Results of compression test

[jbfjr]

Since this Cub hasn't run in about 17 years, I shot a liberal amount of WD-40 in each cylinder. Note in my previous posting I mentioned that I found dirt in the valve compartment due to a missing cover bolt.

Here's what I measured:

Front Cyl. (1?) -- 40 psi
Second ---------- 85 psi
Third ------------- 65 psi
Fourth ------------ 75 psi

I then squirted about a teaspoon of oil into each cylinder and rechecked:

Front -------------- 55 psi
Second ---------- 105 psi
Third ------------ 110 psi
Fourth ----------- 105 psi

I guess there's no denying that the front cylinder has a severe problem. I'd like to hear some opinions on whether or not a complete overhaul is in order, or just a valve job.

Thanks again,
John F.
Richmond, TX

[Donny M]

You can try a leak down test but it sure looks like a valve job is in order. Removing the head will give you a better idea.

[Eugene]

At a minimum I would pull the head and pan. Plastiguage the crank journals. After observation, make a decision.

Actually, I would just pull the engine and do a complete overhaul. Rather than attempting a piece meal approach.

Eugene

[George Willer]

At a minimum I would pull the head and pan. Plastiguage the crank journals. After observation, make a decision.

Actually, I would just pull the engine and do a complete overhaul. Rather than attempting a piece meal approach.

Eugene

I've been told the piece meal approach is like being kind to your dog by cutting his tail off a little at a time.

[jbfjr]

I pulled the head and wire-brushed the valve seats, cleaned the gasket surface and checked the gasket, which looked fine. I torqued the head bolts to 50 ft-lbs, and re-ran the compression tests.

No improvement whatsoever! Looks like I'm in for a re-build.

[TexCub]

Well, on the bright side, sounds like no broken head bolts!!

[evielboweviel]

other option is to buy a good used engine from Ralph. much cheapier than rebuilding one
Ron

[Larry Barb Dotson]

An engine that has set for 17 years could very easily have stuck rings. It sounds like # one cylinder is especially bad. If you have it back together now, I would add cylinder lube to the fuel and run it for a while to see if it might lossen up the rings. If after a few hours of running, the compression starts to come up, then continue with the lube. You may even pull the # one cylinder and see if the rings are stuck. I wouldn't be too quick to replace the engine as new rings and a valve job are not that difficult and you then would know what you have..
Larry Dotson

[Chris Todd]

I'll 2nd that Larry....worth a try

[ScottyD'sdad]

I'll agree with Larry.I had a cub which hadn't run for years. Smoked like you wouldn't believe. Treating the cylinders thru the sparkplug holes ,using Kreen (a Kroil product) and turning the engine over to coat evenly. Did this several times over a weekor so.cleared it up. The rings can stick ,in my case, oil rings, Yours, compression rings . Worth a try. ed

[jbfjr]

Thanks guys, I just happened to have purchased some Kroils, so I'll give that a try. Before I can run the engine, I have to rebuild the carb and do some ignition work, but I'll give it a try.

Before that I think I'll try a leak down test and see what that tells me.

Thanks again,

John F.

[JimT]

If you found dirt behind the valve cover, maybe a small piece of dirt is stuck in the valve seat causing the low reading. Valve could have been in the open position while sitting for 17 years. Now the valve closed and compressed the dirt. Just a thought.

JimT

[Phillip W. Lenke]

A question from the unlearned. What is a leak down test and what does it check .
Phil

[George Willer]

A question from the unlearned. What is a leak down test and what does it check .
Phil

Phil,

It can be a little complex to use one to determine the exact amount of wear in an engine, but in its simplest form can be useful to all of us. Basicly, air pressure is introduced to the cylinder and the sound of escaping air tells where the problem is. Heard mostly from the crankcase breather... Rings. From the exhaust... Exhaust valve. From the carburetor... Intake valve.

A spark plug with the center knocked out and brazed to a male air fitting is all the equipment needed for the simple version.

[Phillip W. Lenke]

Line air air pressure?