Below are threads showing only three 850 owners who have taken a compression measurement and bothered to post on DooTalk in the past 4 years.

Compression
Started By JJJR, Nov 11 2019 04:01 PM

What is the proper compression on an 850?
Started By Crazy_Canuck, Feb 03 2020 09:52 PM

 

I just had a 2018 850 XRS in my garage. He was worried because his last ride he was running down a long rail bed at high speeds and the sled coughed then quit. He said he started it back up but ran on one cylinder so he shut off. After 10 min he tried to start again and it ran so he loaded up and went home. I asked around for base line compression numbers and never got a feel for " normal" . So I did the test with known good gauge (it is a pain in the "edit for bad language" with the airbox in sled). The PTO was 145 and the Mag was 115. I would think 145 is close to " normal".

 

With no pull start. How do you crank over to test. Ussually i would pull with kill switch off.

About to do plugs and might as well check comp as well

I pulled jackshaft so hope it gives enough room

 

It's hard not to be a smart aleck; but how would you run a compression test on a car without starter rope or crank??? I have no problem using the starter button and motor on my sleds. Just ground the plugs with the wires hooked to them, is probably the easiest way. You don't have to follow these guys that say the only way to do it is with a rope starter.

One suggestion I would have... run it as is and if you suspect the readings at all.. squirt a little oil in each cylinder and lay a rag over the spark plug holes and crank it over a few times to distribute the oil and rerun the test. That will tell you how your ring seal is. Providing you are not missing a piece of ring or a have a hole in the piston/or burnt piston edge, the oiil will seal the rings and give you the reading of what a newly sealed cylinder and ring would be. And you can compare your original reading to that. If you don't have a baseline when the sled was new.

A Doodoctor floater makes this an easy task, secondary comes off easy without pulling the shaft. This is the only picture I have showing the floater off the Gen 4 and this was back before all the junk was removed from around the engine. One of Brian's first Gen 4 floaters.

 

So compression tester in cylinder, then leave the plug and wire in the other. The side you're testing, ground the plug? 4hen bump over kill switch off? I was sure I pooched the clutch side cylinder as it died on me. Chugged to start, grabs reverse only every 10 times maybe, otherwise stalls and spits. Compression tested each side is around 120 to my surprise. Maybe as simple as a bad plug? What else could cause such a concern?

 

It's hard not to be a smart aleck; but how would you run a compression test on a car without starter rope or crank??? I have no problem using the starter button and motor on my sleds. Just ground the plugs with the wires hooked to them, is probably the easiest way. You don't have to follow these guys that say the only way to do it is with a rope starter.

One suggestion I would have... run it as is and if you suspect the readings at all.. squirt a little oil in each cylinder and lay a rag over the spark plug holes and crank it over a few times to distribute the oil and rerun the test. That will tell you how your ring seal is. Providing you are not missing a piece of ring or a have a hole in the piston/or burnt piston edge, the oiil will seal the rings and give you the reading of what a newly sealed cylinder and ring would be. And you can compare your original reading to that. If you don't have a baseline when the sled was new.

A Doodoctor floater makes this an easy task, secondary comes off easy without pulling the shaft. This is the only picture I have showing the floater off the Gen 4 and this was back before all the junk was removed from around the engine. One of Brian's first Gen 4 floaters.

That Doo Dr setup is so slick. Is it heavier then stock? It looks like your primary is slipping a bit in over drive? What gears are you running?

 

So what is the correct procedure using the electric start? I’m pulling head and would like to check compression before and after. Thanks

 

I pull the jack shaft for convenience, I always have a pull start cord installed on my sleds. Not going to let an e start problem wreck a weekend trip.
Pull both plugs and ground each electrode to a head bolt or such for e start only. Alligator clips and wire work good or an sp125 spark tester. At that point crank it over equally per cylinder and compare results. Use wot to shut fuel off while cranking.They need to be within 10 percent of each other. Double check if they differ to much.

 

Also pull the y pipe if concerns or unequal readings first, normally you can see most damages from that view.
And don't be a tite wade and buy a decent compression tester, I've seen cheap ones cause false reading and unneeded tear downs because the schrader valve was junk.

 

because the schrader valve was junk.

And FWIW.... if you replace the Schraeder valve on a compression gague, it takes a special one. Standard ones for tires are not designed for the high pressures.

 

Performance 2 strokes are all around 140-150 psi. If both sides are within 5lbs of each other all is probably well. 115/145 is not good

 

Yep, at 170 you better be packing c12