Checking Compression

[Bernoff]

1 Engine needs to be cold.
2 Remove key from ignition
3 Pull Sparkplugs
4 Install tester in one cylinder
5 Hold Throtle wide open
6 Turn engine over 3 or 4 times
7 Record findings and repeat to other cylinders.

Does that sound right? Seen a guy do it on hot engine and thought it was supposed to be cold.

 

[RXN]

I've always done it on a cold engine myself.

 

[Twig]

1 Engine needs to be cold.
2 Remove key from ignition
3 Pull Sparkplugs
4 Install tester in one cylinder
5 Hold Throtle wide open
6 Turn engine over 3 or 4 times
7 Record findings and repeat to other cylinders.

Does that sound right? Seen a guy do it on hot engine and thought it was supposed to be cold.

bingo sir

 

[Jorgy]

You should do it cold, that's what most guys do, however if you do it hot you should be an increase in compression


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[mtncat1]

compression should be done when hot turn engine over until highest reading is shown

 

[Clode]

yup, warm engine, a 2 stroke with low compression will run cold...warm it up and it won't

 

[Lund]

Compression test on a cold engine HAHAHA....that's the first. HOT, 2stroke, 4stroke, gas. diesel, propane all test are to be conducted HOT(operating temp.).
BTW...compression test on a 2stroke mean's squat. You should be more concern about even compression then how much.
To add to it, to do a compression test when you don't really know how to do a compression test will really mean nothing to you unless you know what to look for in a compression test. Kinda like reading a graph on a guage and understanding what is actually going on.
Then to add to the confusion, there is a wet test and a dry test and both real reveal a different story but again that's great on most engine but a 2stroke...hahaha read above sentences..

 

[kingkris06]

read the service manual . ive had atv's where you need to remove air filter to get proper reading

 

[Lund]

If your wanting to know the condition of your sled motor here is what you need to do.
Get it up to operating temperature. Run a basic compression test, both spark plugs remove and at WOT. What you are looking for is a difference of less then 10psi between cylinders. What the actual # are doesn't mean much at this point. (example PTO side 90psi, MAG side 85psi) is considered OK. (PTO side 90psi, MAG side 125psi) NOT OK.
Carbon build up will effect compression ratio on small engines massively and an 800cc 2stroke is not a big motor, carbon on top of the crown of any 2stroke is very common, thus one reason how much compression is not the end all answer.

Next step is to check the piston skirting for scuffing and blow by. The exhaust side is the proper way to do this. You will need to pull the pipe and i prefer to also remove the Y pipe. Using a light and plugs out, you look at the piston skirting for scaring and the cylinder walls. I use a borescope to help do this.
Next you look at how much blow by is occurring, this is black coloring or residue below the rings. The more blow by the more the rings are not seating or worn.
I say not seating or worn because i have seen many rebuild's or even new motors with less then optimal power with not worn rings but unseated rings. In other words the motor never was broke in properly. The most important part of any engine life is the break in.
Again knowing what your looking for will make a diagnose of an engine for proper repair essential. Many shops in the industry fail miserably at this, mostly because of inexperience staff and poor pay, anyone can run a shop but not a good shop.
Hope that helps with your diagnosing.
You should put more emphasis on what you visually see then a compression test. All the info you collect from these will determine what steps you should take, if any.

 

[Caper11]

A lift and blowdown test is a better test than a comp test.


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[RGM]

A lift and blowdown test is a better test than a comp test.


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on a 2 stroke?

 

[lloydguy]

I can agree with Lund a little about examining the skirts and ring's for blow by ect.ect.
BUT, saying the number's mean very little is pretty offside. If my 670 was 90 and 90 they
are equal so, nothing to worry about? Not all engines have same comp.#'s but anything less
than 110 ish on any 2 stroke and you got troubles somewhere in there.

 

[Lund]

I can agree with Lund a little about examining the skirts and ring's for blow by ect.ect.
BUT, saying the number's mean very little is pretty offside. If my 670 was 90 and 90 they
are equal so, nothing to worry about? Not all engines have same comp.#'s but anything less
than 110 ish on any 2 stroke and you got troubles somewhere in there.

I agree but also i think you might have a slight misunderstanding of my post.
Basically what i'm saying is a compression test is not the end all answer and should be used as an indicator of possibly an issue and further investigating is needed.
I have seen MANY misdiagnoses of motors by dealers and back yard mechanics because of relying on compression test's, it also sell's very well to unsuspected customers. Spending $2k on an engine top end with little to no improvements is not a fix.
Compression reading's can be influence by a number of factors and until you eliminate those factor's you might be just wasting coin.
A 2stroke running heavy on oil will give high compression reading's, a simple oil pump malfunction can cause that or misadjustment or the dude mixes.
Carbon buildup is a common issue with 2strokes, that aloan will give high reading's.
In the case of carb., sleds with a sticking choke plunger will wash enough the cylinder wall's to drop the compression to a low reading. Bad seat needle and needle seat will do the same. As for an injected version a bad injector can cause low compression reading's by washing the cylinder wall's. Yet the motor might run properly but not as strong as it should.
Thus, relying on a compression test without a proper diagnose really means nothing. Too many factors can effect the compression of a 2stroke.
I agree with you on only the fact that a worn out motor CAN have low compression, IF and only IF everything else is good.

 

[Bernoff]

I agree but also i think you might have a slight misunderstanding of my post.
Basically what i'm saying is a compression test is not the end all answer and should be used as an indicator of possibly an issue and further investigating is needed.
I have seen MANY misdiagnoses of motors by dealers and back yard mechanics because of relying on compression test's, it also sell's very well to unsuspected customers. Spending $2k on an engine top end with little to no improvements is not a fix.
Compression reading's can be influence by a number of factors and until you eliminate those factor's you might be just wasting coin.
A 2stroke running heavy on oil will give high compression reading's, a simple oil pump malfunction can cause that or misadjustment or the dude mixes.
Carbon buildup is a common issue with 2strokes, that aloan will give high reading's.
In the case of carb., sleds with a sticking choke plunger will wash enough the cylinder wall's to drop the compression to a low reading. Bad seat needle and needle seat will do the same. As for an injected version a bad injector can cause low compression reading's by washing the cylinder wall's. Yet the motor might run properly but not as strong as it should.
Thus, relying on a compression test without a proper diagnose really means nothing. Too many factors can effect the compression of a 2stroke.
I agree with you on only the fact that a worn out motor CAN have low compression, IF and only IF everything else is good.

Thanks guys much more to it than I thought.