Farmall f20 runs poor when hot

[farmboy1215]

Just picked up a nice f20. I notice that it starts run really rough, runs on a couple cylinders at times, then backfires only when hot. When cold, she purrs along. I was originally thinking carb issues, but with it only doing it when hot, I'm thinking it's a mag problem. Just curious what everyone else thinks. Thanks!

[766 Man]

  Are you running straight gas or an ethanol blend?

[Old-F20]

condensor in your mag is the first thing id check.......

[Gabriella Cook]

I had the same prob on my 560 I change fuel line and shes like new, guess it restricted flow so when she sat overnight shed fire right up but burned fuel faster than she dripped in

[farmboy1215]

1 hour ago, 766 Man said:

  Are you running straight gas or an ethanol blend?

Blended fuel. Again, runs good cold, the after 15 minutes acts up bad. Yes, I have good flow to carb. 

[Gabriella Cook]

try new plug wires they may not transfer spark when hot

[MacAR]

Nothing to do with fuel system in my opinion. Condenser is first thing to check, as said above. If that don't fix it then it's the coil. My 300 did that a few years back, turned out coil was toast. Then the Cub decided to do it not long after, was the condenser that time. Symptoms are the same, condenser is cheaper than coil, so try that first. 

[supermechanic]

Since it is new to you, check the spark plugs. Previous owner may have installed hot plugs, and you are getting preignition.

 

[iowaboy1965]

I would also suspect rhe condesor first. Followed by the coil.  

[acem]

I know nothing about a F20 but Ive had coil go bad when warm then seem fine when cold. 

Ideally swap out a known good unit.

 

[39farmalla]

All my Kohler engines would crap out on me when they were hot, but as soon as they cooled down right back to putting along

Replaced all the coils and 1 condenser 

Sam

[snoshoe]

Very limited F20 knowledge or experience. Been holding off to see if I could learn something. Good advice above. Something I think we are missing is these things we're built to run on light oil. This involved a hot intake manifold. Usually there is some means to limit that heat to run on gas. Even if the door or valve is in cold position. That doesn't mean iron hasn't burnt away inside letting exhaust heat the intake. These parts are very hard to come by. My neighbor solved the problem on his 10 20 by mixing gas and diesel half and half. Your mix could vary. I think his mix was a little strong on diesel because it smoked and smelled of diesel but it no longer missed banged and cut out.

I have a different color all fuel burner. At one time it was running 2 gal gas to 3 gal used oil. It would start on it. Pop bang and smoke terrible until you got 100 feet down the furrow. Then nobody would guess you were burning oil.

[timbo1946]

When I first got my OS6 it would do the same thing. I found the manifold was cracked (on the back side) as it heated up the crack would open up snd make it run like that. It has the all fuel manifold. Replaced it and it has run great since

[farmboy1215]

Update on what ive done. Put new plugs and wires on it. I was able to get it started by hand, but it struggled to run on 4 cylinders. Whenever I throttle it up, it falls on its face, runs on 1 cylinder, and backfires. I pulled mag off. It had been rebuilt at some point. All new parts and wires inside. I replaced the condenser, and it still only chuffs off. I checked resistance of the coil, and it was 11000 ohms. Starting to wonder if it's a spark issue or a carb issue. 

[MTB98]

Are the valves adjustable on that? If so check the valve lash when hot. 

[Binderoid]

Is it still doing the same thing with all these new parts, ie runs good  cold? 

A bad coil can have intermittent, heat related short. 

[snoshoe]

2 hours ago, farmboy1215 said:

Update on what ive done. Put new plugs and wires on it. I was able to get it started by hand, but it struggled to run on 4 cylinders. Whenever I throttle it up, it falls on its face, runs on 1 cylinder, and backfires. I pulled mag off. It had been rebuilt at some point. All new parts and wires inside. I replaced the condenser, and it still only chuffs off. I checked resistance of the coil, and it was 11000 ohms. Starting to wonder if it's a spark issue or a carb issue. 

Sounds like valve trouble to me.

[560Dennis]

check rocker arm contacts to valve stem . If worn out the valve lash will be variable. Have them redress at a machine shop . 
no way will a feeler gage be able to set the valve lash if worn out. 
not know the engine you have conditioned . Suggest this 
 

I have brazed them up with oxyacetylene torch in my garage , redress with a radius gage to check . 

[12_Guy]

I assume that the coil was replaced as well as the other mag parts. You could certainly have manifold problems. Does it have a good spark when cold? It should jump an 1/8” gap easily, consistently. What does the brushes on the distributor disc look like? Finally, is the magnet strong? It should pick up a piece of 1/4” plate a foot square easily or support itself on the side of a thin piece of metal like a toolbox. 

[TD25-504]

check your cylinder compression.

[farmboy1215]

Yet another update. I swapped magnetos with another f20 I have that runs good. I got it to eventually fire up, and it ran great. Drove it around some, then parked it and let it run 20 minutes or so at 1/3 throttle. I went to park it, touched the throttle, and it did what it did before. Fell on it's face, ran on a couple cylinders, backfired, blew the rain cap off, and died. After I parked it, I checked and adjusted valves. Very little adjustment needed. Do I have a carb problem that I'm fighting, or am I missing something else?

[jeeper61]

Is it backfire out the carb or exhaust?

It is staring to sound like a sticky valve 

[12_Guy]

What kind of manifold does it have? The all fuel manifold will need to be adjusted to the cold setting for gasoline. Unfortunately most of them are no longer able to be adjusted and may leak even if they are on cold. This overheats the air/ fuel charge and can cause pre ignition. Also check for sticking valves when hot. Just pull the valve cover and watch them once warm. 

[farmboy1215]

35 minutes ago, jeeper61 said:

Is it backfire out the carb or exhaust?

It is staring to sound like a sticky valve 

Out the exhaust. Valves seem free. I oiled them up good when I had the valve cover off. 

[farmboy1215]

39 minutes ago, 12_Guy said:

What kind of manifold does it have? The all fuel manifold will need to be adjusted to the cold setting for gasoline. Unfortunately most of them are no longer able to be adjusted and may leak even if they are on cold. This overheats the air/ fuel charge and can cause pre ignition. Also check for sticking valves when hot. Just pull the valve cover and watch them once warm. 

This one has a gas manifold on it.