nikasil. It's a worth while thing to do just to get rid of the glazing and help re-establish the cross hatch.

How many miles since rebuild? It may take a couple hundred before the rings seat.

Are you using synthetic oil? If so, switch to regular oil for the break in. Sometimes synthetic oil is too slick for the rings to break in properly. Some times they never break in if using synthetic.
When you tested did you hold throttle wide open?
Did you pull it 5 -6 times real hard?

if your cylinders are egg shaped you wont get better compression
I did a leak down with smoke and I think it held well enough. No leaks from the cylinder housing or head. A little leak from hose going into the crankcase. I got maybe 50 miles on it since the rebuild. Two tanks of fuel. I used regular oil. The company was Utah Crank Shaft and Short Block in Springville Utah. The gasket kit was an aftermarket kit. Hard to say if the gaskets are thicker or not. Yes I held the throttle wide open and pulled hard 10 times. I had the other spark plug out. Any thoughts on why it would die when I had been running it and then let it idle for a minute and then give it the throttle again? No one has said anything about machining the head or cylinder housing so I am guessing that is a bad idea. Any thoughts on that?
Machining those parts is not reversible, so I would look at everything else first. Were the pistons factory? Determining which to machine, the head or the cylinder, will depend partly on if the piston is flush with the cylinder at top dead center. If it is flush, you won't want to machine the cylinder, as that could raise the piston in the bore enough to impact the head. The machine shop should be able to guide you. Going to a thinner gasket would be the easiest thing to try, and probably cheapest. I like cheap and easy ;)

I had 4 Polaris Edge sleds, every one of them was hard to start when hot. I had to hold the throttle open about half while pulling the rope to get 'em going most times. That kind of became the norm for starting procedure for me. I feel that when the engine is hot, the gas in the carbs vaporizes and flows into the crankcase, causing an overly rich mixture on starting. Opening the throttle seemed to help that a lot on my sleds.

I would go through the carbs and make sure things are 100% there just to make sure. Pilot jets and the air and fuel passages to the pilot jets are really prone to getting gummed up, which can also cause starting problems.
See less See more
Thanks for the insight. I am not sure how to do this but I will look for thinner gaskets. Is there a particular brand that is thinner than others that you know of? I have cleaned the carbs with carb cleaner and blown out the ports with air. I should be good there. Thanks again.
I've never had a 550 and never worked on one. I know some guys that have wanted greater compression (other engines) and had multi-layer gaskets and just peeled them apart and used one layer of the gasket. I even know one guy that did not use a gasket at all and that engine worked fine. I'm not an advocate of that at all though.

On some of my engines that I had to rebuild, there were steel base gaskets that were thinner than the OEM ones, you would have to buy both and measure. If your machine shop has done some of these engines, they may be able to help you.
I have turned heads to raise compression.
But what I am getting at is if the cylinders are not round the wont seal.
You could take .030 off the heads and may get compression to 110?
But if you bore and oversize pistons and get compression to 120,
Then you can cut heads and go towards 150!
So to check for out of round do you just take a series a measurements with a micrometer or is that not good enough. So bore and oversize would be better than trimming the head. Thanks for all this.
You need to use a specific tool to measure the cylinder. It's called a bore gauge. They usually come as a set and can cost from around 50 bucks to well over $200. The more expensive the set, the better they seem to work, to a point.

Take the measurement with the gauge in line with the connecting rods. Measure at least three places in the bore; top, mid, bottom, and then measure three more times at 90 degrees from the first measurement. That will give you an idea of whether the cylinder is round, tapered, or oval.

Your machine shop should have checked for out of round when they worked on your engine, as it's a pretty common measurement when seeing what is necssary for a rebuild. If you do go over-bore, you will need to send the cylinders off, as most shops won't be able to bore the nikasil and re-plate it .Nikasil is a plating and is very hard. The plating is not very thick, and if it is bored out, it exposes only aluminum underneath, which cannot hold up to the combustion process. Millenium Technologies and US Chrome are two places that have good reputations for doing good work, replating and repair are their specialties. Usually, the cost is about half the cost of a new cylinder.

If your cylinders are out of round, you can also send them off and have them bored back to round and put back to the OEM size by those companies. Send them the pistons/rings and they will set up the piston to the cylinder which saves you a step.
See less See more
Personally to me it sounds like the cylinders are out of round. I would do what me and dan suggested send them in to have them re-done along with the pistons/rings so they can match it all up and find a good gasket set as well to finish it all out.
We still don't know the mileage on those cylinders or the piston clearance. We all suspect the worst; high miles and worn cylinders. You're supposed to sell the sled after a budget rebuild.
Hang on lets not scare him.
That led as is will likely run just fine!
The easy way to tell is to push a ring down about an inch with the bottom of piston.
This will get it square to bore, then simply shine a flash light and look and see if there is any gaps?
You could also measure the ring gap, push the ring to the center and measure, push to the bottom and measure. That would tell you if there's any taper.
Thanks so much everyone. I will put the rings in and slide them up and down and look for light like suggested. I will check ring gap and top middle and bottom. These are great ideas. Thanks so much.
It won't be perfect, but can give you a clue as to what you may be dealing with... good luck!

(Although, I would have used this as an opportunity to buy more tools ;) )
1 - 20 of 43 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.