770 gas dies when moving to low idle

Hello all.

I'm working on a 1962 770 gas. The issue the tractor is having is that it will not idle at any speed below 825 rpm without having the choke about 1/3 closed.

I have cleaned out the fuel tank, sediment bowl, and fuel line to the carb. Pulled, cleaned, and soaked the carb. Cleaned and blew out carb and re-kitted. Problem still persists.

Sprayed ether around the intake and carb, but did not get any change in engine speed.

Checked power to the coil, wound up replacing the wire from the ignition switch to the coil, as well as the wire from the coil to the points.

Installed new plugs, wires, cap, rotor, points, and condenser. It runs a little sweeter, but at the same 825 rpm, it dies just like you turned the key off if you open the choke.

I have tried to find out what the resistance readings for the coil are supposed to be, but cannot find a thing. I have measured the coil primary and secondary windings both cold and hot and have the readings recorded.

My Dad has a '61 770 that does something similar, but only when it's warmed up. This one does it all the time. I also measured Dad's coil. The changes in resistance on his are opposite of the changes on this '62.

Both of the coils get nearly too hot to touch after running for 10 minutes. Is this normal (because they have an internal resistor), or is this a sign that they are bad?

I can post the resistance readings for anyone that would want to know what they are.

Any pointers would be greatly appreciated.

Thanks for your time,

Luke

Timing?

Thanks for the reply, but timing is fine. Points gap and plug gap are to spec as well.

First thing that comes to mind is fuel level in the carb.

Excellent fuel flow to carb. Float height is correct.

how many in of vacuum is it pulling at manifold

would putting the choke on fix an electrical problem??

How much play do you have in the throttle shaft on the carburetor? If you get too much, the engine will just die once the rpm gets too low and fuel mixture gets too lean. Keep us posted on your finds.

Jfred might have a point there but I have seen them fairly loose. I am still wondering about your idle circuit. Make sure the air blows through there.

Based on observation, the coil without a resistor needs to be around 4 ohms on 12 volts to get around 2 or 3 amps through them. Primary to secondary generally runs around maybe 8k ohms give or take some that varies.

(If I remember right 6 volt equipment and mags will check as low as 4500 ohms.)

Dropping 12 volts with a external resistor on a '6 volt' coil is about 2 ohms. This would be a cheaper test.

I am going to comment that there is no resistor in those coils. The way they are made it's actually where they tap the coil. Or, in other words how they wind the coil.

RT (my 2 cents)(I've seen dead coils check perfectly good on resistance.)

Sounds like a plugged idle circuit. I took a carb apart four times on a Super 55 to get it cleaned out so it would run correctly. You might have to run a wire or small drill bit through the passages to get them cleaned out.

Sounds like your coil is bad. The coil should not be very warm in normal use but it will heat up if it is leaking. Simple to check, replace the coil with one that you know is good. If the tractor now idles as it should buy a new coil
To those who wonder how an engine will run with a rich fuel mixture as when you have the choke on just think of why we choke a cold engine. The rich fuel mixture will ignite better than a lean mixture and thereby the tractor will run on a rich fuel and a weak coil but runs weak and stalls at idle. Good luck.

I"ll second what centerplate said.

Thank you for the explanation on the choke helping on an electrical issue. Makes sense.

There is that possibility, but for a cold start, the choke is required for a rich enough mixture in the real world.

Condensation of fuel on cold engine parts and fuel remaining as droplets rather than complete mixing with the combustion air are cold starting problems needing the use of the choke. Really good atomisation of the fuel, using a more complex carburettor would need the choke only for a minimum of time. Think here fuel injection systems over carburettors.

Poster reckons everything is spot on, so ignition should not be an issue '- but, there again, using ether around sparks is not the most clever from a safety point of view.... I've never actually seen an exploded manifold, but I have heard a diesel knocking with the stuff. Bad news.