1978 Yamaha XS750

01:00
02/25/2020


I bought this one a couple of years ago, for a good price. I rode it for a couple of weeks, and it started acting up. First, it wouldn't rev all the way up, and then it kept getting slower and slower. Eventually, it wouldn't start. I looked into it, and the number 1 cylinder had spark, but 2 and 3 did not.

I bought a new (used) control box off e-bay, plugged it in, and the same thing. Spark on 1, not on 2 or 3.

I bought a new (used) trigger plate, installed it, and now I've got spark again.

Started it up, took it for a ride, and it wouldn't rev past about 5,000 RPM (red line is 9,000). Then, I started searching the 'net. Turns out that this bike had an early electronic ignition, done before the engineers had it all figured out. Less than optimal trigger mechanism. Weak coils. And it's now 30+ years old, so what can you expect? While looking around, I found this write-up. Thank you, Dr.Doug!

So, taking a clue from him, I started working on a new ignition.

Got the Mopar reluctor. (At $3.69 each, I bought 2. Stuff happens.)

Got the Mopar pick-ups.

Got the Chevy HEI controllers.

While I was debugging the problems, I got a bit carried away with the e-bay purchases. I ended up with 3 extra sets of coils.

I still have my original (not working) trigger plate.

I bought a points plate, with point cam, from an earlier model of the 750.

While I was starting to collect the necessary new parts, I sorted out all the stuff I had collected, and re-read Dr. Doug's write-up. He did a lot of work making the mounting plates out of aluminum, and he was wondering if making them out of steel might be better in limiting eratic triggering of the pick-ups. One day, I'm looking at the original trigger plate, and got to thinking If I could salvage it to use as the mounting base for my new setup.

Looking at the back, it appears that the units are held to the plate with three screws, and then covered with the insulation material. How to remove the material.....????
In this picture, I have already ground away some of the insulation material, and removed 2 of the 3 screws. More on the process, later.

Here's the nearly final part. I ground off most of the insulation material with the wire wheel (in the electric drill). Then, the plate was attached with the screws in the middle of the picture. They were a #1 phillips head screw. The cross hatch was totally filled with the insulation. I got out the Dremel, with a diamond tipped cutter on it, cut a line, and un-screwed the screws with a straight driver. It all worked better than I had any right to expect.

Even after all that, it was still glued to the plate. I ground out the three square areas (from underneath) as best I could, then 'encouraged' it apart with good old hammer and chisel, judiciously applied. At the time of this writing, I'm waiting on some of the parts, so I'm not doing anything else with the plate until the Mopar triggers arrive.

O.K., the other parts arrived. First thing to do is to separate the pick-ups from the extra mount. They are held on with a brass rivet (hollow in the middle), and a screw. I removed the screw, put the item in the portable vise, and drilled out the rivet with a # 32 drill. Wiggled the drill around a bit, and the rivet was history.

Here's one of the extra plates, after the pick-up was removed (Honest, the photographer wasn't inebriated, just jerky!):

Next, the old Yamaha mounting plate had some irregularities that needed to be removed, There was a bent-in section at each of the old triggers, and a boss for each of the old hold-down screws. A little time with a cut-off wheel and a grinding disk, and they were gone.

Here it is, after I cut off the first bent part:

 


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Revision: 01.0     Email Doug     08/15/2011