MF 135 perkins gas running badly

My fathers 1978 mf 135 perkins gas with zenith carb ran great all summer. In the fall it would not start I diagnosed it to be points that welded togeather. I am a truck and coach tech but am always learning and small gas engines is not my strong suit. I replaced plugs, points and condenser and set gap to spec. When it started it was running very rough, back fired, and huffing black smoke. I tried setting timing by ear to get engine to smooth out and stop smoking but was unable to get it better. I used it a little around the yard and it got to the point were it would stall and be very hard to start and would only run by manipulating the choke on and off.

I had the massey dealer pick it up and they had it for 1.5 months and the bill was $800 parts and labour. The points and condenser was replaced, coil, plugs, carb (disassembled and reassembled) set timing, and a ballist resistor to coil. Service manager stated they were suspicious the tractor was a 12v conversion which is the reason for the ballast resistor. The tractor is a original 12v and never had a ballist resistor previous. I also have lost trust in the dealer as I thought they would diagnose better but it seems they just loaded the parts cannon and fired. That was why I stopped figured dealer would have more experience.

The Tractor ran better when delivered but when driving around yard the rpm s dropped and backfired then rpms picked up again. Talked to dealer they said a spec of dirt could have got in carb. Started in on Saturday and it backfired while running. I shut it off and went to start it later it was very hard to start and as ealier would only stay running by moving choke to on then when it would start to stall taking choke off then it would start to stall and move choke on etc.

I am suspicious it is carb related due to the choke manipulation to keep it running. I am running a small (older ford pickup carb) filter threaded into the card which I know is not advised but it also ran the filter during the summer when it was running well. I am going to clean the screen at the sediment bowl and see if I have good fuel flow through the screens and filter. If that checks out my friend rebuilds marvel and zenith carbs as a hobby and is very good at them so I will see if that helps.

Any opinions or suggestions is appreciated.

Sounds like you have more than one thing going on.
1) There should be a label on the coil and it should specify if it needs a resistor or not. To my knowledge all 135's are 12v. If the label says no resister and they added one then you are starving the spark. If it says one is required then it's proper to have it. When you are running it around and start having problems put your hand on the coil. If it's really hot then it's probably toast. A bad condenser can also give symptoms similar to what you are describing. Modern ones seem to be 50/50 that they are bad out of the box.

2)Choking the engine to keep is running is a sure sign of fuel starvation. Take the filter out and see if it helps at all. I have had very mixed results with them. Also check the sediment bowl and make sure it's clean. There may be a small inlet screen where the line enters the carb, make sure that is clean too. There is a drain plug in the bottom of the carb. Pull it and hold a glass jar under it and you should get a nice steady, good volume, stream that will fill the jar relatively quickly.

On a dealer note... they don't call them stealerships for nothing

This may not be your problem, but it looks like you have replaced most ignition parts except the Distributor Cap. I have seen them have a very
fine crack and the spark will jump from terminal to terminal causing it to back fire and run like hell. Something to consider.

Thank you for the suggestions this far. It is amazing how such a simple engine can be such headache. The biggest reason the dealer upset me is when I booked it in I explained to service manager that my father had passed so I just needed tractor for the winter then it would most likely be sold this summer. I told him do the bare minimum required to get it to run decent. So much for bare minimum $800 and back at square one. I think I am going to talk to them to see what they have to say but it is not going back.

I did replace cap and rotor when I did points and I do believe that was one part dealer did not toss on me. I will check the fuel supply and carb. I also have a new 12v coil on my parked ranger I will try on it as I know it does not require the resistor. I am also thinking about putting a hotter plug autolite 388 in as recommended on another post since it just puts around yard not really working. I will report back with what I find.

Fastest way to get ripped off = take it to a dealer. Honest ones are few and far between.