compression

barkerman77

barkerman77

Member
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Nov 7, 2013
Messages
156
Age
32
Location
Madoc, Ontario, Canada
So I decided to compression test the viper. The sled has 16,000km on it. I believe it's original. I took the power valves out to clean and the pistons looked pretty good still have the machining grooves in them. So the sled was cold when I did it. It's an old tester so I dont really trust it. I got 90, 95, 95 starting with the pto side. That seems kinda low. But the sled runs like a top. I am gonna try it again with a new tester and warm. What you guys think.
 

do it with different tester, and still (ENGINE) cold/ throttle open. than go from there. that is very low. 3:16 (yammie tony)
 
The same thing happened to me with all three of my vipers. I'm at 3600ft elevation and I've always suspected that was the reason why I didn't get 100 + psi. However, with that said the most important thing is the spread between cylinders. Which seems good to me. What elevation are you at?

Another thing to note is that if I remember correctly the PTO cylinder is always slightly low in compression compared to the other two. This is due to it having a slightly larger volume then the other two. I believe this was designed into the engine because the PTO usually runs hotter and they wanted to cool it down while keeping the jetting the same thru out all the carbs. So they increased the volume of that one cylinder which results in a slight drop in compression. This is what I was told a year or two ago if I remember correctly.

If you have the opportunity try a leak-down test if you have access to the equipment. It gives you a more accurate evaluation of your cylinder's health. There's a lot of info about this in the Tech section.
 
barkerman77 said:
I had the carbs off so shouldn't that give wot. I'm nervous about it.
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Yes as far as air flow but fuel/oil will or should bump numbers up a little bit more as the rings seal tighter with it. Whats your elevation? Higher = lower. Have you tried another tester? Compression is only a hint, a leak down test will give you better info. How to is in the tech section.
 
Devilin AblueDress! said:
Yes as far as air flow but fuel/oil will or should bump numbers up a little bit more as the rings seal tighter with it.
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X2

Seems to me if it's running good and the spread is good then I wouldn't worry about it. Do you know if the head gasket is stock? Someone may have put an Opticool gasket in it. If they did be thankful because it provides better cooling to your engine. And from what I saw you'll loose just a little compression with them because they're slightly thicker. I don't remember the measurements but when I was changing to the Opticools I measured them and they were thicker by a little.

If you're still concerned try doing some plug checks. It can't hurt to have some more info to help you figure out what's going on.
 
Try adding a little oil to the cylinders(just a little), if compression #s increese more than 10 psi you have worn rings. I think this trick came out of a old johnson evinrude manual. Don't know how well it works but might as well give it a try and see what happens.
 
I got fairly low numbers with mine too.....100-105-105. Talked to a BRP mechanic and said it was too low then talked to a Yamaha mechanic and said that wasn't bad. Apparently they run lower than most other sleds. Maybe this is why they last so long
 
So it seems to be starting just fine. Then last night I took it out it DIDNT want to stay idle. I had to flip on half choke for a second and it would pick back up. I then took it out for a burn and co e back and its all good now. Maybe just a fuel issue. I just had the carbs off last week so maybe one was sticky or something.
 
don't all these sled run about the same? my buddy used my tester on a srx and it came to 135:dunno:
all a comp test really does is see if you have a bad hole, sounds like a leak down tester is what you really want.
 
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