420 engine problems

I could be all wet with this but if the timing procedure for the 420 is the same as the M and 40, (holding the rotor at full advance & rotating the distributor 'till it sparks with the crank set at the S mark) I've been down this very road! Put a timing light on it & make sure the centrifugal advance is working like it is supposed to!

Our 40 two row utility had a set of springs in the distributor that were way too strong. Result... almost no advance even at fast idle. It had a laboring sound to it, would burn the aluminum coating off of a new muffler in a matter minutes even without a load and it had a harsh bark to its exhaust. The "POP" when you shut it off is unburned fuel in the stack being ignited by the high stack temperature which is working like a hot tube igniter on an old hit -n- miss. With the proper springs and subsequent advance curve, ours smoothed right out & now behaves very nicely.

Worth a peek!

Good luck.

If the exhaust is getting hotter than it should, one thing that can cause that is slow timing. I had my head somewhere else, set the timing way too slow and cooked the paint on the hood.

Mine would pop and scare the dogs out of the barn on shut down. It don't do it now. I have changed to letting the motor idle down all the way and then turn the switch off. My idle on the carb is way lower than I would ever allow a motor to idle, but it works great to get it shut down without ringing my ears.