CA Rebuild - Engine Compression

After putting my 1951 CA back together, it would not start even using starter fluid sprayed into the carb air intake.  The starter works great and it has spark.  Compression is only 15 to 20 psi in each cylinder. Should this be closer to 100 psi?  Should my next step be to replace the piston rings or could the non-starting problem be elsewhere?

The shop manual might help:

http://geraldj.networkiowa.com/Trees/Allis-Chalmers-G-B-C-CA-Service.pdf

Valves not closing completely could kill compression as could very worn cylinders and piston rings.

Gerald J.

Check valve clearance. That is easy to do.

I am sure (if) you have not removed the pistons to check the condition of both the sleeves and pistons that some of the rings are stuck and not expanding to seal the air in the firing chamber. New rings may even not be able to raise the compress to where you get full power. You eyes are never precise enough to know if the piston and sleeves are close enough to spec to reuse. If the sleeves are pitted bad where they were stuck they would always give you problems even it inside diameter is within spec. Without rings and the piston down in the sleeve and you can get a feeler gage of much more than .010 down one side of the piston you will still get blow by out the breather with oil mist. Very hard to adjust a carburetor to solve running problems with low compression.

No way can you guess what condition of pistons, rings or sleeves without measuring in some fashion. A feeler gage, a straight edge, and a flashlight will get you in the ballpark if used correctly, if you do not have access to measuring instruments.

Air coming out the oil breather is called blow by. The term blow by means combustion gasses are going by the rings into the oil pan and then out the breather pipe when the cylinder fires. In your case with it not running it is just air. The rings need to be free and lose in the ring groove to have the ability to expand out against the sleeve wall to seal all air and gasses in the firing chamber. I have used seafoam on a running engine to loosen up oil rings to stop smoking over time. Most of the time when all rings on a piston are stuck they have to be pried out in pieces.

If a valve is leaking the air will either come out the exhaust or the carburetor.

Valves clearance will need to be .010 to .012 at top dead center on the compression stroke. I always check with both rocker arms on the same cylinder in the up position. With both rocker arms up both valves are closed so compression can be built. You can turn the pushrods with your fingers at that point as well as insert a feeler gage between the rocker arm pad and the top of the valve.

When a four cylinder engine sets where water can get in thru the exhaust only two cylinder will fill with water and rust fast normally. No way to know the condition of the engine before it was let to get water in.



Edited by Dick L - 27 Nov 2019 at 3:02pm

Dale,

Dick L: 1-thanks for info on blow by.  Seems to verify problems with piston rings. 2-I had already adjusted the valve stem and rocker arm clearance to .012, but I'll check again if/when I take it apart.  Based on past experience I think the engine should fire even if these aren't adjusted exactly. 3-I bought the tractor 21 years ago and took the head off, had a valve job done, reinstalled the head, and its been stored in the barn since then. If there had been signs of a lot of rust & corrosion in the cylinder heads at that time I probably would have either worked on that problem first or used it as yard art.  My notes from 21 years ago had nothing about rust & corrosion and engine being seized - only that it wouldn't start and a list of replacement parts I needed.  I'm thinking that the engine seized within the last 21 years while under cover. The oil was drained and the oil pan was off during this time which probably didn't help matters. 4- I did adjust the timing (which is another long story).  So I think compression might be the next logical step to address in trying to get the engine to fire.  Let me know your thoughts on suggested next steps.

You have a lot of good help by your side here, follow their advice and you will get it running.

One thing I would suggest is you stop screwing 1/4" NPT nipples into your spark plug holes, before you damage the threads in the head. Either purchase some correct 14 mm to 1/4" pipe adapters or make a couple by welding or brazing a steel pipe nipple to the metal shell of an old spark plug (break and clean all the porcelain out of the shell first). These should also give you a better seal during testing. I'm surprised your compression tester didn't have a 14MM adapter with it, 14 and 18 mm are with most testers. Have you tried your compression tester in an engine, in running condition, to see that there is not a problem with your tester? Is the check valve in it working properly?

When you positioned the engine at #1 TDC to check timing, did you check to see that the valves on #1 were closed (rockers loose, full clearance) and #4 cylinder valves were "on the rock" (no clearance, rockers tight). (I would set the valves at .014 cold as .012 is the hot spec.) Clean the flywheel good when in that position, you may find a faint mark. If not I would at least paint one on it for reference while working on the engine. Rotate the engine clockwise, if your facing the fan, so gear lash doesn't lose a few degrees on you.



Edited by Jim.ME - 29 Nov 2019 at 6:03am

Dale,

Edited by Sugarmaker - 29 Nov 2019 at 8:20am

Thanks again for everyone's input.

Edited by BrianC - 29 Nov 2019 at 3:22pm

I second what Dick L suggests, the marks can be hard to find. Get a brush as he suggests. Remove all the plugs. Working through the timing hole in the torque tube, use a a drill to run the brush against the flywheel while spinning the engine over with the starter. Dick can confirm this, but I believe the timing hole on a CA is on the left (setting in the seat), even though the manual says the right side. It may take several tries to clean the flywheel up, but you should find the marks. Then use the valve rock method to locate #1 at TDC and confirm it is on the compression stroke. Also you can carefully use a wire or plastic tube to reach in through the sparkplug hole and touch the piston. You will feel the piston come up (so you can watch for the marks to show in the timing hole), when it stops raising and starts to drop, you went by TDC, again #1 rockers clear of the valves and #4 cylinder valves are on the rock (both tight as exhaust closes and intake opens), will confirm #1 is on the compression stroke.

Normally with all the plugs out and the transmission in neutral one can use the fan to turn the engine, you might have to put some pressure on the belt if it slips.

You said the pto doesn't work. Did you engage the pto using the lever? You might have to turn the shaft a bit while pulling back on the lever to get it to engage.

If you haven't, I would suggest you save a copy of the manual Gerald J. posted to your computer and print off the CA section to have with you in the shop as you work on your tractor.

jaybmiller: I used a $9 spark ignition tester (connects to spark plug wire and the plug) and the white light flashed constantly on all four cylinders.  There were no directions for its use, so I'm not sure if it can test for strength of spark.  Do you think I should test them manually to see if it has a fat blue spark?