300 seems to miss when idleing

my 300 runs great except when it is running with no load. It doesn't make a difference if it is at low or high idle. Once a load is put on the motor she purrs like no other. The tractor ran like this ever since my dad bought it about 30 years ago. Can anyone think of the possible reason. I cleaned the carb, replaced plugs and wires, and replaced the conventional points with an electronic one. I also put a high output coil on it. None of this seemed to change anything. Any ideas, if not she does great the way she is

Check that the valve gap is .017, as it may be too large, just had to do that on my H with the same symptoms.

I'll have to check that. Thanks

If the engine runs smooth and pulls good under load but breaks up under no load I'd first suspect a vacuum leak.

Check for loose carb mounting and intake manifold clamp bolts. If those are OK, run the engine at idle and play an UNLIT propane torch around the gasket gasket areas. If the engine smooths out/speeds up when playing gas near a gasket it means there's a vacuum leak there. Time to replace the gasket.

Check also for a hairline crack in the intake manifold. Not common but it does happen.

Another possible cause is a bad carb adjustment (excessive idle air flow) or a plugged idle mixture jet.

Suggest also checking the ignition timing. It should be at TDC low idle and advance to 35 - 40 deg BTDC by around 1,000 RPM.

Well here is my take on this if you have a flutter at a no load cond. i would be looking at the dist. as it may have bad bushings or weak or broken springs . Now here is where ya need to make sure that you also have the correct dist. in that tractor for the engine . As there were several different dist. used and depending on what fuel that engine was made to run on . If it came as a distl. from the factory then the timing curve is set different then a gas and at high idle it will throw 30 degrees total advance to much advance with a gas set up as a gasser only gets 22 degrees total .And here again with todays gas even 22 degrees can be to much . With out knowing everything about that engine as to what pistons and what head and so on all ya can do is get them close . A no load miss can cause a good mechanic to indulge in the spirit's trying to make it go away. Heck it may even be back lash in the timing gears . Our like BOB said fuel related . Myself the first place i would start would be the dist. , BUT i have the leg up on most for this as i can pull the dist and put it on the strobe and run it off the tractor and see what it is doing as far as advance curve and points bounce and shaft run out. On my machine i can run the dist up to 9 grand engine RPM . When one buys and old tractor ya never know what all has been done to it over the years . So it turns into a craps shoot trying to make it perfect . SO if ya could find some old geeser like me that has a old SUN or ALLIED dist strobe and knows how to use it then get and I T manual for the 300-450 as the old copy of it has all the spec.'s for the different dist. and you can see if yours is doing what it is suppose to do when it is suppose to do it. once that is correct then ya move on to the next problem area . So for the correct engine timing if it was a Gasser from the factory with a gasser dist and carb then the correct engine timing is 22 degrees at high idle .