'83 750 No spark on the rear cyls and tach stopped working...HELP!!!!!

I rarely ride my old '83. I did take it for a spin about a month ago and it was running just fine. I left in on a 1amp battery tender like I always do for about a month. I went to get it out today and found that the battery tender's switch had accidently been switched to the 6v setting. Anyway, I went to start it. Only the front cyls will run. No spark on the rear and now the tach no longer works. Where do I start? I do have 12v on one of the small wires on both coils. The tach worked just fine when I rode it a month ago. Did the 6v setting on the charger hurt anything?

 

If it helps any, while I was searching threads for my 3rd gen ignition problems, I came across info that may solve your problem. It sounds like one of the PG has crapped out on you. You can check them by checking the resistance between the yellow - white/yellow and between the blue - white/blue. I believe the resistance spec states that it should be 200-400 ohms. Hope this helps with your problem!!

 

...as I recall it, that was a pretty common problem on the first gen's, although I've never had it happen on any of mine. The igniter for those two cylinders took a sh#$. (under seat by tail light) It's probably still pretty easy to get on ebay. I'm sure I have an extra that I'd be willing to part with if you can't find one.

 

Yep the 2-4 cdi box seems bad. I have two of those and would be willing to sell one if needed.

First check and clean the connections at the molex plug. Sometimes they get oxidized and just need cleaning. You can swap the 1/3 box with the 2/4 box just to make sure it's bad. If your 2/4 cylinders fire after the swap then the box is bad.

 

**UPDATE**
You guys were right on!!!! It was the CDI pack behind the seat!!! I switched them even though they have different part numbers and the plugs are different colors. I fired it up and the rear cyls started the front didn't and the tach started working. I called the dealer and they laughed at me...no longer available!!! So, I called the motorcycle salvage yard 50 miles away and they had one...now I have one. I went ahead and bought another one from E-Bay which should be here in a few days just incase I have this problem again. I plan to keep this bike forever, so I guess I should start buying a few parts for it while I still can. Thanks for all the help. This place and you guys are worth a fortune!!!

 

If that's the case. You're going to need a few little things that you should by from one of the on-line OEM parts places while they are still available. PM me if you want to hear about it.