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Discussion Starter · #1 ·
How do you perform a compression test on a 2 cylinder?

Should the engine be warmed up, or cold? Does it matter?

When its a 2 or 3 cylinder, should spark plugs be in the cylinders not being tested, or should they be removed?
 

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spark plugs removed. three pulls. I do it when the engine is cold, i don't know if it matters or not
 

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you should do it with the engine warm, but if it wont run for whatever reason then cold will give you a good enough reading. warm will generally give you a little higher reading then cold. pull until it stops going up, shouldnt take more then 4-5 pulls usually. some people will say to hold the throttle wide open, not sure of the reasoning behind that but i never do. 120 psi or better would be what you want, anything less then 100 and its time to start thinking about a rebuild..all cylinders should be within 10 psi of eachother..
 

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tshack420 said:
you should do it with the engine warm, but if it wont run for whatever reason then cold will give you a good enough reading. warm will generally give you a little higher reading then cold. pull until it stops going up, shouldnt take more then 4-5 pulls usually. some people will say to hold the throttle wide open, not sure of the reasoning behind that but i never do.[/color=red] 120 psi or better would be what you want, anything less then 100 and its time to start thinking about a rebuild..all cylinders should be within 10 psi of eachother..
You hold the throttle wide open so the most amount of air get into the cylinder instead of leaving the carb slide down and choking off the air flow.[thumb]
And if you have more than 10psi difference between the two then you need to look into rebuilding the cylinder that is low.
And take all the plugs out cause it will make it easier to pull over.
 

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awesome, i was just going to check mine and was wondering about the throttle open thing too. thanks
 

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i believe your engine shows lower compression when its been running a little warm. my buds 800 rev showed 90 psi on each, then we let it cool down and she showed 110. that was still low
 

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Engine warm, all plugs out, throttle wide open, pull till gauge stops(4-5 times is good) more than 10% difference between cylinders is reason to check into.
 

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^^^ What he said...

Pull the rope till the gauge stops moving up.

Oh, and to put the question to bed about whether or not to hold the throttle...

YES!!!! You want the throttle wide open. I tested this myself a couple weeks ago, and had large difference with the throttle open vs closed.. (higher being open obviously)

Just for grins on my 800, I checked by hand, by starter, and with the throttle closed/open:
By hand throttle closed = 130psi
By hand throttle open = 140psi
By electric starter throttle closed = 140psi
By electric starter throttle open = 150psi

Case closed...
 

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Ugly_old_Poo_kid said:
^^^ What he said...

Pull the rope till the gauge stops moving up.

Oh, and to put the question to bed about whether or not to hold the throttle...

YES!!!! You want the throttle wide open. I tested this myself a couple weeks ago, and had large difference with the throttle open vs closed.. (higher being open obviously)

Just for grins on my 800, I checked by hand, by starter, and with the throttle closed/open:
By hand throttle closed = 130psi
By hand throttle open = 140psi
By electric starter throttle closed = 140psi
By electric starter throttle open = 150psi

Case closed...
good to know.
so is the compression generally higher or lower when warm?
 

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why warm ive herd soo many diffrent opinions on this..... im curious?!
 

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i always make sure that the cyliders are wet with wd40 or other lubricants.
 

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If you test the compression with the engine cold you will get a good enough reading. If you test it warm it should be about the same. That is the running compression. Most motors that have a hard time starting when they are warm is because the compression is too low. Worn rings is the main cause of this. I got this information from the Chilton's repair manual for polaris snowmobiles. And yes throttle should be wide open and the other cylinders should also be open.(no plugs)
 

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To get a true representation of how the engine is in running condition, you want to test it warm, and clean.. No added oils, etc. Yes, you get higher readings if you add oil, but you're fooling yourself if you think you're getting an accurate reading.

Why?

When you're actually running the engine, it doesn't have that extra oil, and it is hot. Also when you run the engine for performance, you run it with the throttle open, not closed. That is how you want to test it.
 

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ferntfernt said:
i always make sure that the cyliders are wet with wd40 or other lubricants.
NEVER spray wd40 into your cylinders! what ever you do. It is not a lubricant by any means.
 

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wow so many opinions on this i might as well give mine.
yes plugs out throttle open, throttle open alows most air to come in quickly, wont get a higher reading by not opening throttle will just take more pulls.
but yes throttle open.
i say go with cold engine first, get reading if it seams low, add shot of oil to each cylinder, this allows rings to seal better, if it changes reading dramatically ring or other top end problem.
as for the wd-40 i personally and i said personally feel its fine, its a water displacer, with lil lubrication factor but will not harm an engine, i often us it on parts that will be sitting around awhile.
 

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bigwedgies said:
wow so many opinions on this i might as well give mine.
yes plugs out throttle open, throttle open alows most air to come in quickly, wont get a higher reading by not opening throttle will just take more pulls.
I'll have to correct your opinion...

When I was turning over with the starter, it stopped at 140 and hovered, and would not climb any more. (not even a little bit) While still turning the motor over, I opened the throttle, and it shot up to 150.
 

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for one my opinion is my opinion,no one will correct it but me, and as i said, this was MY OPNION!! i have personally never seen the difference that you have this is why this was MY OPINION not stated as fact, but let me ask this
why would you get 2 different readings one by hand one by elec starter??
isnt your motor the same motor no matter how its turned over?
this is also why i stated that there are so many opinions on here let me throw mine in.
you have hundreds of thousands of engineers in this world all with great ideas on how to build the marvels of the world, now theyll each have their own way to get the whatever built and as none will be wrong, they can be different in many ways.
 

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Why the difference in PSI turning by hand or by starter?

Simple, physics...

When you turn by hand, you can only turn the engine so fast. (as fast as you can pull)

When the starter turns over the engine, it turns it over at a higher rate and constant speed.

How does this effect the compression test?

Ports...

When you're turning over by hand, the compressed air from the bottom of the crankcase is released into the cylinder, and because you can only pull it over so fast, most escapes out the exhaust. And since there's not nearly enough speed/pressure it does not get crammed back by the pipe, like a running engine would.

When you're cranking by starter, not as much air escapes out the exhaust port because the piston closes off the port faster while the cylinder still contains more of the compressed air from the bottom.

This is why when you test a piston-port engine with a broken piston skirt, you will not see a difference in compression. You're turning over slowly enough that the compressed air is escaping, and all that's captured for the test is straight piston compression.
 

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An interesting test, would be for someone with a piston port motor with a broken skirt, to test the compression while the engine is running.. I suspect that the difference in compression would be really noticeable then..
 
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