In Reply to: 8N Backfires through the exhaust posted by Jon Bennett on May 19, 2014 at 17:14:54:
" I don't know if it is a 12V coil or not (how can I tell?"
Measure the internal resistance of the coil; 2.5 ohms or more, it's 12v. But, you will still need to use the OEM ballast resistor!
" Is there a good way to diagnose exactly what is wrong, or to differentiate between a timing problem and carb rebuild time?"
Start w/ the simple stuff: check the firing order (1-2-4-3, CCW) then adjust the carb.
Make sure the tractor is at operating temp; that usually takes 10 – 15 minutes at idle depending on ambient temp.
Both Ford and Marvel/Schebler (assuming you have a M/S carb ) say to set both the side-pointing idlemix and the down-pointing mainjet to 1-turn as a starting point. I set the down-pointing mainjet to 1-1/2 turns and don't fool w/ it until the final step.
Then adjust the side-pointing idlemix for fastest idle; not the smoothest idle. Next, adjust the behind the carb idle-speed set-screw for very slow 400-rpms idle. Do that idlemix adjust for maximum idle at least 3-times. Make sure that you turn the screws slowly, like 1/8 of a turn at a time & wait a second or two for the engine to catch up. Take your time!
Do it like this:
1. Adjust idle mix jet until RPM increases
2. Adjust idle-mix set screw until the engine nearly stops (as slow as you can get it unless you have a tach that tells you 400 rpm)
Repeat steps 1 - 3 three times.
Remember the side-pointing idlemix is out for lean, in for rich.
If you do not have any problems inside the carb, it is easy to get the idle down to 350 - 400 rpms.
Your last step is to go back to the main jet. Remember, in for lean, out for rich. If you end up turning it OUT more than ½ turn for max power (remember, you already had it 1 ½ turns out) then stop right there because you have a dirty carb or a fuel problem.
Post back w/ results or more questions.
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