38 zts timing update - Magneto issues?

[TrustyRustyTractor]

I've noticed my Magneto, a Fairbanks k4b, spins over hard at times, it seems to randomly "catch" or lock up and you really need to put some elbow grease to it to get it to spin again. When its mounted on the tractor, I noticed the rotor would stop moving off and on while I was still cranking the engine by hand. After cranking it over several times, I lined the timing marks back up on the flywheel and , of course, the magneto rotor position was now no where near the #1 terminal where it should of been; What's causing this? I previouly posted that I thought my tractors timing issues were do to the magneto drive coupling being off, but it now appears it could be the magneto itself - which I never thought to check since I bought it professionally rebuilt.

 

[Bob]

The "catching" you are noticing is simply the impulse coupling doing it's job.

 

[Ole Shovel]

Like said it may just be the impulse doing what it is suppose to do. Make sure none of the teeth have stripped off the fiber dist gear. If the rotor is out of time email and I can send you a manual for the FM K mag. If impulse parts are worn out not working right and got a back fire that can cause the dist rotor gear teeth to strip too. When all in time and the engine starts the impulse should stop tripping and not hear that clicking sound at the mag if it"s still clicking shut it off the impulse is worn out and if don"t fix will thrash something else.

 

[Ole Shovel]

Magneto manuals, basic parts they don't have all real parts numbers or service info.

http://www.oldengine.org/members/diesel/Magnetos/PetrolMag.htm

http://www.oldengine.org/members/diesel/Magnetos/MagData2.htm

FM Magneto Fundamentals 64 pages

http://www.oldengine.org/members/diesel/Magnetos/MagFmBook.htm

FM Mag Impulse Couplings:

http://www.oldengine.org/members/diesel/Magnetos/FMCouplingMenu2.htm

 

[Anonymous-0]

It could be you are witnessing the operation of the impulse.
I had a Fairbanks Magneto on a '38 Z that stripped the distribuitor drive gear (it was phenolic and pressed on a splined shaft.) That would fire every now and then when the rotor just happened to align with the plug wire to a cylinder that had gas in it. It drove me nuts until I found the stripped gear.

Another thing is the drive gear is pinned to the oil pump shaft. a stripped pin could cause intermittent turning.