TOPIC: 1968 Mercury 650 compression check

I don't have a battery or the controls setup on my Mercury 650 but would like to do a compression check. Can I do this by just rotating the engine at the flywheel?

Thanks

Practically...no. You ever hand started a 64 CI four cylinder?
Heres me testing a 40~CI Bigtwin with one plug out and turning it with a BIG drill.

You need to run the starter. If you don't have the cables do this.
You can just connect the battery to the power connector side of the starter solenoid with jumper cables and connect the coil terminal of the solenoid over to the power side with a screwdriver or something.

Be sure you ground all of the spark plug wires with clip leads to prevent damage to the cap or coil. Or ground the coil output.

BTW it should be 120 psi give take 10 if its a good un.

Yes you can. pull all the plugs and install the tester in 1 plug at a tine. use the rewind starter to pull it over. do it 3-4 pulls till it reaches it highest reading. been there done that many tines even with a 6 cyl.
120 lbs is great.

i've used a good drill and socket to spin some over with all the plugs out.. just made it easier to do, the largest ive tried it on has been a 115 inline 6.. just make sure its a good drill.. dont know if those china dollar store one's have the power or not..

Thanks..
Gonna give it a shoot.

I've rope started my XS when had a flat battery! But just go ahead a yank the plugs, ground them out, get a rope and give it a good pull. Do the reading and move to the next.

Compression is compression. It does not increase with piston speed. BUT, the slower spin will show (maybe, depends on motor, a 2 stroke anyway) lower reading as exhaust overlap can have you lose some. But all your looking for is even numbers anyway, and not one really low. Plus you don't need to open the throttle either a 2 stroke as piston at BDC has the exhaust port open to atmosphere where as a 4 stroke will not, and that need open throttle. My take on it anyway, and the way done me decades with now problem.

Just get a good grip on the drill. I got slapped up side of the head last week with my codeless drill doing a 5.5 Johnson

Have to laugh here on "drill start a motor" My dad LONG ago busted the recoil start his lawn mower. It was one those old school if any remember, ya pulled the handle off to the side and cranked it for a wind up the "spring", then slam it home for a start. Nice invention, but zip wrong a rope pull as faster to start anyway! He showed me (I was a little rugrat then) and he simple took out the 4 rivets holding the recoil, junked that part as had to keep the "cowling" for the flywheel fan to cool the cylinder head. Got out his electric drill (which I STILL have here 60 years later!) Tossed a socket on and soon "fired". I learned a LOT from my dad as he had 50 years knowledge (he was 50 when I was born and bet I was 5 years old then). And I know the new batt powered stuff has some torque, but I got a 1/2 inch chuck Milwaukee variable speed corded drill that if that the drill bit catches finishing the hole? Ya simple going for a "spin!" Or a wrist break! OUCH!

Have not had a chance to go out and check this yet but you say to ground the plugs. Since I am new to outboards, am I going to create spark even though I have no battery and controls hooked up?

I remember going to a ice fishing derby a few years abo and at the start we broke the rope and could not start the auger to drill our holes, so my brother pulls out a drill and we took apart the recoil assembly and put a socket on to start the auger. Got us threw the weekend lol

crowsnestflyfisher wrote:

Have not had a chance to go out and check this yet but you say to ground the plugs. Since I am new to outboards, am I going to create spark even though I have no battery and controls hooked up?

Hmmm..I would think not, but I'm simple not sure.

Been thinking this a bit today. I'm not familiar with your motor (points or electronic ING), but on grounding the plugs? Would simple pulling the coil wire to the cap and ground that out? But as upside down cap the plugs wire should be screw in's but not sure the coil wire if twist in, or push in. I can rebuild a motor sleeping, blind folded, one arm tied behind my back, but I'm damn sure good at letting the "smoke out a wire" electrical! But the coil end wire should be a push in and if so just yank it and take a old plug wire in and to ground it. Just a thought...

Honestly, you can avoid all the nonsense and get better test results with a leak down checker. I have seen many a Merc with bores worn tapered like ice cream cones give excellent compression numbers but run like an engine with.... well, worn out bores! Leak down tests will reveal this, and best of all you don't have to wreck your arm pulling the engine over.