Quick Compression Check Question

Boss Hawg

Boss Hawg

Lieutenant
Joined
Apr 9, 2009
Messages
1,433
Going to look at a boat with a Johnson 70hp motor- Guy says starter is bad but will start right up with rope ---
How can I check compression :confused:
Got burnt once-----never again :facepalm:
 
H

HenryB

Petty Officer 1st Class
Joined
Dec 8, 2012
Messages
244
Re: Quick Compression Check Question

I'd spend the bucks for one of these..

Compression Tester - Special Order est. 10 Days

You should be able to pull crank fast enough for a compression test.

It's a one man operation, a bit pricey, but you will use it often when searching for a used motor and again after the purchase.
 
Boss Hawg

Boss Hawg

Lieutenant
Joined
Apr 9, 2009
Messages
1,433
Re: Quick Compression Check Question

Thanks- I have a good tester but was wondering if the pull rope thing wud be enuff to get a accurate reading--
 
M

mars bar

Petty Officer 1st Class
Joined
Nov 30, 2008
Messages
395
Re: Quick Compression Check Question

Yes, as long as all plugs are out, there won't be any compression on the other plugs and it will pull easy. You want the piston to go up and down about 4 times to get a good reading.
 
D

dazk14

Ensign
Joined
Jul 22, 2008
Messages
966
Re: Quick Compression Check Question

You'll want to pull the rope in succession ~4 times for each cylinder...unless your using a 20' rope and have gorilla arms.

Those are high compression motors and often run up to 150psi.

You will get lower readings by hand, but still usable, since your technique will be similar.

Warm reading are more accurate than cold. Since you have a tester, I'm sure you know the basics.

If your using a tester with a hose (90% of them), it does take time to pressurize due to the added volume.

I'll presume your tester has a Shrader valve at the opening to the hose.

Let us know how you make out.
 
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