How to compression test M18?

[o---o]

What is the basic procedure for a compression test? My M18 is no longer running properly and I want to do a compression test as I work on it.

It was cold outside, about 30* F, and my 1541 would crank but not start. After trying for a bit I quit. There was also a lot of moisture floating in the air and everything in the garage was saturated.

The next day I tried it again and as soon as it cranked it made a HUGE backfire. Wife even heard it in the house! Still didn't start.

Let it sit a few more days until the temp outside was about 50* F and tried again and it started, but ran very rough. Lots of black smoke as I increased the throttle at the beginning, but cleared up after that. When I increase the throttle it seems really sluggish and the engine doesn't have power like it used to.

Any ideas on where to start?

[jsoluna]

I would start by pulling the plugs and taking a look at each cylinder to see how it is firing. You can do a compression test at this time. The Magnum twins do not have a compression release, so you can do a compression test as you would any engine. Wide open throttle, no choke, both plugs removed. Crank until the gauge stops climbing and record. Factory manual states you need at least 90 PSI.

Your backfiring could be related to many different factors including fuel metering, valve sealing and lash, and compression. With the engine running, use a infrared heat gun and point it at the exhaust manifold for each cylinder. If one cylinder is running lower temp than the other, you have narrowed down your lack of power concern. My bet is that the backfire caused a head gasket to blow. If they are both running equally, then you have another problem - I would check your carburetor adjustment and make sure you aren't running lean or rich.

My shop did a video series that goes over doing a full service/refresh on an M18. It may be time for you to do the same.

https://youtu.be/08Y92lGjaKc

[ol'George]

All good ideas posted other that you will likely see about #90 compression which is acceptable, as these engines are low compression ratio.
Remove the spark plugs and lay them on the head with wires attached and crank it over over, you should have good spark, and the plugs should look normal with a color of Nestles Quick (light brown)
They are prolly fouled.
Also check that the oil is not contaminated with Gasoline as the carbs sometimes leak because of crud under the seat as well as the fuel pump diaphragms rupture sending fuel into the crankcase.

Also note Chit tends to accumulate around the magneto coil on the right side under the cooling tins where the wires come out of them.
Every engine that I have worked on that has seen lawn duty, accumulates a lot of chit around the coil causing overheating of it as well as impedes the cooling of that area of the cylinder fins.
Let us know what you find.

[o---o]

Perhaps it's my lucky day?

Throttle open, both plugs out, and crank a couple seconds until the gauge quit climbing and I had 100 on one side and 110 on the other! This is a relief as I was really hoping for no internal problems.

For my knowledge, which is cylinder 1 and which is cylinder 2?

Spark plug test - one plug had decent spark and the other had nothing. I swapped the plugs from side to side and the result was the same. So one plug did spark on each cylinder, and the other sparked on neither. Also, although likely obvious, the plug that didn't spark was black and smelled like unburnt gas.

Maybe all I need is some new plugs?

Pictures below of the plugs and engine general appearance.

[ol'George]

FWIW:
That is a wasted spark system, which means both plugs fire at the same time.
One is firing on TDC, and the other is on the exhaust stroke.
I don't remember which is #1 cylinder, without looking it up.
Your picture of the fins is not where they plug up with Debris.

[jsoluna]

Originally Posted by o---o

Perhaps it's my lucky day?

Throttle open, both plugs out, and crank a couple seconds until the gauge quit climbing and I had 100 on one side and 110 on the other! This is a relief as I was really hoping for no internal problems.

For my knowledge, which is cylinder 1 and which is cylinder 2?

Spark plug test - one plug had decent spark and the other had nothing. I swapped the plugs from side to side and the result was the same. So one plug did spark on each cylinder, and the other sparked on neither. Also, although likely obvious, the plug that didn't spark was black and smelled like unburnt gas.


Maybe all I need is some new plugs?

Pictures below of the plugs and engine general appearance.

Some terminology:

Cylinder #1 is closest to the flywheel, which is the side with the oil filter lines. Cylinder #2 you will notice is further towards the front of the tractor/rear of the engine (the engines are technically mounted backwards in a Cub).

I'd say your spark plug diagnoaia is correct.

I use Autolite 26 or NGK UR4 in the Mag Twins vs the Champions. I've moved away from Champions due to fouling issues.