Basic compression - sucks and blows

[dkgiles7]

Hi,<br /><br />I have a 75 model Electric start 40Hp johnson that came with a boat I bought - the boat was a bargain so I wasn't too concerned about how good the motor was. The motor starts but sounds like it's running on only one cylinder. Realising I had a bit of work to do I am doing some initial work to estimate just how bad the motor is.<br /><br />I don't yet have a compression tester but on pulling the plugs and just pushing a finger into each hole in turn, the lower piston pushes hard but the upper doesn't seem to push quite as hard and also wants to suck air back in on the way back down ... is this normal ????<br /><br />I have only cranked once each time so not sure how much of a 'test' this really is....<br /><br />Any comments on whether this is normal or a useless test? (beyond going out and buying a compression tester which I intend to do)<br /><br />Thanks.

 

[jhreed]

Re: Basic compression - sucks and blows

Good morning, I'm not sure this will help you but, since I'm guessing that you have a twin, both cylinders should push and pull your finger as the ports are covered at the heigth of their travel. In other words they would probably be equal to each other. The other fellows here would surely shout "get a manual" but there are many things that you can test now to tell you whether it's worth even getting one. I'm assuming that the motor is being manually pulled through the cycles. Is there spark to each of the two plugs as they rest on the block? Do you have a clean fuel tank and does the primer hand pump fill the motor until it's hard to pump? My suggestion would be to bring someone else over to help you try to start it with proper fuel/oil <br />mix and then go from there. Of course I mean in a water barrel because if it ran, one could wreck the water pump parts within a few seconds. <br />Good luck, let us know how you do... Jim Reed.

 

[dkgiles7]

Re: Basic compression - sucks and blows

Thanks Jim.<br /><br />The manuals (service, owners and parts) are already on order so hopefully all will be okay and it won't be wasted dollars. I have access to a compression tester but won't get a chance to test until next weekend. I haven't checked spark yet but will let you know how it goes.<br /><br />Cheers.