Compression Test Question

[gooberff]

I have a 92 banshee, FMF fatties, and a pro-design cool head. I don't know the dome size but when I purchased the cool head the domes were supposed to be the equivalent of the stock head.

 

I did a compression test today and my readings were 140 psi on both cylinders. i warmed the bike up and held the throttle open. i am at sea level.

 

My bike will start after about ten kicks when cold but when warm it starts right up.

 

I did this test because of the age of my bike and thinking I might need a rebuild of the top end.

 

Question 1: when doing the test, after kicking to the maximum pressure should the gauge hold its pressure or should it slowly leak down?

 

If it leaks down and it is not leaking from the gauge assembly is this an indication of a problem with the cylinder?

 

Question2: Seeking advice, with this compression reading should I consider the rebuild or leave well enough alone?

[black sunshine]

I have a 92 banshee, FMF fatties, and a pro-design cool head. I don't know the dome size but when I purchased the cool head the domes were supposed to be the equivalent of the stock head.

 

I did a compression test today and my readings were 140 psi on both cylinders. i warmed the bike up and held the throttle open. i am at sea level.

 

My bike will start after about ten kicks when cold but when warm it starts right up.

 

I did this test because of the age of my bike and thinking I might need a rebuild of the top end.

 

Question 1: when doing the test, after kicking to the maximum pressure should the gauge hold its pressure or should it slowly leak down?

 

If it leaks down and it is not leaking from the gauge assembly is this an indication of a problem with the cylinder?

 

Question2: Seeking advice, with this compression reading should I consider the rebuild or leave well enough alone?

 

125psi is a fresh stock motor at around sea level, so I'd say your tester is reading high or you have smaller domes, or your motor is actually gaining compression with age!!!

 

Gauges are usually equipped with a "tattletale" type device that records and holds the highest psi reached.

Compression tests will not tell you if you have a leak cause the cylinder releases all the pressure through the exhaust each stroke. Hold the throttle wide open and kick it a few times til you cannot reach a higher pressure. After that is done to each cylinder, try squirting a little 2-stroke oil in the cylinders one at a time and test again. A noticeable jump in psi is an indication that rings are worn.

[blowit]

125psi is a fresh stock motor at around sea level, so I'd say your tester is reading high or you have smaller domes, or your motor is actually gaining compression with age!!!

 

Gauges are usually equipped with a "tattletale" type device that records and holds the highest psi reached.

Compression tests will not tell you if you have a leak cause the cylinder releases all the pressure through the exhaust each stroke. Hold the throttle wide open and kick it a few times til you cannot reach a higher pressure. After that is done to each cylinder, try squirting a little 2-stroke oil in the cylinders one at a time and test again. A noticeable jump in psi is an indication that rings are worn.

 

 

As stated earlier, the gauge should not leak down at all or your gauge is not working correctly. You would need to purchase a leakdown tester for ring seal information but this still does not tell you the whole story because the entire piston must be within spec for the engine to operate properly. Piston slap or excessive piston to wall tolerance may not be noticed in any testing other than sound and measuring. If you are not sure, open it and mic it. Gaskets are cheap compared to new cylinders and cranks. Factory cast pistons are well know for dropping skirts on the intake side. This will generally kill the crank. $$

 

 

Brandon