I just tested the compression on my LT500 and it reads 120 psi.... this seems excessively low... I expected 165-185.

It's bored .100 over with a cool head, and an unidentified chrome pipe. I don't know the dome size. What would likely (or possibly) cause this low compression reading?

Also, should I:

a) Buy a new compression gauge.

or

b) Tear the motor apart.


I rode last weekend in the sand, and was just barely able to catch a couple banshees that I ride with. They're both lightly modified (pipes, cool heads, aftermarket pistons, advanced timing, swingarm extensions, etc).

Thoughts or comments welcomed!

 

Well if you can.... take off your pipe and look in through the exhaust port with a flashlight. Look for scores (scrapes or scratches that are deep) in the cylinder and on the piston. I don't think your compression gage is off. And if thats the case I think you either over heated it or ran some sand through it maybe.

If your Quadzilla was running right you would walk and talk all over those Banshees

 

What kind of compression gauge are you using?
If it is not a Snap-On, or similarly expensive compression tester, it is more than likely giving you too low of a reading.

Also, the more adapters you connect to the line, the lower your reading will be.

Make sure you have the screw in type (the rubber end type sucks, and won't hold the pressure).

Also make sure there are no leaks at any of the connections including the spark plug hole.

 

What's your elevation????

Silly question??? No! IT makes a big difference in static compression levels. I'm at 7K feet, and my comp will be significantly lower than when at sea level...

If memory serves me well, I'm thinking around 130# is stock.... (Someone correct me if I'm wrong)
Hard to say what dome is in that cool head unless you took it off and looked.

A QUALITY gauge is manditory for accurate readings... but those #'s don't sound too far off.
were you holding the throttle wide open and kicking multiple times??

Ditto Superstars comment: Only when you're looking into the exhaust port... also look for blow-by (Browning/burning gases that are making it past the rings... E.G. shouldn't be any discoloration of the piston below the rings)
It might simply be the way the motor was set up by the builder... A motor that is at .100 over has been apart many times...

If you see much blow by... it's time for a refresh.. Checking ring end gap, piston to bore clearance, etc.

Also, how's your jetting? The little things add up... Banshee's CAN be fast if done right... but so can the 500!

Good luck.

 

Maybe I'm way off here, but I thought 120 was EXTEMELY LOW!??

The stock compression on a TRX250R is around 180 psi.

Stock compression on a Banshee is (I believe) 135 psi.

I would think the 500 would read as high or higher??

 

120# on a worn motor at my elevation is common place.

Stock Honda 250R at my elevation is around 130#... a 19cc dome in an o-ring cool head only gives me 165-170# here at 7000'...

Anything over 180# in a big bore like the 500 would be detrimental to reliability. I've seen as high as 225# on the Honda's running alky.