Question Regarding FAQ - Compression Testing

[pmergler]

I read the Engine FAQ about Compression Testing. I am good with everything except for how to "ground the spark plug lead". Any ideas? Can't I just unscrew all the spark plugs, leave them connected to the plug wires and put them somewhere safe where a spark won't endanger anything? Also someone on the forum mentioned doing the compression testing at WOT, I am doing this on muffs, can't I just do it at 1/2 (aka start position) and that suffice?

Thanks in advance,
Patrick

PS - 1965 'Rude 33 hp.

 

[rickdb1boat]

Re: Question Regarding FAQ - Compression Testing

Just use the kill switch to disable spark. Or run a piece of wire to each wire and then to the block. The motor will not be running anyway, so open up the throttle....

EDIT: Lets make sure you really know how to do the compression test. You comment about being on muffs makes me nervous. Undo all the plug wires and remove the plugs. Ground the plug wires as mentioned above. Screw the tester in one plug hole. Open the throttle up. Turn the engine over 4 or 5 times and takes a reading. Move on to the next hole. Should be within 10% of each other...

 

[tashasdaddy]

Re: Question Regarding FAQ - Compression Testing

take the plugs completely off an locate the wires out of the way, i use a rubber band. shift in forward and move the throttle to wide open.. it is not going to be running when you do this. you just put the gauge in the spark plug hole, and turn the engine over about 4 times and get reading. when running never rev your engine over 1500 rpms. on muffs.

 

[hunt-fish.com]

Re: Question Regarding FAQ - Compression Testing

Just a thought, seems like running water through the muffs will keep the impeller from self distructing while your turning the engine over. I've always heard not to turn the engine over dry. Make sense?

Thanks,
John

 

[tashasdaddy]

Re: Question Regarding FAQ - Compression Testing

won't hurt.

 

[Scaaty]

Re: Question Regarding FAQ - Compression Testing

Your only cranking for a second (one Mississippi) for a compression test. If yanking, good couple 3/4 pulls, and I doubt VERY much that you will get a impeller hot in one second or a couple yanks. Plus it stays wet down there for a long, long time after its been out of the water. And the reason for WOT while doing a compression test is simple. If ya closed the throttle, you are choking off the air, and with less air to compress, just what would you test? Rhyme intended since it came out that way.. 8)

 

[Walker]

Re: Question Regarding FAQ - Compression Testing

Just leave the key off and use a remote starter button on the solenoid or just rig a jumper wire on the solenoid. That is if you don't have a safety lanyard. If you do have one just pull it off and crank with the key switch. You aren't going to hurt the waterpump doing a compression test.

 

[pmergler]

Re: Question Regarding FAQ - Compression Testing

I have a follow-up question. I understand the theory around compression testing under WOT and to get to WOT the motor must be in gear, but I have read and been told (possibly not a reliable source) that you shouldn't put the boat into gear without it turned on? Is that incorrect?

patrick

 

[rickdb1boat]

Re: Question Regarding FAQ - Compression Testing

That would be incorrect. You may want to have someone turn the prop slowly while you put it in gear to help it mesh better, but otherwise, no problem....

 

[pmergler]

Re: Question Regarding FAQ - Compression Testing

Okay so I tried to do this over the weekend and I ran into a snag....my engine won't even attemp to turn over with the boat in gear. So I can't compression test in gear at WOT. Do I have to trick out the boat to make it think it isn't in gear? Is it a big deal to only compression test at 1/2 throttle in neutral.

 

[ezeke]

Re: Question Regarding FAQ - Compression Testing

I don't understand why you would want to be in gear when doing a compression test.

You don't need WOT on a two stroke compression test. You don't need the throttle plates fully closed either. If you have a warm up lever, push it up.

 

[andy6374]

Re: Question Regarding FAQ - Compression Testing

They mean crank it with it out of gear but with the butterfiles fully open...high neutral.

 

[ezeke]

Re: Question Regarding FAQ - Compression Testing

That is not what I mean.

What I mean is that you don't have to be at WOT to do a compression test on a two stroke.

 

[freddyray21]

Re: Question Regarding FAQ - Compression Testing

they won't crank in gear as the neutral safety switch won't engage. Pull the throttle cable off and tie the carbs open.

 

[pmergler]

Re: Question Regarding FAQ - Compression Testing

I got a 100 and a 115 when I did the test in neutral with the throttle up high as it goes in neutral. Not exactly within 10% of each other, but I plan on doing a Sea Foam de-carb anyways. I will retest then. I think worse case scenario everything is okay.

 

[newuserguy]

Re: Question Regarding FAQ - Compression Testing

Just my two cents. Ezeke is correct on compression tests in two strokes. In addition, I was advised to leave plugs in other cylinders for proper compression reading on the one cylinder you are testing. Also, and I may be wrong I but I always thought it was two Mississippi's.

 

[newuserguy]

Re: Question Regarding FAQ - Compression Testing

Just my two cents. Ezeke is correct on compression tests in two strokes. In addition, I was advised to leave plugs in other cylinders for proper compression reading on the one cylinder you are testing. Also, and I may be wrong I but I always thought it was two Mississippi's.

The reason for leaving the plugs in is to create proper back pressure in other cylinders, thereby giving a more accurate indication of engine compression/condition.