Farmall Cub won't start..

1948 Cub - been running fine. Converted to 12V neg ground, but prev owner used a 6V coil. Ran fine. Old 12V battery wouldn't hold a charge. Bought new one, but left the battery switch on by mistake overnight and it blew out the neg lead on the coil. Replaced coil with 12V, wont's start. Changed distributor points, condenser. Set gap. Won't start. Pulled plugs, cleaned them (pretty full of carbon) - checked spark. Good spark at plugs. Won't start. Put in new plugs. Won't start. Pulled carb and cleaned all orifices with gentle WD-40 jet. Won't start. I'm stuck. Ideas?

If you were running a 6v coil previously, whoever converted it probably put a ballast resistor in line to the coil to reduce 12v into 6v. Now that you have a 12v coil on it, you need to remove the resistor so your coil is being supplied 12v instead of 6. Right now you"re feeding 6v into a 12v coil and it probably doesn"t have enough poop to make a good spark.

Thanks. The 6V coil was marked "no external resistor required". There was none. Direct wire from battery positive, to switch, to coil positive. The only 12V I could by was marked "external resistor required". So I wired in a ballast resistor.
Other ideas?

Still doesn't sound right. A 6v coil should have 6v running to it, and a 12v coil should have 12v to it. Maybe someone else can add something here....

Check the fuel supply and make sure it's getting gas to the cylinders. Run a compression test. If you're getting good spark at the plugs, your problem must be somewhere else. rw

I'm trying to figure out; why would you need an external resister on a 12v coil if it's been converted to 12v?
Seems like it wouldn't need it. I'd try jumping around the resister.

Hold your hand over the throat of the carburetor and see if there's good suction on your hand while someone makes an attempt to start the tractor. If the suction is weak pour some engine oil on top of the pistons and crank over the engine a few times and reinstall the plugs. Then see if it start. You need good intake manifold vacuum to pull that fuel into the engine. Hal
PS: Run a compression check on all the cylinders and compare the readings.

More specifics about when the problem started. With new 12V battery installed, but 6V coil, Cub started and ran fine for 6 hours. I received a crank and wanted to crank start it. It started fine with crank, and ran for about 2 minutes then abruptly stopped. I assumed out of gas - that's when I left the battery switch on and blew the 6V coil. Some of the suggestions don't seem to fit with an abrupt stop and then no firing at all.
After cleaning the carb, how do I know if gas is actually getting into cylinders? I get whitish smoke on cranking engine, but only rare occasional firing, sometimes when cranking stops a backfire...
Other ideas?

The new 12V coil I bought is marked on the outside "External resister required", so I put one in.

The 12V coils marked "external resistor required" are really 6V coils. They are marked that way for stupid people with old collector cars.

Have you checked for a good strong spark? Pull a spark plug, put it back in the wire, and lay the plug against the engine block. Crank the engine over. It should spark bright blue.

You should pull No1 plug near the radiator and bring that piston to TDC on the compression stroke. Then remove the cap from the distributor and see where the rotor is pointing. Should be at the No1 plug tower. Are you sure you have the plug wires in the correct firing order? Your plug wires should be in this firing order. Hal

2-1
4-3 as you view the cap.

Bought and install new spark plugs. Check spark with plug out - looks good. The intake on the carb sucks pretty well. Still no starting. On suggestion from colleague at work I tried spraying starter fluid in the carb. No firing...
Could it be the timing off? or spark too weak?

I will check that. Would that be when the timing indicator on the fan pulley is lined up with the timing mark?
However, I was very careful to never remove more than one plug wire at a time from the distributor cap, so order should have never changed from previously running just fine..

So tonight I tried the hand-suction test on the carb intake. Sucks pretty good on cranking.
Then I tried advice of trying starter fluid sprayed in intake. No firing. Couple backfires when I stopped cranking, but no good firing.
Pulled each spark plug one at a time and laid on exhaust manifold and cranked. Good blue spark on every one of them.
Then I tried jumpering around both the battery switch (left it on anyway) and the ballast resistor. Direct wire from battery pos to ignition coil positive. Still nothing.
I tested the compression on each cylinder when I had the plugs out. They read 111, 114, 123 and 118 psi. What should they be?




Other ideas?? I'm stuck.

The attempt to spray starting fluid into the intake may not be telling you anything, because you can easily spray too much fluid, flooding the engine, or spray it incorrectly, against the choke plate for example, and it just puddles up and runs out the intake. If the engine is already flooded, no amount of starting fluid will make it fire.

Check the timing. Rotate the engine by hand until the pointer on the front of the engine lines up with the notch in the crank pulley. Figure out which post on the distributor cap goes to the #1 cylinder (should be uppermost post on the right, facing towards the front of the tractor). Remove the distributor cap. The tang on the rotor should be right underneath the #1 post, or directly across from it. If it's not right under, rotate the engine 1 full revolution until the notch and pointer line up again. If it's still not right under that #1 post, the engine is out of time. Now you gotta figure out why.

Normally, the rotor tang should be pointing roughly in the 2 o'clock direction, as you face towards the front of the tractor. It can be made to run in many other directions, and somebody may have ignorantly rearranged things in the past to make it run rather than setting it up right. What's important is that the rotor tang is underneath the #1 post on the cap when the notch and pointer line up on the front engine pulley.

You may have the spark plug wires out of order.
The distributor may not be properly tightened to the engine, and could have slipped.
One or more gears between the crankshaft and distributor may have stripped or fallen out (sometimes major problem).

Facing the front of the tractor, the four posts on the distributor should go to the four cylinders as follows (IIRC):

3 1
4 2

#3 and #2 may be backwards, because I am going from memory here. The engine will fire and run like a John Deere if #3 and #2 are backwards, but it will run.

OK - Thanks for the checklist. I will check these items. A dumb question (but that seems like all I have left! ;-0
#1 cylinder is the front cylinder, right?

The point about gearing between crankshaft and distributor concerns me a bit.
The distributor rotor turns when the cap is off and I crank the starter motor, so it is geared at some setting, right? Also, I get repetitive spark at each plug when they are removed and starter is cranked, again telling me the electrical is all working on some cycle (maybe wrong timing, granted).

There was one thing I did different just prior to this event - starting the tractor with a hand crank. Just fishing here, but is is possible that starting the engine by turning it over from the crankshaft pulley somehow caused the distributor shaft gears to jump a notch or something odd? I know this would show up as a timing error, so I will check. Just trying to figure out why this suddenly occurred.

#1 is supposed to be the front cylinder.

I was mistaken earlier. The order on the distributor is:

2 1
4 3

FINALLY! I GOT IT!!!
Thanks to those who suggested the following:
- pulled #1 spark plug, put finger over hole and hand-cranked the engine until the timing mark lined up on the rotation when compression occurred.
- then pulled the distributor cap and ... low and behold, not pointing at plug #1 wire, but between #4 and #2 - not 180 degrees off but way off.
- pulled distributor plate out and then removed bakelite rotor cap. Gearing in rotor to distributor shaft very loose and floppy. cleaned all the grease off, and thanks to my Cub buddy Roger's suggestion, noted that both gears had timing marks inscribed AND they were way off. When re-alined properly, the rotor cap now pointed at the marks I made for the #1 plug wire.
- Put it all back together and BOOM! Fired right up like a charm.
Best of all - Cub had previously smoked, and I assumed worn piston rings. Now I think the timing was off by 180 or something, and somehow recently it jumped one tooth on that distributor rotor gear.
Now it runs VERY smooth compared to before, and very little smoke...
Hooray!
Now for fixing the pto clutch...
Thanks to all who helped.
Gevan

Not to rain on your Cheerios, but it will likely happen again, and soon. You'll know what to do to fix it this time, bit it will keep happening, more and more frequently until the distributor won't turn at all.

These things don't just jump time for no reason. Usually something is badly worn and needs to be replaced.