Misfire when hot

As per title, I have now done around 2000kms since the rebuild and the bike has run without fault until now. Seems to run great when cold and even when ridden gently. I have checked the fuel supply and have new Iridium plugs on the way as I wanted them anyway, but I am suspecting pulse generators. If this is indeed the case, are the 85 ones interchangeable with the 84 ones?
Would coils break down with heat?
What does the brains trust have to say?

Is the misfire engine speed or throttle position related?

If it’s revving freely when cold, it’s not likely to be ignition.

If it builds revs gently without misfiring, it’s also unlikely to be ignition.

Pulse generators either work or they don’t, really.

Coils can be troublesome, but again either work or they don’t.

HT leads, however, have been known to break down when they get hot, so that’s something worth looking at. They’re not that costly, and may well be worth replacing. How old are they?

From what you’ve described, I’d be looking at fuel or HT leads.

Thank Chris, will throw the plugs in and go from there and let you know
But first, off on a 2000k 4 day loop on the Blackbird with a few mates.

Not jealous. Much.

This is a favourite trick of the pulse generators,easy to test the resistance on them though.
unplug the pulse generators and check the resistance across the white/yellow and yellow wires then across the white/blue and blue wires,you should get a reading around 470 ohms cold
Get it good and hot and recheck the readings,the value is not all that important only that the two readings are close.
A big variation would suggest this is the fault.
84 and 85 use the same pulse generators but the 85 set will also include a connection to the temperature sender(easily removed)

OK thanks Bif.
If the pulse generators are the issue, is there a reliable way to test my spare set prior to installation? Hot oil on the stove??? My 85 engine is an unknown quantity, looks like it was taken off the road due to a badly scored bore from a failed circlip but it may have had other issues.

Try these-- VF Spark Coxes

I had weak pulse generators on a VF1000F and these worked well enough to fix the problem. I sold the bike after about 10,000 miles and they were still working great,

I think if you check your 84 set cold you will see a fair difference the two.If the 85 set check out cold and the values are the same or very close I would be confident they are ok

Back from the ride and back to the VF. The 84 pulse generator readings are 444 ohms and 446 cold and 570-571 hot. The 85 are both 480 ohms cold.
My new starter clutch springs are due in this week so I will swap springs and pulse generators out then and see how it goes.

OK, job done and it seems to have fixed the misfire. Thanks for the advice Bif.
Interestingly, the old PGs appear swollen in as much the plastic encapsulation is now 0.7mm proud of the metal pickup.
I was also reading that some of the more modern Honda PGs can be modified to fit as well so this may be an option as old parts dry up.

Well it’s back. Sometimes just one cylinder and sometimes now two cylinders. Worse when the bike sits after it is ridden and heat soaks. Normally comes a little better when the speed increases. Coming into the Aussie summer we are above 30 degrees more often than not. The cap resistors check out fine but of course this is when they are cold. Bike has the original plug leads in copper so I don’t expect those to be the issue but I have two metres of new lead ready to go in when the new NGK plug caps arrive. Other than that the coils measure OK coils.
Has anyone had coils give problems when hot?

OK folks, resume normal service. One coil badly cracked and the other with a stretch witness in the plastic so I expect it is not far behind.

I will try to source a new set locally.