84 VFR 750 spark problems

Hello,

Any help would be great.
So my problem is as follows:
I try to start my bike and only have spark from 1 coil. I tested this by placing the plug near the frame and can see a spark. The other coil fails the test. to rule out a coil fail I swapped the power from one coil to the next and repeated the test. The coil that had power now does not and the coil that did not have power does now.
CDI is new as of last summer and spark plugs are all new.

Is it possible that I have a short feeding the coils on only one wire?

Any other suggestions would be great.

 

Honda didn't make a VFR until 1986.

Doesn't your bike have 2 cdi boxes ? check for clean, bright connections at the plugs for the 2 boxes. you'll need to check the pulse generator coils for proper resistances.

 

If it is a VF750 then there are only two coils and each pair of spark plugs fires together. If one of a pair works but the other doesn't, I'd suspect the plug leads or caps as a source of a short. You can check the resistance of the primary and secondary coils, VF750 spec is 2.8 ohms for the primary, and 21-28 kilo ohms for the secondary. Measure the secondary resistance from one plug cap to the other. If needed you can trim the ends of the bare plug cable to make a new fresh contact but don't take much off.

Would be worthwhile disconnecting the plug leads and inspecting the ends and connectors for corrosion. If they're like my ST1100, there will be a resistor screwed into the plug cap, held in by a crosshead screw accessed up the plug hole. Check the ends for cleanliness.

As Squirrel suggests, pull each connector apart and verify shiny metal. Corrosion in main harness connectors will also give screwy sparks.

 

Your test shows that both coils work and their leads are good. Squirrelman pointed you in the right direction. The pulse generators are a common failure item. Check the resistance and you will likely see that one is bad. They seem to be hard to find these days based on other discussions. You might find a good used one. One of the guys who has built an alternate cdi unit was looking into a replacement for the pulse gens using aftermarket units. Not sure how far he got.

 

Check the push on connectors and the crimp on the wire. I had to repair both, wires flexed and broke at the crimp on my 83.

 

Hello,
thanks everyone for the help to date.
To correct my first post yes it is an 84 VF750 not a VFR. Newbie mistake...

So a bit of an update. The bike sat all night outside and first thing this morning it fired up no problem. After the engine warmed in my driveway it shut off and will not fire now.

I am very new at this so excuse any dumb question or comment but could a coil fail after it warms up or the pulse generator? I am thinking the pulse gen is failing as I can switch the leads from one coil to the next and the coil seems fine even with the bike hot.

I am going to pull everything apart again this afternoon and test the resistance on the pulse generator once I figure out how to get to it.

Thanks

 

Ok so I pulled the pulse generator off the bike and when I test it using my meter I get 50o ohms off one set of wires and 0 off the other. I hope I am correct in assuming that this is my problem?

 

all that work ?? sir, testing the pulser resistance is done at the wires--after they exit the cover, not under the cover.

yes, after warming up an ignition coil or a pulser coil can quit working.

 

Yeah I realized how easy the test was only after everything was apart.

Picked up a new set of pulse generators this afternoon and after install plus oil replacement I am happy to report bike started up with no trouble. After a test drive of 100 km still no trouble. Hopefully my troubles are behind me

Thanks