super M dies under load

my super M starts and runs fine untill under a load like bush hogging heavy grass.Then it starts missing and if I don't put it back to idle it will quit. went thru fuel system and cleaned filters and carb. regaped points and plugs, distriber cap looks good. Still does it but only under a load?

I bought a combine that acted like that. There was a kernel of corn stuck on the stem of the sediment bowl. It was getting gas, just not enough gas.

Well a few things here that i can think of , (1) Fuel issue , how is he carb set as in what is the main load screw set at ????? for full load ya want no less the five turns off the seat , Do you have a stupid in line fuel filter on it ?? have you had the sediment bowl out of the fuel tank and check it for and obstruction , Is there a fine screen in the sediment bowl , if not get a NEW factory sediment bowl .That screen will only allow what can pass thru the ports , passages of the carb to go thru and NOT restrict fuel flow.(2) Next up on the list is COIL is your coil getting so hot that you can not touch it ????? (3) your ing. timing is it set to FACTORY SPEC. WITH A TIMING LIGHT or did you just set it by ear , if you set it by ear son you are so far advanced it's not funny . When them OLD engines are set correctly they sound like they are laboring . (4) is your centri. advance inside the dist. working like it should no broken or stretched springs , no frozen up cam . Yea i know you say you have been thru the carb , i see this a lot when people bring a carb here and tell me that it is JUNK as they have been thru it a dozen times . A can of spry carb cleaner or some home made brew is not going to get down to the nitty gritty of eating out years of crud . Sometimes even the stuff i use does not get it ate out even i 24 hours of soaking at 180 degrees then out comes the tiny drill bits and boring with the big auger . Not many check to make sure that the float is not leaking or that all the years of crud built up on that float making it heavier and thus allowing for a bad float level even though you bench set it to the correct height .

Hello LPK welcome to YT! Do you happen to know there should be a screen in the fitting that screws in the carb where the fuel line attaches? Make sure it is clean. I will echo checking for debris in the tank, a good flashlight is helpful.

Plug wires.My Super M acted just like that.After doing everything it still misbehaved. Finally at the suggestion of a froend,I replaced the carbon core plug wires with new wire core.Hasn't missed a lick since.That's 30 plus years ago.

Good point , missed that one only because not one around here runs junk wires all Packard 440 copper core . Just like them electronic ing . last one that i know of around here bit the dust couple weeks ago and i put it back on points .

Carry a hand full of tools, be ready to catch it in the act.

Start with the spark, have an old spark plug with the electrode bent straight or broke off, put it in a plug wire and ground the base, start the engine and see if it will jump the gap. If no, the spark is weak, points out of adjustment, bad condenser(often bad out of the box), bad or wrong coil (it should not be too hot to touch), bad wires, cap, rotor. All ignition components are now poor quality imports. Often they are made wrong, boxed wrong, or slightly different. Don't assume new always means good!

There is a drain plug in the carb bowl. Be ready to quickly pull the plug and see what kind of flow you have. Engine off, fuel valve open, remove the plug and catch the flow in a clean glass if possible. You should get a continuous flow, not slow to a drip or stop. Look at what was caught. If contaminated with rust, trash, water, the same will be in the carb and tank. A fouled tank will cause ongoing problems.

If the flow is insufficient, start chasing the problem back to the tank. There should be a screen in the fuel fitting at the carb, another in the sediment bowl, possibly one standing inside the tank.

About 30 years ago, my M would run fine doing nothing or under a light load. When under a heavy load after a couple minutes it would start to miss. If you would hit the clutch right away and idle it down, you could catch it and after about a minute, it was good to to go for a couple minutes again. If you didn't hit the clutch quick enough it would just die. After a bunch of messing with it, I found that an intake valve stem had a small warp to it. Under a heavy load when it got heated up, it would wedge tight and stay open. Gave it a valve job and one new valve and it ran perfect from then on.

Thanks Kent, that hits it right on the head. You described my problem perfectly.