2007 800R Low Compression After Full Rebuild

RTB · Mar 18, 2014

Alright so i bought a new to me sled (2007 XRS 800R with a 151 track) for 1500 dollars it was so cheap because it had a few problems, but the guy told me he had just completed a full rebuild including crank, spi pistons, and rings. Problem 1 was that the D.E.S.S key was not seating properly. Problem 2 was that the clutching was not perfect. Problem 3 was the track was severely worn. Before buying i tried to ride the sled but could only get it to run for a short amount of time and not very strong because of the D.E.S.S key so i bought the sled and brought it home. Later that day i had the key fixed up so that it would run and took it up and down the road a few times, but the sled still did not have the power that it should. So i went to get my compression checker and checked compression it read 62 lbs identically in both cylinders.. I didnt think that sleds would even run with compression that low?? So my first thought was my compression checker was reading low so i checked my 04 rev 800 H.O. and it pulled 140 which is correct. So i called the guy and told him that it was a done deal anyways and i just needed to know if he really done the rebuild which he still claimed he did and used spi dual ring pistons. I ran the sled for about 6 hours on saturday to see if the rings would seat and compression moved almost to 70 lbs in each cylinder which is not a big difference. So long story short my question is do i tear down and assume that he did not complete the rebuild or perhaps got junk pistons? Or can anything else cause the compression to be that low and still allow the snowmobile to run and idle great but just have a loss of power? I also just cleaned the valves last night and it did not run any better. Thanks in advance.

sledsrock · Mar 18, 2014

Rings are on upside down. Puller down and have a look.

RTB · Mar 20, 2014

Thanks for the reply, I pulled the jugs off and everything looks perfect. Rings are on with writing up and cylinders look really good the sled has 6000 miles on it do you think that the cylinders could be worn down? Also when my 04 rev's compression got low it had a hard time starting and also would die when switched to reverse, but this sled starts first pull every time and also goes into reverse easily. Are there any other factors in the cylinders compression?

RTB · Mar 20, 2014

Another thing is last night before i tore it down i put oil in the spark plug hole and checked compression again and it did not change at all.

whitefish · Mar 20, 2014

Wrong heads installed? Too thick base gasket?

RTB · Mar 20, 2014

I believe they are the stock ski doo heads. I will measure the base gasket after work. Thanks!

RTB · Mar 20, 2014

Any more ideas that i can check out tonight?

whitefish · Mar 20, 2014

Make sure they are the right pistons.

Sling-it · Mar 20, 2014

Once you've had it down that far and not seen the problem, I would start to suspect cracked head, wrong rods, warped heads, pistons in backwards (which way is the arrow pointing?) or something in that line of thinking.

Someone that rebuilds their own motor I don't trust too well. There's exactly 3 mechanics I fully trust to get the job done right, and I'm one of them.

Also check the piston size to bore. I don't know the specs of your motor, but you should be able to find it. Look to see if there is a lip at top of the cylinder. That should give you one clue.

RTB · Mar 20, 2014

The pistons are facing the right way arrow toward exhaust. The ring gap looks good. The gasket is a .8 mm so i think im going to order a .5 mm. Should the piston come all the way to the top of the cylinder ? At top dead center the piston is about 4mm away from the top of the cylinder(4mm of the cylinder wall is still showing)

Sling-it · Mar 20, 2014

I've not seen that engine, so I can't say. Is there a ridge in the top of the bore? Can you see hone marks in the bore still?

"looks ok" is different than is ok. Pull the rings, stick them in the cylinder and check to see if there is about 14 thou or so ring end gap.... I think that's the right measurement.

That in itself isn't really what I would be looking at as much as a large gap indicating he just stuffed new pistons in a worn out bore and called it a top end job (would NOT be the first time I've seen that). If you're not sure, take the pistons and jugs and head to your dealer and ask if they match up. If they can, have them check the head and cylinders for problems too.

I know it's silly, but are the reeds in correctly? I'm assuming it's a reed machine here. Forgive if I'm wrong.

RTB · Mar 20, 2014

There is not a ridge at the top the cylinder is completely smooth. Ring end gap was .10mm which by spi standards is too tight if anything. Right now im leaning toward my piston deck height but im not sure why it would be off like it is. If i were to check the squish The measurement would be over 4mm. I havent really looked at reeds yet but i will tomorrow. Thanks for the replies.

sledsrock · Mar 20, 2014

Squish test

Yes, check the squish using solder, simple to do. 4mm = @.160" that is huge.
Also, .10mm ring gap is way too tight! .1mm = .0039"

Sling-it · Mar 20, 2014

RTB said:
Right now im leaning toward my piston deck height but im not sure why it would be off like it is.

There could be a couple reasons, but it pretty much points to wrong parts across the board. Wrong piston, rod, crank, cylinder, etc. Did the guy say what lead him to a rebuild? Did the motor grenade? I've read those engines were famous for it. If that's the case, who knows what parts he got his hands on as replacements. Time to check some part numbers.

RTB · Mar 21, 2014

I dont even think i could check the squish with a piece of solder haha i know that the squish has got to be way too large for spec. So i called the company who rebuilt the crank and he said that there is only one rod that fits on the 800r crank and that it must be something in the top end. So i will be calling mcb performance to see if i can get some piston measurements. At this point i am sure that my compression problem has to do with the fact that the piston for some reason is not making it to the top of the cylinder.

summit800 · Mar 21, 2014

Did you check to make sure the rings aren't to tight in the grooves of the pistons ? maybe some carbon in the grooves??

sledsrock · Mar 21, 2014

RTB said:
I dont even think i could check the squish with a piece of solder haha i know that the squish has got to be way too large for spec. So i called the company who rebuilt the crank and he said that there is only one rod that fits on the 800r crank and that it must be something in the top end. So i will be calling mcb performance to see if i can get some piston measurements. At this point i am sure that my compression problem has to do with the fact that the piston for some reason is not making it to the top of the cylinder.

Have you pulled the head? If it is the piston, then that would mean the piston must be below the transfers when at BDC.

RTB · Mar 22, 2014

So i measured the wrist pins and they measured 20mm and 800r wrist pins are supposed to measure 22mm so then i measurer the small end of the connecting rod and it measured out to be a rod for an 800 H.O. So at this point i at least have H.O. rods and pistons in an 800r motor...

Sling-it · Mar 22, 2014

Glad to see you got to the bottom of your problem. Hope you find that's the only problems. Make sure to measure so you can get the right pistons, and see if you can find some head measurements to see if it's been planed. You should be up and running before long.

revx827 · Mar 22, 2014

RTB said:
So i measured the wrist pins and they measured 20mm and 800r wrist pins are supposed to measure 22mm so then i measurer the small end of the connecting rod and it measured out to be a rod for an 800 H.O. So at this point i at least have H.O. rods and pistons in an 800r motor...

800r pins are 21mm. Rod end should be 27mm.
If the rods are wrong the crank is wrong.

2007 800R Low Compression After Full Rebuild

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