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MM runs rough with low fuel level?

J

John from NH

Member
This one has me stumped and I'm looking for suggestions.

My Big Mo 500 starts hard and runs rough at idle when the fuel level gets down to below a couple of inches in the gas tank. I have to choke her to get her started and she runs like she's cold, even when she's been running for awhile. She runs OK with the throttle open. I guess this might not be a bad thing since I don't have a functioning gas guage. If I add a couple of gallons of gas she runs fine again. I would think that she would run fine until she actually runs out of gas. The carburetor is clean and the float is set up correctly. All carburetor adjustments seem to be OK as she runs fine otherwise. Is there something wrong or is this considered normal?

Thanks for your comments and help!
John
 
Is there a chance someone has put a riser tube in the fuel tank and raised the outlet level? I put a piece of copper tubing with small holes drilled in it in a couple of my tractors. This prevents pieces of crud in the tank from blocking the fuel outlet.

Also what are you using for a fuel filter? I've heard of some automotive type inline filters causing problems in gravity flow systems.
 
Thanks for the reply!

There is no riser tube in the gas tank and no fuel filter, just a sediment bowl. I can see the tank outlet with a flashlight and it's unobstructed.

John
 
I had that problem with a Jetstar. It turned out that fuel had varnished over most of the screen in the filter bowl. The thing is, the screen looked fine as long as it was wet. But let it dry and hold it up to the light and it was a whole different story.
 

Thanks for the suggestion! I don't remember seeing any screen in the filter bowl when I had it completely apart, either in the bowl or the lines. It's not the original filter bowl. Maybe I should install a screen to keep the bugs and twigs out of the sediment bowl!

John
 
I have the same problem with my ZTU. It has the twist in the fuel line. When the fuel gets down to about 2 inchs it will not run good at all. The problem is that the fuel bowl is down below the carb. If there is not enough fuel in the tank then gravity can not push the gas to the carb
 
I'll try again.
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a41225.jpg
 
We had the same problem with our 602 and 670 Super. We put an electric pump off a combine on the 602, and just kept the 670 at least half full. By the 1960"s MM should have installed fuel pumps on them like other manufacturers.
 
Now that makes sense and I was wondering if that wasn't the case. The sediment bowl was relocated by a previous owner to connect directly to the carburetor. Still this puts the top of the bowl at practically the same level as the top of the float bowl and the bottom of the fuel tank isn't that much higher.

Another person on another board suggested checking the gas cap to make sure it is venting properly. Maybe the vent is that important.

John
 
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