Compression testing a 2 stroke

[tomalt]

I have a 1980 Johnson 7.5 HP 2-stroke. When doing a compression test should the compression tester hold the pressure between pulls or will it leak out do to being a two stroke. My guage doesn't hold pressure so I will take it to work and check for leaks.
Also what compression reading should I see when cold?
And another thing where is a good source of parts for this thing? I'm having a hard time finding a complete impeller and housing kit for it?
Thanks

 

[HighTrim]

Re: Compression testing a 2 stroke

Welcome to iboats Tomalt. Your gauge is obviously shot. After a few pulls the gauge should top out at its compression reading until you manually re set it to test the second cylinder. What is most important is that the compression between cylinders is even, or within about 10 percent. I would guess anything over 80 psi and even would be decent and acceptable, depending on the gauge.

What is the model number?

 

[tomalt]

Re: Compression testing a 2 stroke

The model number is J8RCS.

Thanks for the reply.

 

[HighTrim]

Re: Compression testing a 2 stroke

Here is one example, but there are many online sites. Unfortunately, I dont believe iboats carries the waterpump kit for the 7.5 hp.

"http://www.maxrules.com/fixomccooling.html#379774"

Part number 396644 for the kit 35 bucks.

Part number 389576 for just the impeller, 17 bucks.

Here is another site, complete part list. Water pump kit #58 $31.95 OR Impeller #35 $16.85

"http://www.crowleymarine.com/brp_parts/diagrams/37242.cfm"

 

[tomalt]

Re: Compression testing a 2 stroke

I didn't find one here, thanks for the info.
Tom

 

[F_R]

Re: Compression testing a 2 stroke

Many/most compression testers have a check valve similar to a tire valve core to hold the pressure. And just as easy to replace as a tire valve core. Only thing is those valves come with various spring rates. You need one with near to zero spring pressure. Heavier springs reduce the indicated pressure reading on the gauge. That's one reason for compression gauges being so notoriously inaccurate.