Cub Stuck Valve No Compression Cyl #4

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waynemo
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Cub Stuck Valve No Compression Cyl #4

Postby waynemo » Sat Dec 25, 2021 3:12 pm

After sitting for a month or so this fall, I could not start with a hand crank and magneto. Ran fine at the end of summer and the engine always ran good after an overhaul a few years ago and with very few hours on it. I think the rings are good but there is no compression on cyl #4. I am sure there is a stuck valve and I did get it started a week or so ago when it blew out black smoke before clearing up and ran good. But could not restart after sitting a few days.
What can I do to free up the valve? When it first would not start, I put a new coil in the magneto thinking it needed more spark but now I think that was not the problem and a waste of money!
I got the hood off now and can work on it easier.

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Re: Cub Stuck Valve No Compression Cyl #4

Postby staninlowerAL » Sat Dec 25, 2021 8:50 pm

I had the same problem with #3 cylinder exhaust valve and was able to work through the spark plug hole with a screwdriver and small hammer to tap on top of the valve, lubricating it with PB Blaster through a soda straw to free it up. It took a while but was better than pulling the head. You might also pull the valve cover and spray the lubrication on the lower end. Others have posted on the forum that using some 2 cycle oil in the fuel helps to prevent sticking valves. Might cause it to smoke a little but that's better than sticking valves. Good Luck Stan
:tractor:

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Glen
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Re: Cub Stuck Valve No Compression Cyl #4

Postby Glen » Sun Dec 26, 2021 8:25 pm

waynemo wrote: Ran fine at the end of summer and the engine always ran good after an overhaul a few years ago and with very few hours on it.

Hi,
If you or the shop replaced the valves, and valve guides during the engine rebuild, something that might not have been checked is the valve stem clearance in the guides.
The clearance needs checking, when replacing valve guides, and the hole through the guides needs reaming, if the clearance is too small.
One or more valves can be too tight in the guide if the clearance is too small.
Then a little carbon gets on the upper stem from use, and the valve can be too tight to close.
Or just sitting and not enough lube on the valve stem, and they can stick open.

Below is a page from the Cub and LoBoy service manual telling the valve guide diameters.
The intake is slightly different than the exhaust.
You can't check the sizes with the engine assembled.

For someone putting in new valves or guides, I would check the valve stem diameters too, to see if they are the same as the manual says, so the valves work with the guide sizes shown.
It's possible other brands of valves and guides have slightly different sizes.

http://www.farmallcub.info/manuals/gss- ... 001-07.jpg

waynemo
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Re: Cub Stuck Valve No Compression Cyl #4

Postby waynemo » Mon Dec 27, 2021 12:29 pm

I have a cub and use a mag with no battery with only a hand crank. The engine was overhauled years ago but seldom used. I got it running last week and it ran fine but stuck a valve after sitting again. I am sure the engine is in good shape with the stuck valve my only problem.
I was surprised how little you can see through the plug hole! I cannot see the piston and only a tiny piece of the valves. I rigged up a hypodermic syrenge and used it to squirt kerosene in the direction of the vales. I plan to rig up a test tank with gas and seafoam and run it awhile if I can get it running. I hope I do not have to remove the side valve plate. The manifold and tail pipe are in the way too. I am letting it soak and tapping with a wood dowel, it is hard to know if the valve stem is pushed against the lifter or not. I would think that it would be good if it was sitting with lots of clearance with the lifter and the spring assisting valve closure while soaking.

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Re: Cub Stuck Valve No Compression Cyl #4

Postby Gary Dotson » Tue Dec 28, 2021 9:14 am

Tap the valve closed and rotate the engine, if the valve doesn’t close, do it again, and again, and again, until the valve closes well on it’s own. Your’s having run recently, probably won’t take much. You might put some Sea Foam in your gas to help prevent future sticking.

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Re: Cub Stuck Valve No Compression Cyl #4

Postby tst » Tue Dec 28, 2021 10:49 am

hood is off, run the engine on a pony tank with the spark plug out of the cylinder with the stuck valve, now you can tap on the valve with a screwdriver and squirt oil on the valve stem it will free up


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