compression test

[tsnobaddays]

I was wanting to know the proper way to do a compression test on a 2 and 4 stroke motor. Is this a reliable method of testing wear on a used engine. I am looking to buy a used dirtbike.

 

[IndyMX]

It's one way to test for wear, and probably the simplest way to get an idea about where your internals are. But only a good test if you have a baseline on a fresh motor. Otherwise the numbers are just a guess.

What I understand, once you have the compression tester connected, you kick the bike thru the stroke until the needle stops rising. Should only take a few good full kicks. That should give you a good number to work with.

But again, if you didn't do a compression test with a fresh top end, it's only a wild assed guess.

Another good method is a leak down test, however it requires different equipment. And will give you a more accurate picture of how seals and whatnot are working.

There's really no good, accurate test other than just tearing it down and measuring everything.

 

[2strokerfun]

If possible, it's best to test after engine is warmed up. As Indy said, it will give you an idea of the top end. More obviously will tell you if a top end is worn out or ill fitting, but not really tell you how good of shape it's really in. On multi-cylinder engines, they are really good for looking at discrepencies between cylinders.

 

[tsnobaddays]

I thought that you ran the cylinder dry initially then added a small amount of oil and looked at the differences between the two. The way that I understood it is the oil filled any spots in the cylinder wall that are worn and the compression test would come out higher, meaning that there are signs of wear. Please correct me if i'm wrong. I remember readings this in a Dirtwheels Mag I may be wrong on the procedure.

 

[jsantapau]

if you have a multicylinder engine with one low hole and you add some oil to that cylinder and compression jumps up considerably, that tells you you do not have bad valves but a worn /piston cylinder.


with a single cylinder you need a baseline when fresh and you can measure as it wears ........ you can confirm low compresion if you think the kickstarter seems easy... and if you are talking about a modern four stroke with an automatic decompression release just throw it in the garbage can

 

[tsnobaddays]

jsantapau what do mean by, if you are talking about a modern four stroke with an automatic decompression release just throw it in the garbage can. Not sure that I understand this comment. Are you saying that a compression test is worthless in a enginge has a decompression release.

 

[jsantapau]

yes that is exactly what I meant the compresion release will not let you get an accurate reading

 

[helio lucas]

jsantapau what do mean by, if you are talking about a modern four stroke with an automatic decompression release just throw it in the garbage can. Not sure that I understand this comment. Are you saying that a compression test is worthless in a enginge has a decompression release.

no, what he means is really throw it in the garbage and get a two stroke :nener: :laugh:

 

[jsantapau]

now that is funny