100 psi compression good or bad?

[grotto]

Is 100 psi compression on a 9.2/1 4 stroke single good or bad (tested cold).
I know my 2 strokes are 150 + psi, but don't know if 4 strokes are normally lower

[Jantar]

Is 100 psi compression on a 9.2/1 4 stroke single good or bad (tested cold).
I know my 2 strokes are 150 + psi, but don't know if 4 strokes are normally lower

Atmospheric pressure is 14.7 psi, so 9.2:1 would be 14.7 * 9.2 = 135 psi.

100 psi is very low.

[bogan]

Did you test it while holding the throttle wide open?

A hundy does sound low, but check the manufacturers specs for what it should be.

[Katman]

You are not going to get a true compression figure with a cold engine.

[Ocean1]

Atmospheric pressure is 14.7 psi, so 9.2:1 would be 14.7 * 9.2 = 135 psi.

100 psi is very low.


You are not going to get a true compression figure with a cold engine.

You're not going to get a meaningful single figure at any temperature. There's just too many variables, including plug and guage deadwood.

Now, if it was 120psi with that guage on a warm engine last week and now it's 100psi you've got a problem. Squirt of light oil down the plughole and another test might tell you if it's a rings problem or a valve problem.

[FJRider]

You are not going to get a true compression figure with a cold engine.

Even 100 cold ... ain't that great ...

To achieve acceptable (as opposed to respectable) ... it would need to be bloody hot ...



But (for the benefit of the OP) if it runs ... and the other option is walking ... carry spare oil ... you'll probably need it ... if you check the oil level.

Don't check the oil ... best you wear comfortable footwear ...

[nodrog]

You are not going to get a true compression figure with a cold engine.

what the fuck would you know about fixing motorcycles?

[Katman]

what the fuck would you know about fixing motorcycles?

I'm just here for the pizza.

[Flip]

I'm just here for the pizza.

What te fuck would you know about Pizza?

[Katman]

What te fuck would you know about Pizza?

Well I know it doesn't need a capital P.

[FJRider]

Well I know it doesn't need a capital P.

What the fuck do you know about P ... ??

[Katman]

What the fuck do you know about P ... ??

Comes between O and Q.

[FJRider]

Comes between O and Q.

The thousands of $$$$ your parents spent on your education wasn't wasted then ...

[grotto]

It's on a bike I was thinking of buying.
39,000 km on the clock, starts & runs real good.
I couldn't find manufacturers spec of what compression should be.
I didn't have throttle wide open (forgot). Would compression be higher or lower with throttle wide ope?
Thanks

[bogan]

It's on a bike I was thinking of buying.
39,000 km on the clock, starts & runs real good.
I couldn't find manufacturers spec of what compression should be.
I didn't have throttle wide open (forgot). Would compression be higher or lower with throttle wide ope?
Thanks

Compression would be higher, as more air can get to the cylinders to be compressed.

[FJRider]

It's on a bike I was thinking of buying.
39,000 km on the clock, starts & runs real good.
I couldn't find manufacturers spec of what compression should be.
I didn't have throttle wide open (forgot). Would compression be higher or lower with throttle wide ope?
Thanks

What size/model engine ... ??

[SMOKEU]

Comes between O and Q.

WTF do you know about O and Q?

[neels]

carry spare oil
On a 4 stroke, only if the oil ring is leaking, if the compression rings or valves are buggered it could have low compression but not burn any oil. It will, however, blow out the crankcase breather like crazy.

100psi cold is probably not too bad, run it and get it hot and take off the engine breather or if too hard the oil filler cap, if it's puffing like a steam train the rings are sad.

[FJRider]

On a 4 stroke, only if the oil ring is leaking, if the compression rings or valves are buggered it could have low compression but not burn any oil. It will, however, blow out the crankcase breather like crazy.

100psi cold is probably not too bad, run it and get it hot and take off the engine breather or if too hard the oil filler cap, if it's puffing like a steam train the rings are sad.

It could be the valves fucked as you say ... wont blow smoke ... just wont pull.

I haven't seen what size engine it is. On a smaller one ... 100 may be ok ... sort of ...

To follow it and smell the fumes ... if there is a small amount of "burning" it would be noticed then. I had an XJ750 that did that. It went ok .... but (and) used oil by the gallon.

[bogan]

100psi cold is probably not too bad


100 may be ok ... sort of ...

Add another 30 odd psi for cold engine and closed throttle, and its probably about bang on for a 9.2:1 engine.