24 replies to this topic

[670blues]

Hello ski doo experts of the world,
I have a '99 summit x with 1-2k miles on a used but rebuilt (top-end, so I was told) 670 from a 97 or 98 summit. It's gotten progressively harder to start over the last 2-3 seasons. It goes from too lean to flooded without really firing. Worse when its cold. It also started bogging at WOT. I cleaned and went over the carbs, and they seemed to check out. Currently not getting spark but that could be due to fouled plugs. The compression is just over 100 and fairly even between the 2 cylinders. Based on all this and knowledge gained from this forum, I am rebuilding the top end. Totally happy with stock performance just want it to start reliably. Cracked it open tonight and the cylinder bores look clean, no visible lips/steps. I will attach some photos of these. My question is what do I need to replace for a top-end rebuild. Seems like I could get away with a honing of the cylinders, new rings and gaskets. Can I hone it myself or is it better to have a machine shop do this? Any recommendations of shops in Missoula Montana? Do I need new pistons? New needle bearings? Any tools beyond a torque wrench and a ring compressor? Any important tricks to rebuilding these things?
Thanks in advance!

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[Lloydguy]

There was no blow by,from the looks of the pic's.Take the ring off of the pto side
piston and place it 1/2" from the top of the pto side jug and use a feeler gauge to
measure the amount of end gap.Then comapre that to factory specs.Then do the same to
the other ring and jug.DO 1 AT A TIME AND PUT PARTS BACK WHERE THEY CAME FROM.
Torque wrench,feeler gauge and mocrometer is about all you need.
When you were checking the compression were you holding the throttle wide open?

[670blues]

2 more photos:
will check the feeler gauge tomorrow

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[670blues]

OK, thanks for the info Lloydguy! I measured compression twice and I think the first time we had the throttle open and got ~110. The second time was with a different gauge and I did not have the throttle open.

Ring end gap is between 0.011 and 0.015 for each of the 4 rings, which is well below the wear limit of 0.039. Ring/groove clearance was also within spec. Pistons have 0.50 stamped on them and measured 78.35mm wide, but that was with a crappy micrometer that may not have been deep enough to get a proper measurement. Stock bore is 78mm, so I'm guessing that means my pistons are 0.50mm over? Huh, was guessing things would have been way more out-of-whack given the low compression reading. What do I do now?

[sledsrockcolorado]

Post a picture of the head, combustion chamber side showing. My first thought is a std 670 head with HO pistons = low compression.

Also - how many miles on this rebuild? Sure looks very minimal.

[670blues]

Here is a photo of the head. 1-2k miles on the rebuilt motor.

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[sledsrockcolorado]

Here is a photo of the head. 1-2k miles on the rebuilt motor.

That is a std. 670 head. I can not tell from the pictures, are the pistons a flat top or is there a slight dome to them? The std. 670 piston will be a dome piston and the HO/X will be a true flat top. Those pistons/cylinders look very clean for that many miles.

Might need to check the squish next.

[meandmyz]

Post a picture of the head, combustion chamber side showing. My first thought is a std 670 head with HO pistons = low compression.

Also - how many miles on this rebuild? Sure looks very minimal.

x2 and that is a non ho head...i don't know for sure how to verify the pistons thought...maybe someone else could verify those from the pics....but definitely a non ho head...only one sensor too...the pistons kinda look like there is a dome and the are non ho pistons too...maybe a carb jetting issue setup for ho...not too sure if the jetting is different too on those 44's...but could just be a jetting issue and thats why the plugs are also pooched...too much go go juice!

[670blues]

The motor should be non-ho as it's from a 97 or 98 non-ho summit. My carbs, ignition and HAC are also from the 98. Went over the carbs pretty carefully and set them to factory specs. Does seem to run a bit rich. It may also have electrical problems beyond fouled spark plugs, but I think I need to deal with the top end before I get into that.

So if the ring gap is to spec and everything looks good, is 110psi compression an issue?

[sledsrockcolorado]

The motor should be non-ho as it's from a 97 or 98 non-ho summit. My carbs, ignition and HAC are also from the 98. Went over the carbs pretty carefully and set them to factory specs. Does seem to run a bit rich. It may also have electrical problems beyond fouled spark plugs, but I think I need to deal with the top end before I get into that.

So if the ring gap is to spec and everything looks good, is 110psi compression an issue?

A couple questions:
1. What is the elevation you are at? (where you checked compression).
2. What thickness is the base gasket? (how many small indicator holes 4,5,6,7,8)

[670blues]

I'm at 3200'. Mostly riding at 7000'. Couldn't find any indicator holes in the base gasket, but its not particularly thick or anything. Some pictures below.

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[sledsrockcolorado]

Hmmmm......you should have higher compression. I am at 4850ft and would pull 150psi. BUT my head was shaved, ALTHOUGH I did have extensive porting (raised exhaust ports). Are the rings free in the piston ring grooves? Your pistons do look right as well (slight dome, std. 670 piston). This is interesting.

[670blues]

The rings are free and the ring/groove clearance checked out. I guess the next thing would be to check the piston/cylinder wall clearance, but its starting to look like I cracked open a perfectly good top end. I'll have to get it back together before I can look at any other issues. Can I use the existing parts and some new gaskets?

[Lloydguy]

If your plugs are over gapped,that could have been part of your hard starting problems all along.If you didnt check the compression with the throttle wide open,and a decent gauge
your compression numbers could have been much higher,but now that the topend is opened up
it's too late to geuss.

[670blues]

Ah well, next time I do a top end it will be that much easier! Looks like I'll have to slap it back together and run the electrical diagnostics. Thanks for the help!