Compression tester?

[dirkdiggler]

How far off can a Walmart $25 compression tester be???

I bought a jet ski couple weeks ago and the guy that I bought it off of had a decent(I think) compression tester. Both cylinders ended up at 150 psi on his compression tester.

Today, I checked the compression in the same ski with my compression tester and it came out at 115 psi. Is this possible??

On my Walamar tester

1991 Superjet 650 = 125 psi in both cylinders
1994 FX-1 = 120 psi in both cylinders
1989 650sx = 115 psi in both cylinders

should I just add 35 psi to what my tester shows and assume that is the correct psi???

What are the original compressions when new for each of my ski's???

 

[marz831]

150ish is good for all three of those assuming stock heads. Sounds like your gauge is 35 lbs off

 

[Freakrider]

I once tried a $25 one from auto zone it had low readings also...I took it back.... I now love my Sears craftsman tester...

 

[augustaf]

Most inexpensive testers have schrader valves which have too much spring pressure (like a tire valve). So even if the gauge is accurate the poorly designed checkvalve will cause a low reading. Also if you have an aftermarket head you need to use a gauge adapter that's long like your sparkplug (stock is short) otherwise your reading will be about 20 psi lower due to the increased chamber volume. Don't worry too much about your reading as long as both cylinders are reasonably close and the boat has good power.

 

[Shonuff]

Go get yourself a good Craftsman gauge.

 

[sneaky]

dont forget snap on or mac? i use mine often tho. .