[Rocketman]

When you guys are talking compression, are you talking a warm engine or a cold engine? What's the best way to do a compression test?

 

[Steimy]

You buy a gauge that screws in where your spark plugs screw in, (There are also other types) and pull your pull start and it will give you a compression reading.

 

[Rocketman]

No No...I knew HOW to do the test....but do you do it with the engine warm or cold? airbox on or off?

 

[mxz7]

It Doesn't matter if the engine is hot or cold.No reason to take out the airbox or hold the throttle open either.Just put your gauge on and start jerkin the thing over.

 

[Steimy]

Sorry, didn't understand your Ques. right, mxz7 took care of you though.

 

[kid rocker]

Here is the way I do a compression test.  Leave the airbox on,  kill switch off, wide open throttle, pull five times.   And I do it "cold".  "Warm" will give you a "higher" reading.

 

[FINNBEAR]

Do it with the engine warm, remove the plugs, place back in the spark plug wire caps and ground them, wire tie the throttle wide open, make sure the kill switch is off, pull on the rope hard and as many times as it takes until the gage will not increase in pounds any more - look up the spec for that engine what the difference between the two  or three cylinders should be.
You could do it cold but you will not get as good reading as warm.

 

[Zagg]

Seems to me doing it hot would be better. I mean when its running is when the compression matters and its going to be hot then  

Zagg

 

[Machzzzz1]

I like to warm up the engine.  That way you get oil on the rings and i think a more accurate.  

Doing it like that with wide open throttle should give the best results.  But theres nothing wrong with doing it cold.

 

[lil indy 500]

Rocketman,
When you come out for the break we will check the compression.  I think kid rocker does it right.  

 
              lil indy 500

 

[Dooer]

you will get a higher reading when cold. always warm engine up and give it 5-6 pulls. Readings should be within 10% of each other.

 

[midnight screamer]

Looks like this is well taken care of!

 

[idooski]

There's no need to put the plugs back into the caps and ground them. You have the kill switch off. There will be no spark to the plugs. I like doing the test cold for one reason. Consistency. If you do it warm, just how warm was it? Can you get it to exactly the same temp next time you want to do a check to compare to the first one? Same goes for the # of pulls. It's all about being able to get a consistent accurate reading. If you just want to take a quick reading to see if the compression is OK, then it really doesn't matter how you do it as long as you do it the same for all cylinders.

 

[kid rocker]

You may do a compression test warm, hot, or cold.   Your most accurate test is cold.  Just like Idooski says, you will probably never get all the  cylinders the same temperature.  A warm engine will definately read a higher compression number than a cold engine.  So the most accurate test is cold.  If you want to see if the rings are bad after a low reading, add some oil in the cylinders and if it raises the compression, your rings are probably o.k. Most engines should have at least 65 lbs. or more.  If the reading is less than 40% of the engine rated compression, the engine may need new rings.  (Note, if the compression is low it would be advisable to squirt some oil through the spark plug hole and turn the engine over a few times by hand, and then check with compression guage. If it does not improve, it is a sure sign for new rings.) This is right from the instruction sheet from my compression guage.  It also says to do a compression test cold.  My sled had 65#'s on one side and 72#'s, and the sled still started and ran.  This is a great site   for accurate information, just trying to be accurate, and help.  Piece out.  

 

[ballsout1]

Allright rocker dude, if the comp goes up after the oil it means the oil is taking up the seal  in the rings and the rings are bad. Also I`m not entirley sure what this means but I had a 670 Mxz  that didnt exhibit anythinng weird as far as running (it actually ran real good) and it had 180 cold and 165 hot, doo you have any  speculation as to what  is going on there? Also whats up in your post about 65lbs??? is that for one pull?? Maybe i miss  read your post.  I`m going to check the comp on my 700 again it pulled down 160 to 165 cold, now I`ll check it hot also, for some reason all these years I thought that comp  READING drops hot because the air density drops inside the cylinder at warmer temps but rinng and or cylinder sealing will ACTUALLY increase as would be indicated on a leak down test. i apologize in advance if  I `m reading to much into this or like I said maybe I misread the post. I better go peace out now....

 

[kid rocker]

ballsout1, I did indeed have 65#'s on one and 72#'s on the other after 5 pulls.  Yes, the rings were bad and so were the pistons.   I couldn't believe the stupid thing still ran.  Yes on mine it reads higher warm than cold.  Go figure.   However, you are right about air density.  Remember that is probably how you and I  jet our sleds rather than temperature.  I sure would like to see that head design on your 700, it has me really thinking.  Piece out.

 

[670HOTH]

I just checked my 99 MXZ 670HO.  I have a new, high quality gauge and I pulled it as hard as I could 5 or 6 times with the throttle wide open.  The engine was semi-hot.  I got 150 mag and 149 pto.  Just for reference!

 

[mr670]

The best way to check a engine's condition is with a cly leak down tester.  The best diognostic tool  you could have in your tool box. Summit racing sells one for $70.00, Snap-on also has a nice one, for way more $$$.