Cub Compression Questions

equeen · Jan 2, 2013

1. Approx how much clearance between top of piston and bottom of head above piston when flat-top piston at TDC?

2. Carbon deposit on top of piston would reduce this clearance, thus increase compression (yes?). That's bad? Why?

3. If everything else in order, should a '50 Cub engine start (no matter how weak it ran) with cyl compression of 70-65-80-75 cold and dry?

4. Is it reasonable that question #3 compression would rise to 90-80-95-90 for wet test? If everything else in order, should the engine start?

Trying to learn in general and in regards to a specific problem - engine won't start.

Thanks

buzzinduzzin · Jan 3, 2013

#2: yes, bad, glowing carbon can cause detonation & pre-ignition #3 : yes #4:wet? oil squirted in plug holes?,yes

Oldiron29 · Jan 3, 2013

I have one I dragged in that has wors compression
That that and it runs. With bad compression you have a hard time getting it to suck up gas to fire. If you flood the carb it will start. I have adjusted the valves no better. So either it needs rings or valves and seat ground or both.
oldiron29

El Toro · Jan 3, 2013

You need to check the fire to your sparkplugs should have a hot blue spark. Hold your hand over the throat of the carburetor while a helper makes an attempt to start the engine. Low suction means low intake manifold vacuum. If you have a distributor make sure you're getting battery voltage to your coil. With the ignition switch in the "on postition" and the points open you should have voltage on both small terminals on your coil. Take some 320 wet or dry and rub your points until they're shiny. Hal

rustred · Jan 3, 2013

if you are confident everything is to spec, give it a pull with another tractor. i have seen this before where they wont start when sitting for a long time. then after the first start they start good.

screwstick · Jan 3, 2013

The clearance between flat top pistons and the head is the thickness of the head gasket.
With (relatively) low compression, it will be harder to start.
Do you have a Magneto Ignition?
I agree with rusted, probably gonna have to pull start the tractor to get it to run.

equeen · Jan 3, 2013

The tractor is now running. It is magneto ignition.

Thanks VERY much for all the responses.

I was especially interested in the "clearance" question because for years I've been thinking that with all that cylinder cavity, etc. that there was lots of cavity for the ignition phase. Then I remembered that I had a head ground on an 8N and was told that as long as the piston didn't bump the head, without gasket, then there would be adequate clearance with gasket. Thus just thousands of an inch space between top of cylinder and head when ignition occurs. Heavy carbon build-up would reduce space even further.

Yes, we cleaned carbon off piston heads, valves and the head.

Son and I are happy campers to have this old engine running nicely even if the engine is essentially worn out. Good oil pressure as well.

Cub Compression Questions

equeen

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buzzinduzzin

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Oldiron29

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El Toro

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rustred

Well-known Member

screwstick

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equeen

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