1983 VF750 Sabre ignition question

Hey there everyone! I have a 1983 VF750 Sabre and just finished rebuilding the carbs. Hit the start button and it fired right up and went on fast idle. After about 30 seconds I turned off the choke and it idled perfectly!! I checked all 4 exhaust pipes and they were all the same temp. Things were just how you would want right!! All of the sudden the tachometer quit and the back two cylinders quit firing! After looking at a wiring diagram I determined that the rear cylinders are 1 and 3 and that particular coil controls the tach pulses! I noticed that the two spark units had the same part number so I switched the connectors. I started the bike again and now the tach works and the opposite two cylinders are not firing! Can one of the spark units go out just like that? I checked the pulse generators and they were both at the correct resistance. Can I assume that a spark unit somehow puked that fast?? Could anything cause one to fail like that? They are all over E-bay but would really like to know what caused it. The bike only has 14000 miles on it. Thanks for any suggestions!

 

Check all the related wiring, tap on the spark box wiggle the connector see if anything changes while running.

 

Pulse generators (pick up coils) commonly fail as engine heat increases but may be fine for the first few minutes.

 

Thanks for the repllies! I went out there today after work and hit the start button and all the sudden it ran great! What the heck is going on with this thing? It's been runnin real good since then but now I am thinking it's a connection problem. I will start from the pulse generator connectors and clean every connector in the system and eliminate that part anyway! Could the igniter have an intermittent problem? I am going to be gun shy riding it now!

 

Probably not, since cdi failures are usually terminal and final , not intermittant.

 

Well I purchased one on E-bay for 40 bucks so if it turns out to be something else I guess I will have a spare! Does anyone on here happen to have a copy of a Honda service manual for this year Sabre? I hate the other manuals!! Thanks

 

This is a good site for Sabers and older V4's. The Compleat® SabMag Links