Johnson cd-12 restoration....Sunk?

[Tech Tony]

Hi everyone, When I first got this motor, I tested compression and it was 40 in both cylinders and I figured since both cylinders were so close, I'd hone and re ring it. So I did. everything looked goodthere was a little rust in the top cyl, but it honed out pretty good. and I put brand new rings. After many hours of hard knocks learning and basicly restoring the entire motor, It was time to test the compression. when I assembled the powerhead, I used tcw3 oil on the piston rings and wrist pins, along with assembly lube...so I test it, and viola, 40 psi on both cylinders,.. I know it is better than it was, it has to be. I'm using a new harbor freight compression tester . maybe the oil dried up or ran out, it was 3 or 4 weeks ago that I put the powerhead back together, when the cylinders aredry do they get that low? thanks..tony

 

[flyingscott]

Have you tried to start the motor yet could just be the gauge. These motors do not have high compression ratios will run good with 70-80 PSI.

 

[Tech Tony]

I don't think Its the gauge, I tested it on my lawnmower, and it has 60 psi, the I put a shot of tcw3 oil in the cylinders and got 45on the lower cylinder and still 40on the top. I think im in trouble here,..

 

[racerone]

I think the gauge is junk.

 

[Tech Tony]

I hope your right, I tested the lawnmower again and got 75 psi this time, where i wish the motor would be, the cylinders on the outboard hiss when you hold your finger over the holes, I will call my friend who works with me that has a good compression tester..but I haave tha tsinking feeling

 

[Tech Tony]

I wonder ,the lawnmower is 4 stroke and thecd-12 is two stroke maybe the gauge cant trap the pressure enough geeze, you need an engineer for this stuff.

 

[nabiul]

Did you do a proper ring job?

Check the gap between cylinder and piston, check the ring end gap when inserted into the cylinder, check the ring groove width and clearance with the rings. Just slapping new rings onto worn out pistons and cylinders isn't going to do anything.

Did you check the head gasket for leaks, check the flatness of the cylinder heads?

I'm no expert but I think a motor that shows wear to 40 psi on both cylinders is so far gone internally that it's not worth trying to repair, even if you got the compression fixed the crankshaft bushings and bearings are probably worn to hell. After my last experience with a johnson 9.5 that read 60psi, I will never bother with a motor like that again as it's at the end of it's useful life.

 

[flyingscott]

Get it put back together and start the motor see what happens.

 

[flyingscott]

I wonder ,the lawnmower is 4 stroke and thecd-12 is two stroke maybe the gauge cant trap the pressure enough geeze, you need an engineer for this stuff.

What, the gauge only measures compression doesn't care if it's 2 or 4 strk. If you put oil in the cylinders and it made no difference your gauge is bad like racer said.

 

[Tech Tony]

OK, flyingscott, I'm with you and racer, Its all back together and when you pull the starter with the plugs in, you get a good thunk, thunk. and its not that easy to pull. I've pulled ona few motors in my day and this one feels good to me. Its ready to run and i'm just waiting on a few parts for the tank. they will be here this week. I will start a new thread when that happens. thank you all for your help.

 

[HighTrim]

Hopefully it is the gauge.

For future, before spending money, determine WHY the compression is low. If you determined that the bore was good, and within spec, yet the rings were worn, then you went in the right direction. Just hate to see people waste money on a block that is NFG. Always test compression first, then if low, ensure you know why its low.

The easy way is to stick a good ring in the bore, and measure the end gap. If above spec, bore is worn too much.

 

[Tech Tony]

just borrowed my friends good snap on compression gauge, and i wasn't expecting much, but when I pulled the rope, I gor 77psi on top, 80 on the bottom. I think Ill be taking that other gauge back to harbor freight. And thanks to all for your expirence and help. When I get it started I will Post a new thread.

 

[the machinist]

The THUNK THUNK tells me that you may have one or both pistons in upside down & they are hitting the head when you pull it over