compression test

[maxwell]

i was going ot look at a snowmobile this weekend, 2002 summit 800 with 2500km, the machine is about 2 hours from my house so i asked for compression numbers over the phone, the guy said they were both at 95psi. which is not good at all for a sled with 2500km, but he also said the sled had sat for over a year, so my question is if i go to look at this thing and i put some oil in the clys and let it run for a couple minuts and get the oil injection working will it see higher numbers, maye the cylinder walls are just so dry?

 

[Rucky]

i was going ot look at a snowmobile this weekend, 2002 summit 800 with 2500km, the machine is about 2 hours from my house so i asked for compression numbers over the phone, the guy said they were both at 95psi. which is not good at all for a sled with 2500km, but he also said the sled had sat for over a year, so my question is if i go to look at this thing and i put some oil in the clys and let it run for a couple minuts and get the oil injection working will it see higher numbers, maye the cylinder walls are just so dry?

That very well could be the case, but this sled doesnt sound too good...i couldnt see you gaining more than 20 psi by doing what you stated...so 115 is still pretty low...then again i could be completely wrong!

 

[new24stroke]

The thing with doo's are that the rings start to flake and you will loose compression.It is probably in need of new rings.Ski-doo had to go to cromoly rings.Also doo only runs a single ring.When I redid my 800 ho I went with wiesco piston.If you can get it cheap my be then it would be worth buying.

 

[Summitric]

Lots To Consider: 1) Is The Compression Guage He's Using Accurate, Or Old As The Hills, Sitting In A Drawer Rarely Used. 2) Is The Schraeder Valve Inside The Compression Guage Sealing Properly. 3) Possibly True About The Oil And Running It For A While. 4) Did He Pull The Engine Over With The Throttle Wide Open(doesn't Sound Like It, So In This Case The Compression Will Be Considerably Lower. 5) Did He Doo The Compression Test With Both Spark Plugs Out(quicker Pull Over, Of Course). 6) Old Broken Down Gas(not Fresh) And If He Ran It And It Mixed With Last Years Oil, Will Be Considerably Depleted Of Natural Sealing Properties, And Result In Lower Compression Readings. 7) How Many Times Did He Pull It Over To Attain The Compression Readings. And There Are Many Other Factors To Consider.
I Personally Would Be More Concerned That The Compression Was Over 100psi And That Both Cylinders Are Very Close In Compression To Each Other(which They Are).... Normally 1 Cylinder Will Go Before The Other, If Theres A Problem In The Engine, And Its Usually The Pto Side Cylinder That Will Go, Due To The Heat From The Clutches Transferred Through The Crankshaft.
2500kms Isn't Much For Todays Sleds, But Then Again, We've Seen Stranger Things, Right??? Good Luck And Let Us Know The Results.

 

[maxwell]

yah, well its such a good deal im hoping to get the rest of the season out of it, because i rebuilt my 600 doo with wisecos and never looked back, still seing 130lbs after 3000kms, they were still only single ring pistons. though.

 

[maxwell]

Lots To Consider: 1) Is The Compression Guage He's Using Accurate, Or Old As The Hills, Sitting In A Drawer Rarely Used. 2) Is The Schraeder Valve Inside The Compression Guage Sealing Properly. 3) Possibly True About The Oil And Running It For A While. 4) Did He Pull The Engine Over With The Throttle Wide Open(doesn't Sound Like It, So In This Case The Compression Will Be Considerably Lower. 5) Did He Doo The Compression Test With Both Spark Plugs Out(quicker Pull Over, Of Course). 6) Old Broken Down Gas(not Fresh) And If He Ran It And It Mixed With Last Years Oil, Will Be Considerably Depleted Of Natural Sealing Properties, And Result In Lower Compression Readings. 7) How Many Times Did He Pull It Over To Attain The Compression Readings. And There Are Many Other Factors To Consider.
I Personally Would Be More Concerned That The Compression Was Over 100psi And That Both Cylinders Are Very Close In Compression To Each Other(which They Are).... Normally 1 Cylinder Will Go Before The Other, If Theres A Problem In The Engine, And Its Usually The Pto Side Cylinder That Will Go, Due To The Heat From The Clutches Transferred Through The Crankshaft.
2500kms Isn't Much For Todays Sleds, But Then Again, We've Seen Stranger Things, Right??? Good Luck And Let Us Know The Results.

wow thanks for the info, clearly i have been doing it wrong, i always take both plugs out but never hold it wide oopen, either way, i will bring 3 compression testers with me and try some differant things, should the comp test be done on a cold or warm engine?

 

[Summitric]

wow thanks for the info, clearly i have been doing it wrong, i always take both plugs out but never hold it wide oopen, either way, i will bring 3 compression testers with me and try some differant things, should the comp test be done on a cold or warm engine?

THEORETICALLY A WARM ENGINE WILL GIVE A HIGHER COMRESSION READING AS THE PISTONS/RINGS/CYLINDERS EVEN THE OIL/FUEL HAVE EXPANDED DUE TO HEAT. I PERSONALLY LIKE LUKE WARM, TO WARM AS THAT'S HOW WE ALWAYS TEST AUTOMOTIVE ENGINES. MORE THAN ONE TEST IS A GOOD IDEA...... SO BE SURE TO REMOVE BOTH SPARK PLUGS AND HOLD THE THROTTLE WIDE OPEN(BE SURE TO DISCONNECT THE IGNITION COIL, HOWEVER FOR OBVIOUS REASONS). AND PULL THE ENGINE OVER SEVERAL TIMES TO GET YOUR READING.

 

[BIGFOOT]

What Summitric says is correct..... It will take bout 5 pulls to see the correct compression.... You will see the gauge build rate level off.

 

[99summitx]

I just put my 670 back together today, just put new gaskets in and that was it, 6700km on the engine never been opened untill last weekend. I got 120 and 122 on the cold engine and an even 133 when warm. 5 fast pulls, wide open and both plugs out.

 

[glengine]

On the 670 it would be time for new pistons if it were me.. Just as a preventative maintance thing. two pistons are alot cheaper than if a skirt breaks off or something like that.. Did you happen to measure the piston to cyl wall clearance? Or did you measure the ring end gap?( end gap will probably be not to bad)

 

[99summitx]

On the 670 it would be time for new pistons if it were me.. Just as a preventative maintance thing. two pistons are alot cheaper than if a skirt breaks off or something like that.. Did you happen to measure the piston to cyl wall clearance? Or did you measure the ring end gap?( end gap will probably be not to bad)

I have a 1mm overbore kit for it and was going to do it while I had the jugs off but I wouldn't have gotten it done by the Mountain Mania weekend so I just threw it back together and will do a rebuild in the spring.

 

[glengine]

Oh yah.. Glad to hear that some people do think about preventative maintanance... I've always found it to work good for me.. I've never had a toy leave me on the mountain do to a failure...

 

[Als]

Hello;
Here's how I do it.
Yes you want to do the test on a warmed up engine.
This will eliminate false readings due to oiled down cylinders.
Use a good proven gauge.
Have the engine at operating temp.
Plugs removed, but back in the wires and grounded to the engine.
Ignition off.
Throttle wide open.
Pull as hard as you can 5 times.
Take reading.

On good cylinders the reading usually tops out in 2-3 pulls.
watch the gauge as you pull if you can.

In my experience a cold engine will make 3-5 lbs more compression
than a warmed up engine. Some are the same.

I have never seen higher compression on a warmed up engine over the same engine when cold.

The speed that you pull it over at makes a difference.

Again as others have stated use a good gauge. I have seen gauges that would not read over 100 lbs
on a 150 lbs engine.
Good luck.
Alan

 

[99summitx]

That's funny I got a lower reading on a cold engine, maybe it had to do with that it had not been run yet since I put it back together??

 

[Summitric]

That's funny I got a lower reading on a cold engine, maybe it had to do with that it had not been run yet since I put it back together??

YOU WILL ALWAYS GET A SLIGHTLY LOWER READING ON A COLD ENGINE, THAN A WARM ENGINE......... HEAT HELPS EXPAND COMPONENTS AND SEALS BETTER, ESPECIALLY THE PISTON RINGS.... THEY HAVE AN END GAP FOR A REASON, SO YOU CAN'T SQUEEK ENGINES WHEN THE RINGS EXPAND...........

 

[Shredder]

I Personally Would Be More Concerned That The Compression Was Over 100psi And That Both Cylinders Are Very Close In Compression To Each Other(which They Are).... Normally 1 Cylinder Will Go Before The Other, If Theres A Problem In The Engine, And Its Usually The Pto Side Cylinder That Will Go

This is a great piece of advice. There is no set reading to speak of like 125lbs, every engine will be a bit different. I always take a bend-a-light. Pull the plugs, and within a few minutes you can see exactly how that engine was last running.

 

[glengine]

Yah and one thing to keep in mind the 04,05 rev800's had crappy rings from the factory so if comp is down on one of those it may very well just be the rings.... But if a sled is down to like 100psi it is definitely time for a rebuild on that one... Some of the one's now aday's that we are putting heads on they are up all the way to like 160psi.. On a turbo sled though you dont want comp over like 120psi.. That is because the boost makes up for that easy.. the best way to lower comp for a turbo is to get your head cut, but it needs to be done in a certain way for the best results.. But for an average if you have 125 or more you are pretty good.... And the bendy light is a really good point cause you should be able to actually see part of the cyl wall, for and scorring..

 

[OVERKILL 19]

Lots To Consider: 1) Is The Compression Guage He's Using Accurate, Or Old As The Hills, Sitting In A Drawer Rarely Used. 2) Is The Schraeder Valve Inside The Compression Guage Sealing Properly. 3) Possibly True About The Oil And Running It For A While. 4) Did He Pull The Engine Over With The Throttle Wide Open(doesn't Sound Like It, So In This Case The Compression Will Be Considerably Lower. 5) Did He Doo The Compression Test With Both Spark Plugs Out(quicker Pull Over, Of Course). 6) Old Broken Down Gas(not Fresh) And If He Ran It And It Mixed With Last Years Oil, Will Be Considerably Depleted Of Natural Sealing Properties, And Result In Lower Compression Readings. 7) How Many Times Did He Pull It Over To Attain The Compression Readings. And There Are Many Other Factors To Consider.
I Personally Would Be More Concerned That The Compression Was Over 100psi And That Both Cylinders Are Very Close In Compression To Each Other(which They Are).... Normally 1 Cylinder Will Go Before The Other, If Theres A Problem In The Engine, And Its Usually The Pto Side Cylinder That Will Go, Due To The Heat From The Clutches Transferred Through The Crankshaft.
2500kms Isn't Much For Todays Sleds, But Then Again, We've Seen Stranger Things, Right??? Good Luck And Let Us Know The Results.

Curious, What is the reason for "Throttle Wide Open" I did a compresion test both ways, and I was within a pound of one another? Thanks

 

[glengine]

That is so that you get all the air in as possible... With the throttle closed you can get a false reading cause it might not be able to draw in enough air.. It will not always give you a false reading, but it is just kinda a safeguard to be sure that you get an accurate reading..