No spark at plug

Hi folks, I have a 1941 Allis chambers b. I have a problem with it losing spark, I would remove just the distributor cap wipe it off and it would start right back up. After a few times of that, I removed the rotor body checked the points and condenser. ( of course I can't find no marks on fiber gear) it be nice if I could find picture of a new fiber gear I would just count the teeth)). So I went through standard procedure making sure number one cylinder was coming up to top dead center via thumb in the spark plug hole, checking the fly wheel line center mark . Then lined the rotor button up on the timing boss Mark in the rotor body. I advanced the engine around and observed the rotor at each magneto snap ended at the next cylinder according to the distributor cap. I replace the rotor body put a Phillips screwdriver in the hole touch the coil and rotated the engine and he had spark at the coil. I clean the contact rod that goes through the rotor body to the coil . And replaced the distributor cap. My problem is I'm losing spark between the rotor button and the spark plug and I don't know why? The rotor button looks new but distributor cap is not, but the contacts do not look worn.

Rotor MAY have a crack in it, allowing spark to sneak to ground, but it's probably just timed wrong.

Might be a bit tough to figure out if the marks are REALLY gone.

As the impluse spring is winding up, the rotor tip will be stopped ahead of a high-tension terminal, then, as the impulse "trips", at the instant the points open, the spark is created, and the rotor needs to have moved in line with a high-tension terminal, after impulse has tripped, the rotor will be past the high-tension terminal.

What is the make and model of your mag?

Also, you can GOOGLE the make and model, and likely pull up a factory service manual that's been uploaded.

First of all timing that way using the center line/mark it will never start. All static timing is done at top dead center line. Always.

All of my tractors set outside year around. I have to make sure the magneto's have good gasket under the cover and cap or moisture gets in. As Bob said check for cracks. Also look for carbon lines.

I think your right, it passed the terminal then fireing that has to be problem , all tho when the rotors at rest it lands at terminal. It was running well before it lost spark. Went and checked mag. Model number. It's Fairbanks morse and the only letter it has it J. I have 3 of those fiber gears are the letters on the shaft side or backside?
Thanks for your reply.

Thanks for the reply. So the center mark on the fly wheel is not top dead center for number 1 cylinder?

Lacking a timing mark on the fiber gear, why not make your own mark, then set it back a tooth at a time 'til you get spark out of the 4 high-tension terminals?

I LOVE challenges like this, and have gotten many old engines running using simple common sense, and figuring out how to address these timing issues.

It ain't "rocket surgery", and using common sense can usually fix it!

I found this picture online is not the model of my mag. Don't under stand why there's not one photo of this gear w/ labels <brsrc="https://www.yesterdaystractors.com/cvphotos/cvphoto9759.jpg">

The center mark is 30 degrees advanced. When using a timing light the center line would e what you would line up in the center of the inspection hole at high idle. throttle wide open.

You can find the center line when setting the magneto on the tractor when the magneto is ready to fire number one plug. Then when cranking and it reaches top dead center it will fire number one plug.

It becomes confusing for people reading the Allis manual on setting the magneto that does not know that virtually all four cycle engines are static timed to number one cylinder on the compression stroke at top dead center. The mechanical advance changes with the speed of the engine.

To the left of the number one terminal there should e a lug or whatever you call it the rotor should be pointing toward. When hand cranking the impulse spring needs to spin the armature to produce power for spark before the rotor passes the number one inside terminal. If you set the magneto to the terminal like a distributor the armature would spin after passing the terminal.

Sorry

If you set the magneto rotor to the terminal like a distributor the armature would spin after the rotor passed the number one terminal.

I will check the tomorrow, I m using that lug as as timing reference mark, the magneto seems to be snapping at that Mark. I think tomorrow I'm going to back it up to make sure it's not going past it.
Thanks for the reply dick

Turn the mag upside down so the impulse doesn't catch. Turn it so the points just break. That is when the rotor has to line up with a terminal in the cap.

Wait a cotton minute. I am the one confusing you! TDC and Center is the same thing. You were correct. When I read center I was thinking the Fire line. You would have been in time! Where I typed center above I was thinking FIRE.

Using that lug as you stated should also be correct.

First of all thank you for all the replies. tired of freezing my booty off trying to get this thing timed, I decided to remove the magneto, after reading through the owners manual and number of online references on this magneto is stated where the steel gear mark should be. one tooth has chamfer and some are also marked with a red paint. When I found was the red painted tooth and a stamp mark/ chamfer were a tooth off from one of another. With the use of my cell phone camera screen mirroring and a 50 inch TV I blew that fiber gear large enough to see the C..
Got to use what you got LOL . Cleaned points reassembled the magneto and has spark out the end cap, going to replace on the tractor tomorrow see how it goes . Thanks again for the help. Newbie

IT'S RUNNING I got magneto 101. I was amazed how smart the madneto was engineered. I found this tractor free on Craigslist last fall not running, guys said he rebuilt it few ago, Been setting now locked up. Pulled pan check what was visible, didnt see anything. Cleaned oil pickup. Then I pulled the starter, must have been the bendix hung up, now I can roll it over. But it had a knock, I found front drive pulley loose.
We have a huge tractor show ( fly wheelers) in our area maybe take there.
Thanks for the help

Michael, Michael, Michael, you is one lucky dog. Have you tried the lottery lately? :idea: