Cub engine questions

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Cub engine questions

Postby carmanic » Thu Dec 06, 2018 1:37 pm

Hello All, I'm in the process of resurrecting a '58 Lo-Boy that "ran when parked" (unfortunately that was outside and 15+yrs ago), and have a few engine-related questions;

1. What is generally considered an "acceptable" low-end compression reading ?
2. Same question regarding oil pressure ?
3. Can the valve clearances be set w/o removing the carb or manifolds ?

Thanks in advance, more to follow.

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Re: Cub engine questions

Postby inairam » Thu Dec 06, 2018 3:35 pm

See what to do with the cub you just brought home


1. What is generally considered an "acceptable" low-end compression reading ?

When it is extremely hard to start, have heavy blue smoke or when is does not have the power to do what you want.
The most important thing is the number is consistent in all 4 cylinders. 85 83 87 85 is better than 100 80 45 95
If it has not run for 15+ years I would get it running and use a lot of seafoam in the gas and oil for a while and then see what the compression is.

2. Same question regarding oil pressure ?

Again get it running and loosed up before you worry about the pressure. As long as the needle moves.

Make sure you prime the oil pump. It has been sitting for a while. I would also change the oil.

3. Can the valve clearances be set w/o removing the carb or manifolds ?

you do not need to remove anything but the valve cover but is it a lot more difficult with the carb in the way. You do not need to remove the manifold.

I think the manual says 0,013" I tend to go 0.015. I would adjust the valves after it ran for a bit with seafoam in the fuel and oil and before I did a serious compression test.
When you only have 9 horsepower you need to know the names of all of the ponies!

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Re: Cub engine questions

Postby Glen » Thu Dec 06, 2018 7:48 pm

Below is a post I made about priming the oil pump, it is about half way down the page.


The Cub service manual says a Cub engine has 120 lbs of compression, that is probably for a new engine. Some less is ok, they still work.

I would give the engine a tune up, Cubs need a tune up, like cars years ago that had distributors, and ignition points. The points could be corroded from sitting, then it won't run.

Below is the 1957 Cub LoBoy owner's manual, the experts on here recommend reading it. It has lots of info about operation, maintenance, and lubrication. There is a table of contents on page 1. ... index.html

It probably has a Battery Ignition unit, the info for those starts on page 42. It shows the points there, and setting the point gap.
You can file the points if they are not burned too much, or replace them. I usually take them out to file them, the metal is usually hard, I hold one piece at a time on a hard surface and file it flat with a large flat file. Wipe off all filings with a clean rag.
It could have a magneto, they are in the manual too. Identify which it is, and disregard the info for the one you don't have.

Below is a listing at TM Tractor for new points and condenser, you can look at them.

The engine needs the timing set with a timing light after setting the point gap, and replacing other parts. The distributor cap, rotor, spark plug wires, and spark plugs, might need replacing too.
You should take out the spark plugs and see how they look, clean them, and set the gaps. Or replace them if needed.
Clean all dirt from around the spark plugs before removing them, so it doesn't fall into the engine.

Lube the fan, if it still has the original style fan. They use motor oil. It shows how in the owner's manual. People have posted on here about fans that seized from low oil in them. Good luck with the LoBoy. :)

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Re: Cub engine questions

Postby jsfarmall » Thu Dec 06, 2018 8:27 pm

Just unbolt the valve cover and let it drop down. If you have touch control it will sit right in behind the hydraulic lines. Will give you room to adjust. Absolutely prime oil pump first as stated above. As long as your oil gauge moves out of the red you have enough pressure.

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