Fluctuating tachometer

I got a bug up my arse and went looking for the site I got the above info from. The first link is his motorcycle page. The second is ignitors. With more info than above and another good read. Checkout the carb rebuild section.

http://users.rcn.com/kochc/moto/moto.html
http://users.rcn.com/kochc/moto/spark/spark.html

 

I wish Hawkrider had followed up on this. I've got the same issue. Good thread but the links are broken.

 

I can swap the boxes on my 750 and my tach works with either.. I have have two of each box and get the same results on any of them.

Only times my tach was being sporadic or non functioning is when the 1-3 pulse coil was kicking out. Whats of course different from the OP for me is that my 1-3 coil was also kicking out and he claims to have no change in the bike other than the tach.

 

I think I have the same problem as the OP- No drivability issue. After 10 miles on the freeway going 70+, the tach started jumping erratically and at one point went to almost to 0. Turned around and on the trip back it only dipped once. Road for an hour on back roads and it never acted up.

 

Well, what do you think, should I buy this ebay tach just to have on hand?

$15.00

I assume the face plate and needle can be changed easy enough?

 

Robvg, I read the majority of this thread and didn't see any mention of the fuel pump relay. Assuming that the VF700 bikes have a similar electric fuel pump and fuel cut safety relay then your yellow/green tach signal wire will go from the cdi box to that relay by the pump. Now the wiring diagram shows the tach signal to T off before the safety relay, but on my bike if I disconnect that tach signal from the relay the tach goes dead. It shouldn't go dead, but it does.

If the if your bike is similar, then check the tach wire connection to the safety relay.

 

Don't count on it. Everyone I've tried the needle broke. They are so fragile to start with, then 30 years of drying out and getting brittle if you sneeze wrong they break. Plus never did get the base off one needle.

 

Well I think I got it covered then. There was a complete instrument cluster on ebay for $10 plud $12 shipping. For that price I just had to bid. I was the only bidder.

 

As long as I got you here GreyVF750F, if it still runs fine, then the coil is ok and it must be in the wiring or tach itself?

 

I don't think it's in the wiring unless it's at the dash harness/area. The 1/3 ign box controls the tack/coil period. The 2/4 controls the fuel pump relay/coil period.
The 2/4 will not work the tach. I've swapped boxes on about 6 different bikes and they all act the same. This is for 83-85. Plus all my research in to them says the same.

The blue wire on the 2/4 triggers the fuel pump relay. The yellow on the 1/3 triggers the tach. Now if it's not the dash/tach, which it sounds like. AND if you don't have any running problems or power loss then that means your ign box for the tach is working fine for firing I would think. Though the coil is only looking for a pulse. The tach could be looking for a particular wave shape or amplitude. If it's not correct then your tach might jump around. I saw one ignition box work fine for firing the coil but the tach didn't work. Changed boxes and all was well.

Hey it's electronics and there's this thing call FM that even logic can't explain.

You really have to make sure that the coil, pulse generator and stuff are working fine. It may be hard to feel a power drop for an instant. When the tach acts up nail the throttle and see what happens.