MF 135 Strange Gas Tank Problem

Neighbor Has strange problem. Tractor runs fine at low speeds but at full throttle runs rough. Thought gas cap was problem. Instead of vacuum in tank and not letting gas flow if you loosen cap it wants to blow the cap off. Never heard of this. Any ideas......

If the tank is getting hot due to a heat shield not there that will cause that type of problem

If the fan belt is slipping it will do that as it needs the good flow of air to keep it cool. Check for a loose or glazed belt.

How does it run with the cap off...

Yes, gas tends to expand when it's heated.

Some caps have a check valve to let air in but not let air (and gas) back out, which can be a real problem with a full tank and a hot engine!

And even a vented cap can get be clogged. I once bought a new 'vented' cap, it wasn't! Try a run with the cap loose.

It is possible but unlikely the pressure in the tank would be enough to over ride the needle valve in the carb. If that were the case, once the engine were shut off hot, the carb would begin to overflow and the engine would be severely flooded on restart.

I would look more for a weak spark, points out of adjustment, worn distributor, bad condenser, bad coil, worn plugs, bad wires, cap, rotor, etc. Check the spark at the end of each plug wire. It should give a 1/4 inch spark to ground.

Could also be fuel problems, check the fuel supply available in the carb bowl, remove the carb drain plug and check the flow. Engine off, fuel valve open,should give a steady flow with the plug removed. Not slow to a drip or stop. There is a screen in the carb inlet elbow, a common place to clog.

Catch what is caught in a clean glass, look for water or trash. If the sample is contaminated, so is the tank and carb, both will need to be cleaned. A rust flaking tank will be an ongoing problem.

How is the air cleaner? There is a wire mesh inside the canister, it is often neglected. If hard packed with dirt it will have to come apart and be replaced. Not an easy job, but if clogged it will not run top end.

Finally, the main jet may need to be adjusted. Last thing to do after everything else is right. Start about 1 1/2 turns off the seat, idle the engine, then suddenly accelerate. It should take sudden full throttle without hesitation. If it balks, back the main screw out 1/4 turn. If steady black smoke, turn it in some. Keep adjusting until it will accelerate smoothly. A single puff of black smoke is the goal.

(quoted from post at 21:07:30 09/30/21) If the tank is getting hot due to a heat shield not there that will cause that type of problem

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Exactly. That would be my first guess. Fix it before it explodes.

We had this exact thing happening with our Massey Super 90. I finally found an original Massey gas cap that has the cone on it. Solved the problem. The other gas caps dont let the pressure out but these do. I got this one from a salvage yard about an hour away. Cheap fix and it now has an original cap.

Good morning, all: I am just passing through, my experience may (or may not) be useful. My 9N ford gas tank is right above the hot engine, seems bad to me, so when I am using the Woods finishing mower, I unlatch the fuel/battery access door on the top of the 9N's hood. This is to improve ventilation under the hood, at least I think so!

Dennis M. in W. Tenn.