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Yachtwork, Marine Engineer
Category: Boat
Satisfied Customers: 1048
Experience:  MEC3 and RYA Yachtmaster Ocean
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Will my compression show lower as a result of a weaker tur

Customer Question

Will my compression show lower as a result of a weaker turnover?
Submitted: 2 years ago.
Category: Boat
Expert:  Yachtwork replied 2 years ago.
Yes it will. The RPM of the engine during a compression check should be about 250. If you pull the plugs and have a good battery connection you should have about the right RPM's, but if you hear the starter dragging then you will get a lower compression for sure. One thing that can help, is even if you get a low reading, but all the cylinders are about the same then you should still be okay as far as compression goes. So, be sure you have all the cylinder plugs pulled during the test. This helps a lot to keep the RPM's up and the presure standard. I think that answered your question. Scott
Customer: replied 2 years ago.
Thanks, Scott!
You're totally right! and With the plugs pulled the rpm on the flywheel does go way up. Turns out my compression is really bad and my head gasket is shot. I'm happy to replace it as this is a project motor. Finding lots of resources on how to get that done .However, Once it's done, I'm still going to have that slow start. I checked all my ignition coils with a multi meter and it looks like only 2 out of 4 are functioning and even the ones showing a reading are quite dodgy. I haven't read that you get slow crank from bad ignition. What do you think? I've tested batteries, troubleshooted the solenoid, Bypassed the solenoid to attempt power straight to the starter. Haven't taken starter to get tested. <-- that seems to be the popular one. I've done a visual inspection of all wires but I'm realizing it might come down to digging into some cables with the multi meter.Where would you start?Just so you have some context. I bought a used 1984 boat with same year Evinrude V4 140. It started up right away after sitting for 5 years although the head gasket was already worn out I didn't know any better so I put about 120 kilometres on the thing before I overheated it severely due to my attempt to bypass the VRO and who knows what else. Now I'm gonna clean out the cylinder heads and try to get some compression. After that's done I'll have the ignition and starting problems!Thanks for your time!
Expert:  Yachtwork replied 2 years ago.
To test the starter I use a Clamp Amp set on DC. Be sure to buy an AC/DC version. They sell for about 60 bucks at Sears, and 250 from Fluke, and you can buy them at a tool store for about 30. Clamp the positive cable going to the starter and it should show about 120 amps for that little starter. Slow cranking normally means a bad armature that is slightly shorting to ground and turning amps into heat when we want a magnetic field.
You can also check at the starter lugs with a regular multi meter set on volts. You should see above 9.5V. If not then its the battery or cables. Thus if you show 9.8v at the starter and you have a slow turning engine then it is most likely the starter, but a clamp amp will tell you for sure.

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