farmall 240 engine stumbles after 30+/-minutes

Help. I have replaced points,condenser,rotor,cap,wires and plugs. I have rebuilt the carburetor. I have pulled the head, pistons and sleeves. New pistons,rings,sleeves and had the head done with new valves.It starts easy and runs perfect until.....Thermostat works. Gas is good. Air filter clean. Coil is wired direct from battery with switch to eliminate old wiring issues. I have had and used this tractor for years.Need advice. Thanks
 
30 minutes just sounds like a vapor lock. Is it a good steel or copper line,or a piece of hose?
 
No answers, just questions:
Did the problem exist before ANY of the work was done??
Did it show up when SOME of the work was done (assuming the engine internal repair was done, and the ign work done to try to cure the stumbling that appeared after the internal engine work)...
When stumbles, any smoke (intermittent flooding (black)or sucking oil somewhere (blue)?)...
Have you had anyone check for good blue spark when stumbling?
Plugs stay nice and clean??
I'm assuming no water loss from inhaling water somewhere, and "stumbling" is intermittent misfiring (not on any specific cyl) and/or loss of power/rpms...
Fascinating...
 
The problem existed before any work was done. No white, black or blue smoke ever. I do not know how to check for blue spark when stumbling. Plugs stay clean. Your assumption is correct.
 
A couple of things to check:

The fuel line, is it routed as far away from the exhaust manifold as possible? Could be vapor locking, boiling the fuel in the line.

There should be a drain plug in the carb bowl. Have a wrench ready, when the problem occurs, quickly pull the plug and see if there is a steady flow of fuel. The flow will slow as the bowl empties, but it should continue as a stream, not slow to a drip or stop. Try catching the first fuel in a glass container, look for water or trash settling in the bottom.

Gas cap vented?

The new coil, is it the correct resistance for use without a resistor? Should be 2.7 to 3.3 ohms measured across the + and - terminals. If it needs a resistor it could be overheating the points and the coil itself.

Is the distributor in good shape? Weights free and working, no excess wear in the bushings, all connections clean and tight.

Best time to solve the problem is when it's acting up, let us know.
 

You don't state if you have checked gas flow. It needs to run strong, not for just two seconds but for twenty.
 
(quoted from post at 12:03:24 07/29/14) Help. I have replaced points,condenser,rotor,cap,wires and plugs. I have rebuilt the carburetor. I have pulled the head, pistons and sleeves. New pistons,rings,sleeves and had the head done with new valves.It starts easy and runs perfect until.....Thermostat works. Gas is good. Air filter clean. Coil is wired direct from battery with switch to eliminate old wiring issues. I have had and used this tractor for years.Need advice. Thanks

The vent in the gas cap is plugged. Loosen the cap, and see if the problem goes away.
 
I ran the tractor until it stumbled then opened the bowl drain. Gas flowed freely. gas cap is ok. Coil is marked 6 volt no external resistor. I felt the coil and it was never hot.I have an ohm meter but it is not sensitive enough to detect 2 or 3 olms. The distributor has been gone thru. Everything is ok. I have tried another new coil, condenser, and points. Did not change.
 
(quoted from post at 16:08:21 07/31/14) I ran the tractor until it stumbled then opened the bowl drain. Gas flowed freely. gas cap is ok. Coil is marked 6 volt no external resistor. I felt the coil and it was never hot.I have an ohm meter but it is not sensitive enough to detect 2 or 3 olms. The distributor has been gone thru. Everything is ok. I have tried another new coil, condenser, and points. Did not change.

Does the gas flow freely for twenty seconds or just two?
 

Are the valves adjusted correctly? With the proper gap? If the gap is too small, it will go away completely when the engine gets warm, and then the valves cannot close completely, and the engine WILL stumble and run rough.
 
I would like to thank everyone for all the tips.I did all the testing that was recommended.No luck.I am leaning toward an electrical problem. The tractor has 6volt system. New battery,coil,points, condenser,rotor button,cap,wires and plugs.Points and plugs gapped by the book. Timing is adjusted with a timing light. The coil is wired directly to the battery thru a separate switch. What could cause electrical failure after 30+/- minutes?.
 
The valves were adjusted by the book when the motor was put together. It has been run enough that I need to check adjustment.
 
(quoted from post at 07:43:47 08/01/14) The valves were adjusted by the book when the motor was put together. It has been run enough that I need to check adjustment.

I like to add .002" to .003" to the recommended gap. It is almost impossible to set the valves, start the engine, allow it to reach operating temperature, and then remove the valve cover and re-check the clearances, BEFORE the engine cools down again.
 
I totally agree with your statement. Official IHC book says adjust valves hot. It defines hot as any temperature above 32F. Makes it easy.
 
Might connect the volt meter to the coil, run and watch, see if the coil is loosing power. Something else to try, not sure this will work, but rig up a 6 volt test light (# 51 lamp), wire it to the coil on the point side terminal to ground. It should flash every time the points open. If it will run this way (it might drain away too much current) you can watch when it acts up, see if the flashing and voltage are any different when it stumbles.

One way to check for running spark quality is to slightly pull the coil wire so you can see and hear the spark jump but still keep it close enough for the engine to run. This really only works if you can recreate the problem without having to be under a load.

Back to fuel issues, have you tried partial choke to see if the problem is affected?
 
Special thanks to Steve@Advance for the last tip.I hooked a timing light to each plug one at a time and watched the flash. All cylinders show the misfire. The stumble is spark related. What would cause this misfire only when things get hot?
 
(quoted from post at 07:27:21 08/02/14) Special thanks to Steve@Advance for the last tip.I hooked a timing light to each plug one at a time and watched the flash. All cylinders show the misfire. The stumble is spark related. What would cause this misfire only when things get hot?

Coils are known to rarely develop an open when they get warmed up. It happened once to me. The tractor was out in the field and quit. after checking everything I could think of nothing else and replaced the coil and it fired right up. A new one would be that much more unlikely to open but it is still possible.
 
voltage is to high going to the points ,you dont need but around 7 or 8 volts. sounds like the points are getting hot and breaking down, you might need a inline resistor going to the points. I had that same problem drove me bat vrap crazy for a week till i figured it out..
 

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