New issue with the resurrected VF500f - Backfiring and erratic tach readings

With help from this forum, I got my fire-damaged VF500f back on the road, and the bike was running very well. Only known issue was that it was charging at about 15.2 volts when rpms were up. Ordered a used MOSFET rectifier, but wanted to run the bike a bit to see if I could get the fan to turn on. Several minutes into my ride, the bike started backfiring pretty badly, running very rough and the tach started shooting up and then dropping to 0 rpms, but the bike was still running. Best guess is that I damaged some of the ignition components with the high charging voltage? Received the R/R and that will go on tomorrow. But any ideas on what components I need to check out? Battery? Coils? Spark Units? Anyone experience anything like this? Thanks for your help.

 

Wait til you are chargimg correctly amd have known good battery and see whats going on then.

 

Will do. Will follow up after those two components are sorted.

 

Jury rigged the new R/R this afternoon. Checked battery voltage off the bike and found reading of 13.1 volts, so looks like it was overcharged with the old R/R. Bike started right up but wouldn't really idle well. Voltage at idle 14 volts and 14.3 volts at about 4000 rpm, so the charging issue is resolved. But the bike wouldn't idle and when revving was still popping and backfiring. Tach was mostly reading zero, but with intermittent little fluctuations, but nothing very high. Shut her down, and found the two rear exhaust pipes cold, so the rear cylinders were not firing at all. Just for kicks, swapped ECUs and started her again. Rear exhaust headers started to heat up, tach was jumping around a lot more and rising higher, but still bouncing all over the place. Couldn't tell if the front cylinders were also firing as the pipes might have still been hot from the previous run. So will try again tomorrow after she cools down.
So I guess next step is to check both coils to see if they check out. If they seem good, then I would think I damaged one of the ECUs? Clymer says that the tach is driven off of the rear (cylinder 1 and 3) ECU? So a zero tach may be telling me that the ECU driving the rear two cylinders is fried? Anyone have experience with a bad ECU and have seen these conditions?

 

Although relatively rare, they can go bad. See if your issue has swapped cylinders. Honda coils on the other hand rarely go bad. Sometimes they crack and can ground out, but they are still "working".

The good news is there's a bunch of parts out there.

 

I've repair'd 4 or 5 Ignition Boxes. The FET usually goes (2SD1071 or equiv ($1 each)). Did one a month ago. Also beef up some of the solder. New FETs also instal'd with Heat Sink Compound. Transistors driving Inductive Loads take a beating (Capacitive Loads too).

 

Retested today with the ECU plugs still switched. seemed like all exhaust pipes were getting warm, but the bike was still stuttering, backfiring and the tach was bouncing all over even at relatively even idle. So before troubleshooting, I replaced the jury-rigged wiring to the new R/R and tidied that up. Then swapped the ECU plugs back, and started her again to check charging voltage. Charging at about 14.1 at idle and about 14.3 at revs, but the backfiring was totally gone and the tach was now acting like a tach. Took her out and she is running fine again. Best guess is that the higher than normal battery voltage was causing some disruption in an ECU? Letting her sit off the charger for a few days let the voltage drop a bit. That's a total guess, as I didn't change anything besides the R/R. I will watch her to see if the symptoms return at some point. Now I will just put some miles, and some new tires, on her. These VF500fs are so light, they are wicked fun to ride!

 

That's awesome that you know how to do that. To most us, those are mystery boxes that either work, or don't! I daresay most dead electronics just need a really cheap part and some know-how to put right.