Don't you just love tracking down problems?? I have a ferd 460 on propane, for pumping water. It has the electronic ignition, and Impco carb/mixer..... and a million hours.
last couple days, been getting tougher and tougher to cold start. It sneezes back through the carb. eventually it will start and run. This app has been tough on it's engine controller, (ford uses a little aluminum box to manage the distributor) So replaced that right away. not much help. put a new dist cap and rotor, spark plugs too. Plug wires were new this spring.
It was 'dropping a cylinder' last week, thats when I started 'tuning'. (pretty hard on crankshafts when ya got a miss)...This motor is hooked straight to the load, only cushion is the springs in the clutch hub.
I got the dropped cylinder taken care off with the tune up pieces, but now with the popping @ cold start....something has changed!
I rebuilt the vaporizer/regulator a couple days ago.
All the reasons I can come up with for a back fire are: too lean, too rich, vacuum leak, timing. oh, and possibly a exaust man leak? The manifolds are fabricated 'retangular tube' header type. The gaskets between the head and manifold have eroded on a couple of cylinders and are leaking. I am wondering if on cold start, partially unburned gas is hitting that leak, and with more oxygen and heat, it's flashing back, through the now open intake valve and pop?
As for other vacuum leaks, there really isn't any more lines to check, The distributor is mechanical only. I suppose it's possible the intake man gasket is bad. or the carb to man gasket.
ooohhhh... i hate these "could be ignition/could be fuel' type scenarios.
another possibility is the carb. It uses a diaphram to regulate fuel. If that ol dog is stiff, torn, then the signal at startup will cause that too!
Dang it, I have maybe 2 weeks of running that pump and will be done for the year.
Before next summer's season, I think will put in a Pertronix Ignitor electronic ignition. And get rid of that 'mystery ford box'. That will be about 65 bucks/year saving. ( averages one per year).
Any other area to check for hunting down cause of backfire? might as well pile it up!!