Cub compression

waugh621 · May 14, 2023

I have a cub that will smoke while running (worse under load) i had a freind of mine tell me it may be the cylender rings and to test the compression. So he brough his tool out and checked the cylenders and got
1. 120 psi
2. 110 psi
3. 120 psi
4. 120 psi
He said those numbers didnt seem low and based on what i see online are pretty close to what they need to be. What should me next step be from here. I know it is burning a little oil cuase the spark plug in cyl. 3 is oily after running.
Thanks Nate.

RBnSC · May 14, 2023

An oil ring could be stuck or an expander ring broken or a bad valve guide. Normally I would say valve stem seals but I think most cubs were flat heads may be wrong about that.
Ron

grandpa Love · May 14, 2023

Cubs are flat head

grandpa Love · May 14, 2023

That's good compression.... I've had them crank and run on 60-65! As far as the smoking, work it hard for a while. Lots of our cubs smoke for a while after we bring them back from the dead! Running them almost always makes the smoke go away.

We also use 15-40 diesel motor oil in ours.

waugh621 · May 14, 2023

I wouldn't say i have ran it hard but i have run a yard roller around with it. It probably ran for a couple hours and the smoke didn't clear.

grandpa Love · May 14, 2023

I've had to mow for 2-3 hours on some....

warbaby · May 14, 2023

Your compression is fine so I'd suspect whopped out valve guides and maybe a clogged (or wrong type) breather cap.

Also make sure the crankcase oil isnt overfilled or diluted with gas or coolant.

wore out · May 14, 2023

Good compression indicates the ''compression rings'' are sealing.

It has little or no bearing on the condition of the ''oil control rings'' which may be stuck, broken, or simply worn so the ''scraper edge(s)'' are no longer sharp.

The one cylinder you mention having an oily sparkplug has an oil control ring issue and is likely ''pumping'' enough oil to cause most of the exhaust smoke.

Janicholson · May 14, 2023

Flat head engines generally do not have oil consumption from valve guides. The valves are hanging downward and get lubrication from oil mist in the tappet chamber. So I would think it needs to be worked hard enough to open the governor for several hours. If then there is no change, I would use a bore scope, or inspection camera to see if there is a dramatic top ring ridge. These are not sleeved engines, and would need to be bored and honed for oversized pistons. Jim
Inspection camera less tham 20

Cub compression

waugh621

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RBnSC

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grandpa Love

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grandpa Love

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waugh621

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grandpa Love

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warbaby

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wore out

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Janicholson

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