Compression in a 140

[Jamepc]

I need to do a compression test on a Mercury 140 in a Sea Doo Islandia. What psi should I get on good cylinders?

I've done them before on my trucks and car. Is it basically the same? Remove all the plugs first? What's the easiest thing to disconnect to prevent it from starting? Should it just turn over until the gauge stops climbing or just a certain amount of turns? What's the plug hole size in these? Any other tips?

Thanks!
James

 

[Chris1956]

Re: Compression in a 140

Same as a car. Remove all plugs, stick the gauge in to a hole. Jumper the starter solenoid so it will crank. Let it build to max. Write it down. Compression s/b at lease 90psi, and all should be close to each other.

 

[Jamepc]

Re: Compression in a 140

Thanks for your reply.
I thought I read here on the board that compression should be around 150-160. Maybe that was for the inboard small blocks though.

 

[Jamepc]

Re: Compression in a 140

It's actually a 240 HP Merc, not 140. Maybe that's why you said 90 PSI, or maybe that's still correct?

 

[Jamepc]

Re: Compression in a 140

Please pardon the bump.

Regards,
James

 

[Chris1956]

Re: Compression in a 140

James, If the motor is a sterndrive, the compression should be 125PSI+ and even on all cylinders. You should be able to get an official number from a MerCruiser manual.

 

[Jamepc]

Re: Compression in a 140

It's actually a 2001 2 stroke 240 HP Mercury Outboard (well an Mercury power head mated to a jet drive). Is that still +/-125 psi?
I don't have access to a manual. I don't have the boat yet but I want to check the compression.
Thanks,
James

 

[Chris1956]

Re: Compression in a 140

James, Normally 2 cycle outboard manufacturers do not cite specific compression readings. Since your power is produced by a 2 cycle engine, this applies to you. I would expect that your compression should be at least 90 PSI and more importantly, even on all cylinders.

 

[Jamepc]

Re: Compression in a 140

Thanks for your help Chris.
James

 

[redjmp]

Re: Compression in a 140

More like 125 and within 10% of each other worst case.
My 115 (Modern day 140) gets 135 to 140 lbs.

 

[hkeiner]

Re: Compression in a 140

Compression gauges are often not very accurate. Your gauge reading can easily be 10-20 psi off (high or low). As long as the PSI numbers are close on all cylinders, you are most likely fine.