Mule Stopped Running

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RBW
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Mule Stopped Running

Post by RBW » Sun Jun 26, 2022 4:30 pm

After running like a sewing machine for months, my Mule is running like garbage. I first noticed it a few weeks ago - sounded like it was running on only one cylinder after initial start. Seemed to notice a bit more valve noise than usual. After running for a bit, it smoothed out and had plenty of power.

After sitting for a few days, same problem again. After a few days more, same issue, but took longer to resolve. It had sat for about a week and I tried to start it today - worst it has ever run. Stumbles, won't stay running, intermittent acceleration.

Swapped plugs with new, no change. Swapped plug wires with new, no change. Adjusted points, no change. Replaced condenser, no change. Bypassed fuel tank and filter, no change. Checked carburetor inlet screen - all clean. Removed fuel pump return line and checked pressure - when engine is at stumbling idle and with finger over return line, there is still pressure. Engine will accelerate briefly, but eventually stumbles and dies. Removed rocker arm covers to check valves for movement - lifters appear full and valves do not stick when engine is turned over by hand. Magneto timing is good. Checked compressions - left is a little low at 90. Right is 110. Still should be plenty to permit normal operation. Engine has always had good power before all this and was not burning oil.

Really at a loss as to what is going on. I have been running the machine on aviation gasoline. I though maybe the lead is causing the valves to stick, so thought I might change out with auto gas. I have had that issue with small engines before.

Would appreciate any ideas. I've been at this thing all day with absolutely no improvement.


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Re: Mule Stopped Running

Post by Chuck W. » Mon Jun 27, 2022 7:32 pm

How old is the carb?

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Re: Mule Stopped Running

Post by RBW » Tue Jun 28, 2022 3:07 pm

Carb is NOS. Worked great when I installed it.

Does the carb incorporate a sort of accelerator pump? I did notice a fair amount of fuel being admitted into the intake tube when cycling the throttle shaft - enough that it would actually leak out of the tube with the J tube removed.

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Re: Mule Stopped Running

Post by Chuck W. » Tue Jun 28, 2022 3:22 pm

Does the carb incorporate a sort of accelerator pump?
Yes.
What brand spark plugs are you running? I've had a few engines that just didn't like Champions, changed to Autolites and ran fine.....

To me, the carb is the weak link, when all else fails, try another carb.

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Re: Mule Stopped Running

Post by RBW » Tue Jun 28, 2022 4:04 pm

Autolite 2223. Swapped with new just to make sure this wasn't the issue. I don't have a spare carb, but may have to get one.

Odd to me that, leading up to this, it would miss and run like crap until warmed up, then it would run fine. I am still wondering if it has valve issues.

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Re: Mule Stopped Running

Post by Grummanflyer » Fri Jul 01, 2022 4:00 am

If it accelerates briefly then falters, it's losing fuel. Pull the choke about 1/4 to 1/2 way out and see how it acts. The idle/slow speed circuit and the main fuel metering in the carb are two different circuits. The accelerator pump actually squirts raw un-atomized fuel right in the carburetor throat to allow a temporary rich mixture until the engine RPM comes up enough to to allow fuel to be pulled into the venturi through the main jet. If the choke trick changes how it acts, pull the carb, pull the bowl off the bottom and check for debris. In cases like this, it's really helpful if you don't turn the carb seven ways from Sunday after you remove it. Try to keep it upright as best you can and remove the bowl screws and bowl with it upright. That will allow you to see whats in the bowl when you take it off. Lots of times , if you roll it around all over the bench, while you disassemble it, the trash in the bottom gets lost or dumps out on the bench and you never are sure if there was anything in it or not. Plenty of times I have seen a piece of garbage laying in the carb bowl and it will idle all day long but as soon as you stand on the gas, it gets sucked up and obscures the main jet and it won't run right.You can screw the brass jets out and blow them out with compressed air. Do not run a drill bit thru either or you will affect the mixture.
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Re: Mule Stopped Running

Post by markrdje » Fri Jul 01, 2022 9:01 am

I would say maybe bad gas maybe? Aw, you already used different gas, but the gas nowadays ain't like it used to be.I have a mule but I am not a mule man.
change user name to mark

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Re: Mule Stopped Running

Post by RBW » Sun Jul 03, 2022 8:42 am

Think I figured it out. Should have known when I first put an automotive compression tester on it and it took a few revolutions to build pressure. Not an ignition or fuel problem at all. Left exhaust valve is intermittently not sealing, possibly due to a lead deposit under the valve face. Engine will run okay, but misses and backfires, then gets much worse and eventually quits.

Put a differential compression tester on it and found it will only hold 20 psi over 80 when the valve is not seating correctly. Just wiggling the valve stem will cause it to seat, and then the compression is great.

I have been using aviation gasoline and think I need to switch back to ethanol free automotive gas. Put a borescope in the spark plug hole and found quite a bit of lead deposited there. 100 low lead just seems to have too much lead additive for these low compression engines. I have had a similar issue with a pressure washer engine.

Could also be tired valve springs from sitting up for many years, possibly with that exhaust valve spring compressed constantly.

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Re: Mule Stopped Running

Post by RBW » Sun Jul 03, 2022 11:33 pm

Running again. Removed left cylinder and separated cylinder head. Exhaust valve stem had a bunch of carbon buildup that was preventing the valve from closing consistently. Cleaned carbon, lapped intake and exhaust valves, and put back together. Runs fine now. Think I'll just get two spare cylinder head assemblies for when something like this happens again.

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Re: Mule Stopped Running

Post by Chuck W. » Sat Jul 09, 2022 9:16 am

You probably already know this, but make sure you know what heads you buy, most on e-bay are 2AO-42 gen set heads. viewtopic.php?f=148&t=262382

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Re: Mule Stopped Running

Post by RBW » Sat Jul 16, 2022 7:09 am

Chuck,

Good info - it did not even occur to me that the genset heads had different length valve stems. I'm going to pull my rocker arm covers again and see what I have installed. The assembled heads I got from eBay are definitely generator heads. The complete head assembly P/N on the box matches what is listed in the M274 AO42 engine parts manual, but the valve stems are definitely shorter.

My cylinders (at least the left one) do have the copper spacer. If the valve stems are the short type, perhaps that's why the old lifters seemed to be stuck at full extension and why I get so much valve train noise, especially after cold starts.

The machine is certainly running better. Now to do the hub seals - it is puking out the drop axle oil at a prodigious rate, so I think the old leather seals have had it.


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Re: Mule Stopped Running

Post by RBW » Sat Jul 16, 2022 1:31 pm

Turns out I didn't know about the generator heads. I have generator heads installed with copper spacers. That explains all the valve train noise - the lifters have to fill all the way up to take out all the slack in the valve train. Guess I need to pull the spacers the next time I have the engine off.


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