100 psi compression good or bad?

**grotto** · 18th October 2012, 10:12

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** · 18th October 2012, 11:09

Originally Posted by 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

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

100 psi is very low.

**bogan** · 18th October 2012, 11:28

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** · 18th October 2012, 18:52

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

**Ocean1** · 18th October 2012, 19:01

Originally Posted by Jantar

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

100 psi is very low.

Originally Posted by Katman

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** · 18th October 2012, 19:03

Originally Posted by Katman

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:Me** · 18th October 2012, 19:21

Originally Posted by Katman

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

what the fuck would you know about fixing motorcycles?

**Katman** · 18th October 2012, 19:39

Originally Posted by nodrog

what the fuck would you know about fixing motorcycles?

I'm just here for the pizza.

**Flip** · 18th October 2012, 20:32

Originally Posted by Katman

I'm just here for the pizza.

What te fuck would you know about Pizza?

**Katman** · 18th October 2012, 20:35

Originally Posted by Flip

What te fuck would you know about Pizza?

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

**FJRider** · 18th October 2012, 20:36

Originally Posted by Katman

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

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

**Katman** · 18th October 2012, 20:41

Originally Posted by FJRider

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

Comes between O and Q.

**FJRider** · 18th October 2012, 20:44

Originally Posted by Katman

Comes between O and Q.

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

**grotto** · 19th October 2012, 11:09

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** · 19th October 2012, 11:12

Originally Posted by 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

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