Here's my story it's sad but true.... (or low compression)

[CharlesFCook]

So my 1947 Cub got to the point where I could not get it to start without pulling it. Then it did not have much power. SO this could be several things. I had rebuilt the carb a few months ago and cleaned out the fuel tank. Then the cub sat for 2 years because I was on the road non stop. So I finally had some time and started to look at the cub to see what might be the problem. I pulled the spark plugs and found big drips of water on the ends of the plugs. The coolant was down about a quart. This made me think I have a blown head gasket. The oil was not creamy however. I did not have access to a compression tester at the time so I just took the head off.

Here is what I have done so far...

Broke a head bolt taking the head off. Had it drilled out. I wanted the guy to weld a washer and nut on it but he did it his own way.

I replaced all 4 intake valves 3 had very pitted heads and 3 had cracks in the steams.
I lapped all of the valves.
I adjusted the lash on all of the valves.

Put everything back together.
When I torqued the head down the drilled out bolt hole bolt did not feel like it was going to take all of the 45lbs so I stopped on that bolt at about 40lbs.

Now I have low compression on all cylinders:
1 - 30 lbs
2 - 30 lbs
3 - 10 lbs
4 - 75 lbs

I wonder if I lapped the valves to much or too little? I just did the lapping until I had a grey strip around the valve face.
Could the rings be bad? The cylinders look smooth.
Cracked / warped head? I checked it with a steel straight edge and a feeler gauge and it seemed OK.

What should I do next to figure this out?

Thanks for your help.
Charlie

[Dave (69 lo-boy)]

Try doing a wet compression test (with a bit of oil in the cylinders) and see if the numbers are different, if they are higher it is probably the rings.
Dave

[CharlesFCook]

I did add some oil to the cylinders but I did it up front so I don't know what it was before...

While I had the head off I put some oil in the cylinders to keep it from rusting and I noticed the oil was gone in a few hours. I wonder how long it should take for the oil to drain away?

[bob in CT]

You need to adjust your valves (double-check this) and do a dry compression test, then a wet one. In that order. If the numbers come up with the wet test, that indicates rings. If there is no change then you problem is likely with the valves. Those numbers are exceedingly low.

[ScottyD'sdad]

Rings could be stuck, from sitting the 2 years,so they don't expand to tighten the gap, around the pistons. I've had some so stuck, I've had to chip them out of the piston grooves, with a small chisel. Bore was ok, I honed the bore, and replaced the rings.
Ed

[CharlesFCook]

I did re-adjust the valves before the last compression test (what I posted).

To do the dry test now that I have some oil in the cylinders should I spray some carb cleaner in and crank it over a bit?

Any Ideas?

[Eugene]

As I read the posts, the compression test results were the wet test. Results are so poor - no need to do a dry compression test.

Suggest pulling the engine. Plastigauge the rod and main bearing, then remove the pistons. Measure everything to determine any needed machine work and parts.

[ScottyD'sdad]

Eugene is right. If it tested that poorly, it needs attention, not more tests.
Ed

[bob in CT]

I doubt if you will find a 68 year-old engine that does not need to be "freshened up". That said, I would still do a wet/dry to eliminate my valve job as the problem, however unlikely. It costs nothing and only take a few minutes. Then I would take it completely apart and go whole hog. If you tear the block down, I would put in hardened valve seats, too, for the exhaust valves.

[CharlesFCook]

I like Bob's idea to eliminate my valve job as the problem before I go whole hog.

How do I get back to the Dry test stage?

[Clark Thompson]

When replacing valves with new valves the seats in the block have to be properly ground. Lapping in valve is not the way to go.Waste of time. I would remove the head again. Take all the valves out (intakes also) inspect the seats. I have yet to see any cub engine not need the valve seats properly ground or replaced.Then after the seats are properly repaired then a lapping can be used to indicate the position of the seat on the valve contact. Optimum contact would be in the center of the valve angle contacting the seat. Or close to it.If reusing old valves they must be ground also to take out any out of round wear and or wear in the valve face. Keep in mind that you must maintain the proper margin on the valve or the valve will not last long. Then when that is all proper the valve lash must be reset on all valves. Grinding seats or replacing seats changes the valve lash.

[CharlesFCook]

Thanks Clark and all others for your help.

Where / what kind of a shop can do valve seats? Locally I don't have much for repair shops. We do have a machine shop and a small auto repair shop.

[Bob McCarty]

I take mine to a local Carquest, but any auto parts store should be able to recommend someone.

Bob

Edit: Some auto parts stores have a machine shop (like my local Carquest). If they don't, they should be able to give you suggestions.

[jljmonky]

My mechanical experience has been mostly learned in books, I have yet to tear down my cub (cuz it ain't broke) and the last small engine I tore down had a hole in the block so I didn't win that rebuild but, just a thought... if the timing were off couldn't compression be greatly affective? Engines don't just drop all compression without burning holes in pistons and since the head was off that would have been noticed...

[Dave (69 lo-boy)]

Ignition timing will not affect compression, valve timing could do it, but unless the engine was recently assembled that should not be an issue.
Dave