Why won't it fire?

[Glen]

Hi,
Thanks for answering about it being a USA Cub.
The way they say on here to check the spark is to pull the center spark plug wire out of the magneto cap, and hold it by the insulation, with the end of the wire 1/4" away from a place with no paint, on the engine block, turn the ignition switch to on, and run the starter, there should be strong blue sparks.

It should also have blue sparks if you remove a spark plug, and connect it to the ignition wire, set the bottom of it on a bare place on the head, and run the starter.

Yellow sparks are weak, and the engine may not run.

If the spark is yellow, the ignition points, condenser, or coil could need replacing.
New ignition parts can be defective, and not work right, something in the magneto might need replacing again, if it's not making blue sparks.

Below is a pic of the magneto drive, governor gear. It should be in this position when the engine is at number 1 cylinder TDC.
Sometimes people have the governor off the engine, and turn the gear to the wrong position.
The timing gears have marks to align them to the right position.
The 2nd pic, from TM Tractor, shows the gear teeth, and the timing mark, on the left side of the pic.

[Moray]

I spent some time on this again tonight.

Took the magneto off, removed the rotor arm cap and made sure the marks lined up.
Spun engine so no.1 was on compression/TDC. Made sure rotor arm pointed roughly upwards, and fitted magneto in to slots, with top of magneto fully in towards engine.
Rotated engine one revolution until crank pulley TDC mark lined up with pointer, then pulled the top of the magneto away from the engine, until the impulse tripped, and tightened nut.
Also checked the HT leads are where they should be.

I've not got a compression tester (I couldn't remember if I had one or not, as it's been a long time since I've used a petrol one!), but when turning the engine by hand, there is noticeable resistance as each cylinder is on it's compression stroke. I know I can borrow a tester from work, but I'm not sure if it'll be suitable for the bigger spark plug threads.

Tried tipping some fuel down the bores, and LPG, but still nothing.

I'm pretty confident nobody has been at the internal timing, as it doesn't look like any of the front cover nuts/bolts have been off for a long time. The magneto slots are at roughly 2 and 8, as per Rick Spivey's and Glen's posts, and the valves seem to be opening and closing when they should.

[Glen]

Hi,
A problem the starters can have is the brushes and bushings wear out, then it won't work right.

The starter has a bushing at each end of it. If they wear, then the armature can rub inside the starter.
If the armature is rubbing, it makes the starter run slow, and use more power than normal.
It only takes a small amount of wear, the parts run close to each other.

If you remove the hood, the original Delco generator on a 1948 Cub has a cutout, or Relay, IH called it, on top of it. The model number on the plate on the 6 volt gen was probably 1101355.

Below is a page from the 1950 Cub operator's manual, showing the electrical system with the hood off.
It might help you identify if your Cub is original.
You can look up under the hood, using a light from the left side, and see if the gen looks the same as the pic, and has the Relay on top of it.

http://farmallcub.com/rudi_cub/www.clea ... e%2033.jpg

[Waif]

Have you double checked spark plug gaps?

[Dale Finch]

As suggested above, did you disconnect the kill switch wire at the mag? I recently had a SA with a mag that wouldn't start. FINALLY (after rebuilding the mag!) disconnected the switch wire on the side of the mag, and it started right up! I replaced the wire from the pull switch on the panel to the mag, and no more problems.

[tmays]

He says it fires, so kill wire is not the problem

Governor timed to the engine?

[Dale Finch]

Ah Yes!! DUH!

[Gary Dotson]

Sometimes they can be just plain cantankerous. You might have to pull start that first time. I was once visiting George Willer and he had a Cub that he had just assembled and was ready to fire up. We cranked on that thing till we turned blue, checked and re-checked everything, still wouldn't fire a beat. George had a very large electric drill and he had made an adapter to fit the crank opening. We started spinning it with that thing and after a lot of spinning, it finally fired off. After that, it was one of the best starting Cubs George had. It made no sense at all!

[tmays]

Agree with Gary. When they won’t start up and I feel everything is right, we pull it to start

[ntrenn]

Gas fouled plugs? Have had that problem a couple of times....didn’t believe it until I had it happen to me. Will cost about $10 to put in new plugs.

[Moray]

I've not had any time today to try anything, but it's got a new set of plugs in it.

Attempting to pull start it was something I thought about, but I wasn't sure how well it would pull start.
If it's dry tomorrow, I might get a towrope out and get somebody to give me a tow around the field, and see if we can eek it into life that way.

[Eugene]

Tow the tractor from the drawbar and not the bolster.

[k hutchins]

Eugene
Having never done it, please clarify. You tow it backwards then using the draw bar? Or reverse the draw bar and run your rope/strap under from the front? I'm confused.

[Bob McCarty]

Run a chain or tow rope from the draw bar forward, under the front axle to the pulling vehicle. Have the Cub in 3rd gear, gas and switch on, and let the clutch out after a little speed has been picked up.

[Dale Finch]

Just to add to Bob's post, you don't need to turn the drawbar around.