American Bosch timing

[ssimntih]

got a mag for a C case, a bosch mja4b-108. while im letting the engine soak i figured id get a head start by fixing the dead mag, so i tore it all apart. little did i know there are two settings for 3 different pieces. theres a timing mark on a small disk on the metal gear that meets up with either an "A" or "C" printed on the rotor. there are two keyways on the impulse back plate, and two keyways in the cam that runs the points. i didnt pay any attention to them taking it apart and now im not sure what settings to use for my tractor anyone know how to set this mag up for my case?

[Rodrigue Caron]

Hi, A =anticlockwise C =clockwise. Assemble all parts throuth magneto using same marking. Good luck Rodrigue

[ssimntih]

i can time the rotor that way but the points cam and the impulse have no markings

[Dempster]

There should be A or C mark on the cam, impulse drive hub has keyway in it, impulse weights & dist rotor. Buff the rust off and look harder I've never seen them without marks yet. Your cam should have two key slots and the impulse should also marked A or C on one side or the other. On the impulse weights you put those back in with the C side facing the drive cup has the spring in it since your Case C would use a CW drive mag. The MJA, MJB, MRF, all shared same type markings on rotor and cam and also others like MJH & MRD used same type impulse with same markings. Can't tell you right now without looking up the individual part numbers on which side the marks would be because there lots of different cams and impulse hubs and marks could be on either side when facing the slots. Buff them off and look harder bet they are there.

[Dempster]

MJA4B-108 is base mount, CW drive, 30 degree impulse. Should use a ICB2A2 impulse with a HB7328 hub. On that impulse hub with the tab pointing away from you and upward then facing center with key slots to the bottom the RH keyway should be marke "C" and LH "A". You want to use the "C" keyway. That mag should have a timing lever on front cover being what is called a timing range mag using a CA525 cam. Facing the keyway slots on the cam with slots pointing upwards the RH slot is "C"and LH "A". If has no timing lever and fixed timing mag it should use the CA52106 cam and slots should be marked the same. I checked those on new parts today. If you still can't ID it send me a drawing, pic, email or pm here. Have a chart is actual size you set cams against to ID them but it doesn't show them all as there many other cams used over the years too. There was a MJA version A,B,C, & D. The MJA, MJB, MRF can be used to build up a 1,2,3,4,5 or 6 cylinder mags is why there were so many cams, impulses and rotors for CW & CCW drive.

[ssimntih]

its hot again thanks for your help guys. the reason i couldnt read the markings i discovered today was because someone had beaten on a few of the parts and filed them smooth or round again, and by said filing had removed some of the marks.

[Dempster]

Glad you got it going and hope no parts were too worn or damaged. I have lots parts for American Bosch yet new and used some the factory no longer has or will ever make again.

[ssimntih]

i did leave a part out when i put it back together, there was a little thin disk on the steel gear with a timing mark to match it to the rotor, it was loose and had started to eat up the teeth on the rotor so i made a mark in the gear and put it together without the disk, i hope this doesnt hurt anything

[Dempster]

It's fine without that just uses that slot to line up the gears and suppose to help keep carbon from brushes out off the gears. Carbon or broke off brushes can still get in there with that washer on anyway. Keyway slot wears in that piece if it comes loose and those use to come with a overhaul kit.