Ford 641 carb

I have a 1958 641 that came with a Marvell TSX-765 carb. A few years ago it started leaking fuel out of the gasket, so I ordered a rebuild kit. It was really dirty, so I cleaned it, replaced the butterfly on top because the old one was chewed up for some reason, replaced the needle and seat, ran wire through the main jet, etc... After that, it ran perfectly, but I have to set it to idle at about 8-900 rpm or it would die.

Some time after that I picked up a Zenith 13878 carb on sale. It is the kind with only a fuel adjust screw (no air adjust). With it installed, the tractor idles perfectly, but it seems to run too rich, and I don't have a manual for how to adjust it properly. After I get it where I think it needs to go, it runs good for a week or two, and then starts running rough, and the plugs get dry and black.

Could someone tell me what I'm doing wrong on either carb in order to get the tractor running the way it should?
 
#1 when was the last time you serviced the air cleaner oil cup??
#2 what happens if you try to run it with the air cleaner unhooked??
#3 on the M/S carb the main jet should be set at about 2 turns out form bottom as a starting point then adjust it so it runs well and higher RPM.
Running rich can be a simple air cleaner clog or the carb set to rich or carb still dirty. If you can not cause the engine to die by closing the main jet then the carb still need some cleaning done
 
At 8-900 RPM the M/S is not really running on the idle circuit.
For that one it probably just needs a better cleaning.
Running a wire through the idle circuit ports would likely help.

With the Zenith, two things I would try.
I would first check the quality of spark from the ignition.
Make sure it is blue/white in color and will jump a 1/4 inch gap.
If that checks out try turning the fuel adjustment in until it starts
to bog down, then open it up just enough to achieve full throttle
and full power without hesitation.
Count and record the number of turns to where you have it set.
Then if it acts up in a week or two, make sure the adjustment
didn't move from where you had it set.
 
Ok, here's what I found.

On the Marvel... I took it back apart, cleaned everything, ran a small wire through all the orifices, and reassembled. Then I noticed something that looked different than the Zenith. On the Zenith, the butterfly sat so the secondary idle orifice just peeked over the butterfly when closed and the primary was completely clear. On the Marvel, the butterfly sat completely between the two; the primary above, and the secondary below. I reinstalled, and the idle/air adjustment did nothing, and the tractor ran about 700 rpm with the idle rpm set screw completely backed off.

Now the Zenith... I cleaned and reinstalled it, and a little reading, I realized that I had been setting the idle/air adjustment a little too rich. I had been turning the screw in until the engine started to roll, and backing off just enough to stop the roll (apparently on the rich side). After reading about adjusting a Ford Model-T that had the same type adjuster, I realized I needed to adjust in both directions until the engine started to run rough, and then set the adjustment half way between. Hopefully this will fix my problem with fouled plugs.

Now, one other question. Originally I had a 6v coil running on a 12v conversion with a coil resistor. Last year my coil died, and I needed to cut hay, so I went to autozone and got the closest thing they had; but it was a 12v coil. Being in a hurry, I just put it on the way the other one was with the resistor still hooked up. Would that make a significant difference in the fire? If so, should I bypass the resistor?
 
On the coil it all depends on if it is a true 12 volt coil or if it is a 6 volt one they cal 12 volt. A true 12 volt coil will say something like external resister not needed but a 6 volt coil they call 12 volt will say resister needed so if you have a true 12 volt coil yes having he resister can cause a problem
 
Curious.. What are the idle and main jet sizes in both carbs? What are the carb intake and throttle opening sizes?

As to the ignition coil, why not drop by the auto recycler and pull a coil with wiring connector from either a Ford Ranger 2.9L, or a 89-95 Taurus 3.0L. They are both epoxy coils and should last a long time, plus they easily generate 30Kv and above where a standard cylindrical oil filled coil typically generates about 20Kv and burns out easily producing that voltage.

Remember to pull the coil wire too.
 

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