Compression update

[cub.bub]

I've been trying for a week to do this, tough time of year to be a home hermit but the wife was called into work
I felt somewhat guilty......but I'm over it now
So here it is.....

Compression test warm
Cyl. 1. 3. 4. 2
Dry. 95. 92. 92. 98
Wet. 95. 95. 93. 98

Plugs were black with minor soot on them but dry.
Guys I'm getting smoke out the exhaust, not real bad , but.....
I'm thinking fuel too rich? I do feel better after the compression test
Any opinions,suggestions are welcomed and appreciated, well back out to put the hood back on...they're calling for snow tonight. Merry Christmas! Bruce

[staninlowerAL]

Just curious, soot on plugs = incomplete combustion. What heat range plugs are you using? Might consider a hotter plug. Also maybe a tank of higher octane fuel with some seafoam to clean things up. Stan

[Bigdog]

Sounds about right. I wouldn't let a little black smoke bother me. Work the tractor, add a little Marvel Mystery Oil to the gas and oil and keep on keeping on.

[cub.bub]

I'm running a tank of 92 octane with sea foam in her right now with champion D-15 Y plugs.
I've only run around half a tank thus far and was thinking the Seafoam might be the cause of the soot? Or plug range.
But feel better than when I started.
Bruce

[gitractorman]

The black soot is really common. As I said, these Cubs tend to run pretty rich. I think it's due to the updraft carb setup and they just suck up so much fuel and don't burn all of it. Anyway, I'd definitely go with the Sea Foam in the gas and maybe a set of D-21 plugs. They're a bit hotter and may just due the trick for you.

[Barnyard]

Compression test warm
Cyl. 1. 3. 4. 2

Just curious, The cylinders are numbered 1 -2 -3 -4 from front to back. Is this what you mean or did you check each cylinder in actual firing order

[cub.bub]

Barnyard, I took them in firing order....don't ask me why ....
And I think I have a set of the D-21s I bought for another tractor, I'll stick them in and see what happens.
BD, the (soot)is kinda like when you don't have your torch set right.....bad example, but it's all I got
Again Thanks to you All for the help.
Bruce.

[Scrivet]

Personally, I wouldn't worry about a little smoke with compression numbers like that. If you can't see where you're driving through the smoke then I would START to be concerned. The time will come when you've got it apart for something else and the "might as wells" or "mushroom theory" will take over.

[ShakerPrairie]

I haven't looked at compression specs or figured the percentage of difference in your readings but to me that looks pretty decent.
The D-21's seem to be one of the favored plugs on this forum, I'm sure for good reason.
They're pretty high in the heat range and for general light use they would be a good choice.
On the only Cub tuneup I've ever done I removed some old D-21's and used Autolite 3116's as I try to keep a few sets of those onhand.
They're a colder plug than a D-21 but the Cub runs good and burns a pretty clean stack.
How much colder they are than a D-21 I'm not sure because spark plug cross reference charts seem to make less and less sense as time goes by.

[Clark Thompson]

The compression is the bare minimum. with optimum compression (110 to 130 ) and if everything else is correct the cub engines do not tend to run rich. If running rich and soot build up on the spark plugs is happening then there is something wrong. Carb, ignition or a valve problem. I have yet to see a cub engine that didnt need a proper valve job. If your getting smoke out the exhaust that is black then that would tell me its a carburation / ignition issue. If your getting blue black smoke with your compression numbers so low you have a ring problem. But I dont know too much about sea foam! Other than years ago I used some in a 302 ford engine. with in 30 minutes the rod and mains were shot!

[Eugene]

I dont know too much about sea foam! Other than years ago I used some in a 302 ford engine. with in 30 minutes the rod and mains were shot!

Really. Gonna have to explain. Couldn't have been some other problem?

[Boss Hog]

I've been trying for a week to do this, tough time of year to be a home hermit but the wife was called into work
I felt somewhat guilty......but I'm over it now
So here it is.....

Compression test warm
Cyl. 1. 3. 4. 2
Dry. 95. 92. 92. 98
Wet. 95. 95. 93. 98

Plugs were black with minor soot on them but dry.
Guys I'm getting smoke out the exhaust, not real bad , but.....
I'm thinking fuel too rich? I do feel better after the compression test
Any opinions,suggestions are welcomed and appreciated, well back out to put the hood back on...they're calling for snow tonight. Merry Christmas! Bruce

the numbers are in line but maybe a little low, I am betting you checked them with the starter and only one plug out at a time. Check them one at a time with the coil wire in it should start up. I will bet the numbers come up.
You will here all kinds of ways to check compression, I have found this to be the most accurate way . That is running,
If it starts and runs good I would not worry too much about it. The D 21s are the way to go.

Boss

[Bus Driver]

Since the Cub has a compression ratio of 8 to 1 or less, the 92 octane is not at all needed. And since the higher octane ignites slower, it might contribute slightly to the sooting problem.
The compression testing technique typically is to remove all the plugs, open the throttle fully, be sure that the battery is in top condition and test with the engine spinning freely, no compression resistance from the other cylinders.

http://www.linnbenton.edu/auto/perform/compress.html