Low Compression

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Wbdisco
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Low Compression

Post by Wbdisco » Tue Apr 04, 2017 3:54 pm

I've been lurking here for a while and decided to go ahead and ask a fairly straightforward question, I think. I have sort of inherited/borrowed a mule from my grandfather on the condition that I could keep it as long as I needed it if I could get it running. Needless to say it has been sitting for years. Over the past year I have replaced most parts on the motor (carb, fuel pump, plugs etc). A majority has been bought from John Emery. I got the mule running and driving but it is very underpowered, it has to be in low range to pretty well move without bogging. I noticed a hissing sound coming from be head and checked compression 110# on left and 75# on the right. I did the squirt bottle test and found a leak between the head and jug. I pulled the head and found a copper spacer and John said they were used on mule heads to compensate for a longer valve stem. After doing some inspecting, I found that I think it has the newer 2AO42 heads with the almost flush valve stems. Would there be a problem with removing the spacers? Also the cross hatching on the cylinders looks almost new. I talked to my grandfather and he said he had a guy in NC do some work on the engine probably 20 years ago. Am I wrong in thinking he may have put the newer heads on and left the spacer in? Could that be causing my loss in power? I couldn't ever get the leak to stop unless I removed the spacer and torqued the head down. If the spacer was left in I had to over torque the bolts to stop the leak. I realize that diving into someone else's project is usually a pain, but I am not really doing a restore, just using the mule around my property for hauling firewood and stuff. Thanks for the help.

Brad

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Re: Low Compression

Post by muleman7 » Wed Apr 05, 2017 8:29 am

Brad

You found the problem. The 2AO42 head must be mounted WO the copper head spacer.

The AO42 heads used longer valves [keeper groove to top of valve stem] and used the spacer to compensate. I understand this was to create lower compression thus providing higher low end torque for use in a vehicle.

The 2AO42 engine WO the spacer was 6.9 to 1 compression ratio while the AO42 engine with the spacer was 6.5 to 1 compression ratio.

Using the spacer with the 2AO42 head will cause the hydraulic lifter to fail to maintain proper functioning and cause running problems and possible engine damage.

See page 9 of Lance's My First Mule viewtopic.php?f=79&t=267953 describing engine damage due to running 2AO42 heads with spacers.

You are correct that the 2AO42 valves are nearly flush with the top of the valve rotator where the AO42 valve is above the rotator. The AO42 valve is 0.050 longer

See my Post AO42 Mule vs 2AO42 Power Plant Engine Differences viewtopic.php?f=148&t=262382

I would install the head WO the spacer and then check compression. You should get about 145# if new.

The only problem if it can be called a problem is one side could have the AO42 head at 125#s while the 2AO42 head at 145#s.

If concerned I can supply the 2AO42 valves so you can change the AO42 head to a 2AO42 configuration providing a balanced system.
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Wbdisco
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Re: Low Compression

Post by Wbdisco » Wed Apr 05, 2017 10:28 am

I pulled both spacers out, and in process of piecing everything back together. The shroud drives me nuts, I should have just pulled the motor. Hopefully there isn't any permanent damage. Thanks for the help, I'll keep an update when I get everything back together.

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Re: Low Compression

Post by Wbdisco » Wed Apr 05, 2017 4:31 pm

Quick question, does the engine have to be warmed up for the rings to expand to test compression correctly? I seem to be hearing a little leak down coming from the rings. Everything is right on the top end, but I can still hear a hiss. I may try to put a little oil in the cylinder to test it out.

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Re: Low Compression

Post by muleman7 » Wed Apr 05, 2017 7:10 pm

Both pistons travel together TDC and BDC thus 42 cu in displacement each 1/2 revolution. You might be hearing crankcase air exchange.

allowing for ring end gap you may notice a hiss as you turn the engine slowly by hand. 110-125#s should give you good performance.

Oil will help to seal rings on an engine that has been sitting for a while.

Have the throttle wide open to allow a full charge of air when checking compression. If the magneto is not off install spark plugs on the ignition wires so the spark has a path.
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