Question1983 VF750F interceptor 10,000 miles on it. I have tested the main coils, plug wire and resistors, changed spark units twice, tested pulse generators all by the shop manual and all has checked out good. The bike will start and then quits when it warms up. I fully charged the battery, drained the gas. The bike just started skipping one day and i have another interceptor and i tried the spark units in it and found that they were bad. I then replaced them 2 more times thinking that was the problen and still i have very weak spark. Is it the pulse generators? they test out ok for resistance but what i am thinking is that as soon as the oil circulates in the motor and splashed on the pulse generators they short out. I'm not sure.
AnswerPulse generator (pulser) failure is a pretty rare problem, so I don't think that is very likely. Besides, if a pulser wasn't working right it would be more likely to cause no spark than weak spark.
I think your problem is more likely to be in the plugs, coils or plug wires. Your bike uses the "wasted spark" principle to fire 4 cylinders from 2 pulser/spark unit/coil sets. Each coil has two plug wires and actually fires both plugs at the same time, but since one cylinder is at the top of the compression stroke and the other is at the top of the exhaust stroke, the spark in the cylinder that just exhausted is "wasted".
This means that the ignition's secondary circuit consists of the coil's secondary winding connected in series between the two plugs with the plugs connected to each other through the cylinder head. In other words, if a bad wire or cap or plug fouling keeps one plug from firing the other one probably won't fire either.
The first & easiest thing to try is a set of fresh set of DPR8EA-9 plugs. Before you put them in make sure the gap is the correct 0.9mm (0.035"). NGK plugs almost always come with exactly the specified gap, but it doesn't hurt to make sure - the package might have been dropped or something.
Next, I would think about the plug wires. Even the best silicone rubber insulation only lasts so long before it starts to crack from years of exposure to sunlight & air. Tiny cracks that you can't see can let moisture in which can short the spark on rainy days, so even if the wires aren't contributing to your present problem, replacing them is a good idea. Go to a good auto parts dealer and ask for Delco brand Packard 440 spark plug wire. This is 7mm, copper cored wire like your bike is supposed to have. Don't get automotive type resistive wire - see this article: http://www.motorcycleproject.com/motorcycle/text/plugwiretech.html
Don't forget to get new caps while you are at it.
It is possible that one of the coils is breaking down and produces a weak spark when it warms up. You can check this by running the bike until the problem starts and then pulling a plug from each coil to see which coil is causing the problem.
Or you could just change out the coils with the ones from the other bike and see if that solves the problem.
