M274 A5 Help

We have a A5 variant mule at the Museum I work at and I have been fighting it consistently.

When we got it it ran fine but slowly started to get extremely temperamental. Start misfiring and back firing through intake and back firing out the exhaust quite nasty like.

We took the plugs out and cleaned them and I re timed the Mag according to the several info threads and website I have seen. Ours seen happy with the V marks lined up versus being off a tooth. For approx two weeks the mule ran perfect no surging no bucking just perfect.

Now, it is going back to the misfiring and back firing out through the exhaust and intake again. I verified the timing again, and adjusted up and down to no effect. We checked the fuel filters and found crud and cleaned it all out. I checked the fuel return line to find no blockage and plenty of fuel flow.

The carb looks new with paint marks on the adjustments. I haven't really messed with them as I have not found alot of info on what exactly needs to be adjusted.

Other info...

I have been using Non Ethanol Fuel, with some lead additive (was suggested). Oil has no gas smell in it. Filters are clean and tank has been drained and refilled with fresh fuel.

I have noticed there is discoloration in the muffler where it looks like the exhaust is extremely hot. To me this shows a lean running engine but with no major info on carb adjustment I haven't been able to look into fixing that if that is not normal. I am going to pull the plugs again soon see how they are and post on here if needed.

Any help is most thankful and I am also looking for any contact info on mule supplies (parts) if possible as well.

Mine has done the same thing.

I had sooting and wetness on my plugs.

I messed with timing with not improvements.

The hottest xd21 plugs help some.

I cleaned my magneto points and it helped some.

I really haven't used it much since last tinkering with mine, to know if my issue is resolved.

Thank you! Will look into new plugs!

I ordered new plugs and while waiting on them I figured I would tinker with the mule a bit. After cleaning the old plugs which were heavily fouled I got it started back up with much protest to find a new noise which sounded as if it was coming from the magneto. On closer inspection I found the grounding harness plug (I assume) loose in the housing and when I pulled it out the mule ran rough, back firing like it did before. When I held it firmly in place smoothed out and ran happy.

So, either the little tyrant is playing with me or the loose plug was the culprit which is now fixed. I spent an hour driving it around with no issues. I still plan on seeing if the plugs will foul over before installing new ones.


I have yet to figure out though my the exhaust seems so hot. My muffler is actually discolored where the exhaust enters it from the heat. I used a temp gun on the cylinder heads general area and did not find anything over 120-140 degrees.

Exhaust on these gets really hot. If you are driving it around when it is dark/at night, the exhaust glows red. Exhaust pipe and muffler is discolored on ours as well.

Check your intake manifold rubber connector tubes and the gaskets.
They are a pain to get to and often missed. I have had 2 mules with that problem and gave the same symptoms you are discrbing.