Farmall Cub

My 49 Cub with belly mower will run about 20 minutes,then lose all power. It may run at idle but sometimes doesn't have enough power to move with PTO off. I can get some power by chocking along with black smoke from exhaust. This has been happening all summer probably 15 to 20 times. Have tried many fixes but none seem to work. Does anyone out there have a possible diagnosis?

Andy, If you'll list the fixes you've tried, it will make it easier to narrow down possible problems.

Bob

andy7102 wrote:My 49 Cub with belly mower will run about 20 minutes,then lose all power.

First thought, coil breaking down. Magneto or battery powered ignition system?

It may run at idle but sometimes doesn't have enough power to move with PTO off. I can get some power by chocking along with black smoke from exhaust.

Indicates fuel/carburetion system working.

Pull the main jet out of the side of the carb and make sure the holes are FULLY clean. They get varnish that builds up on them and small bits of crud plug them up but float away when the flow drops off. I catch flack for suggesting using a straight pin, but it is strong enough to wipe the gunk from inside the tiny holes. Since I did that on mine, my low power issues are far fewer and farther in between.

Hmmm 49 Cub, probably magneto. Probably ignition but could be fuel starvation albeit intermittent at that.

As ntrenn mentioned, check the main jet - especially the tiny longitudinal orifice. Do not use anything made of steel in that orifice. A nice piece of a single strand of twisted pair (telephone wire) will do the job perfectly without scratching or scoring the brass jet. Also check the needle and cage. Hold the empty cage up to the light - if it is kind of opaque then there is a bit of varnish in the cage which may be causing the problem.

Also, Eugene is correct as well. Sounds like ignition so check the coil. Make sure it is putting out a very bright blue spark. Also check the condition of the spark plug wires and ends/boots ensuring that all are good.

And of course if you let us know what you have done and what the results were that would be of help.

Black smoke is the primary indicator of unburned fuel, technically referred to as incomplete combustion.

There are only 2 causes.

One, too much fuel being delivered to the cylinders and the excess fuel that can't be burned is going up the stack in the form of black smoke, a carburation issue, or...

Two, ignition failing to burn the proper amount of fuel being delivered to the cylinders.

Troubleshoot your ignition system thoroughly before messing with the carb.

Thanks for ideas. As for fix attempts; New coil, wires, spark plugs, rotor, points ,condenser, clean fuel tank, sediment bowl, fuel line, carb (only blow out I'll try telephone wire) making sure gas cap breather hole isn't plugged, After each fix it runs fine so I think the problem has been found but after about 20 minutes of hard work it quits again! I wonder why the time factor as that would seem to coinside with the time needed for the engine to get warmed up. another fact:: The tractor was converted to 12 volts by a previous owner at least 20 years ago.
The black smoke only occurs when I overchoke to get the cub to get a little more power than idle.

andy7102 wrote:After each fix it runs fine so I think the problem has been found but after about 20 minutes of hard work it quits again!

Eugene wrote:

andy7102 wrote:My 49 Cub with belly mower will run about 20 minutes,then lose all power.

First thought, coil breaking down.

20 minute run time - common symptom of coil break down.

Heat related, electrical cause. Ballast resistor on/at ignition coil? Bad new ignition coil? Wrong ignition coil?

And another thought. When the engine starts to falter, check for battery voltage on the ballast resistor input side, voltage drop on output side and output voltage at both coil terminals. No ballast resistor, check for voltage at both coil terminals. Battery voltage at input terminal and voltage drop at output terminal.

|Had a small mishap with cub , managed to hit rad with something and now must remove rad to get repared. Other problem at standstill for a couple of weeks.

Problem solved. New coil and the cub runs fine. The coil was new and was either faulty or not correct one. Anyway I cut 5 acres of grass this afternoon so I know it's working. Thanks for suggestions. n