Low hr M20 issue

[OldSkull]

After "swapping" this low hr M20 in my 1872 (Now 2072), I found out really quick, it's not going be a turn key swap. The carburetor got issues, some sludge where hiding behind those shield but no oil leak from both crank oil seal and head gasket. I clean the engine put it back together and in the 1972 he goes.

I decided to do some tuning and testing, I manage to make it run smoother but the carb still need to be restore. no big deal since the M18 got one running well I swap that later if need. Things get wear when I did a compression test here the result after 10 minutes warm up.

First test cylinder one 105psi

First test cylinder two 75psi


I warm up the engine again and give it another try:

Second test cylinder one 95psi (sorry this one is out of focus)

Second test cylinder two 89psi


I think it's a bit low for a 200hr engine, I suspect a valves leak so as soon I found out how to find the dead center position on this engine I will do a leak test. If my suspicion are right I need to remove that engine from the cub and open it, This need to wait a bit... story to followed soon...

[jimbob200521]

Head gaskets are another good thing to check, if the engine has a blown gasket it can cause bad readings on a compression test. Just something else to check out.

[OldSkull]

Originally Posted by jimbob200521

Head gaskets are another good thing to check, if the engine has a blown gasket it can cause bad readings on a compression test. Just something else to check out.

Good thinking Ryan! Having variable result like that make me think "Sticky valves", since that engine was sitting in a corner for few years it make sense but I'm not ready to remove the engine from that cub, my expensive Canadian made cherry picker hydraulic cylinder crap out! Kapout! I will pick a cheap Harbor freight air/hydraulic Friday ($72 with a 20% coupon), no way I spend $500 on a new OEM cylinder or lost my time rebuilding the old one .....To be continue

[jimbob200521]

Originally Posted by OldSkull

Good thinking Ryan! Having variable result like that make me think "Sticky valves", since that engine was sitting in a corner for few years it make sense but I'm not ready to remove the engine from that cub, my expensive Canadian made cherry picker hydraulic cylinder crap out! Kapout! I will pick a cheap Harbor freight air/hydraulic Friday ($72 with a 20% coupon), no way I spend $500 on a new OEM cylinder or lost my time rebuilding the old one .....To be continue

It may or may not be it, but something to check since you're taring it down anyway. You'll know right away once you remove the cylinder heads.

[ol'George]

ol Skull,
Been my findings that the M-18's run in the #90 range.
I have 4 engines that reflect that.
One engine has new .010 overbore, pistons & rings and about 50 hrs running time and both cyl read #92
The M-20 is stroked a bit, so it could run higher, they say + or - 10%
and with carb butterfly wide open

You can check valve clearance, but I say run it a bit and get a "feel" for it.

[OldSkull]

Originally Posted by ol'George

ol Skull,
Been my findings that the M-18's run in the #90 range.
I have 4 engines that reflect that.
One engine has new .010 overbore, pistons & rings and about 50 hrs running time and both cyl read #92
The M-20 is stroked a bit, so it could run higher, they say + or - 10%
and with carb butterfly wide open

You can check valve clearance, but I say run it a bit and get a "feel" for it.

Now I'm confused! John Hall did a partial rebuild lately (tittle M-18 running rough/missing) and he report 130psi on cylinder 1 and 120psi on cylinder 2 and this is before he open this engine! Check post number one of this thread:
http://www.onlycubcadets.net/forum/s...ghlight=Magnum

Now you tell me you got four M-18 in the 90psi and among them one partially rebuild with 50hr on the counter!

G...I think I have a brain fart.....Cricket.....cricket

[john hall]

Originally Posted by OldSkull

Now I'm confused! John Hall did a partial rebuild lately (tittle M-18 running rough/missing) and he report 130psi on cylinder 1 and 120psi on cylinder 2 and this is before he open this engine! Check post number one of this thread:
http://www.onlycubcadets.net/forum/s...ghlight=Magnum

Now you tell me you got four M-18 in the 90psi and among them one partially rebuild with 50hr on the counter!

G...I think I have a brain fart.....Cricket.....cricket

George has posted before that these engines don't have really high compression. If I had to guess, mine or his compression tester is out of calibration. The one thing is certain though is that you have a noteable difference between the 2 sides. FWIW, mine was about the same before and after the "rebuild". I'm not near my service manual to see what Kohler has to say is a normal range. Know anyone that will loan you their compression tester to see how the readings compare to yours?

[jimbob200521]

Originally Posted by john hall

George has posted before that these engines don't have really high compression. If I had to guess, mine or his compression tester is out of calibration. The one thing is certain though is that you have a noteable difference between the 2 sides. FWIW, mine was about the same before and after the "rebuild". I'm not near my service manual to see what Kohler has to say is a normal range. Know anyone that will loan you their compression tester to see how the readings compare to yours?

I want to say that the manual states less than 90psi requires servicing. I'm 99% sure that's what the manual says.

Edit: Found it; if the difference between cylinders is greater than 15% or the pressure is less than 90psi, service is basically required.

[J-Mech]

I'd run it a while and see if it comes out of it. Make sure it's tuned up right, fuel system clean.... then run it. Work it hard. It may just surprise you.

[OldSkull]

Ryan is right about the manual saying 15% or under 90 psi engine need services, that why George statement surprise me a bit but he is not a young spring chicken and have done a few before and even if his tools are old as he is, they are like him build from a era of quality and perfection...( I'm such a..hole licker, you can pay me later George)

Joke aside I really don't know what to think, for me 90psi is way lower then anything I work on before, these are very low compression engine so I walk in darkness territory here and I try to avoid stumbling on the carpet flowers...

For now my cranium Hamster need to be feed before he spin the wheel on high gear so i'll be back in a jiffy!