Gen 1, no fire, whacky tach....Spark unit(s)?? help?

83 VF750F

Trying to get my 2nd Gen 1 to light, with no luck. Not even a pop when cranking. Also, the tach jumps all over when cranking.. 0-RPM to 4K and up and down. This is weird.. Looking at the elect schematic I see it is controlled by the spark units.

Here's what I've done:

Cleaned carbs - they were pretty good from the start but really spotless now.
Set the valves. Inspected cams.. they are dandy.
Gave it the "compression thumb test". My compression tester is still at a buddy's place, but the thumb test says there should be plenty to at least fire!

Visually checked spark on all cylinders. All have spark..

Confirmed cam timing when I set the valves.

Checked the exhaust for obstruction... all clear.

I don't have a timing light on hand or the special timing check cover, so I haven't checked the timing.

I did check primary and secondary resistance on the coils, and the pulse gen resistance. All is good.

So I have fuel, compression and spark, but not even a pop.

At this point, with the tach acting all freaky and spark but unknown timing, I suspect the spark units are screwed.

Also noteworthy is I have no headlight, blinkers or horn. All fuses OK. Also, temp gauge just pegs (max) when its connected to the sender. I don't think these items are related to not starting though?

Neutral light and oil light DO work.

 

One more thing. I even sprayed either right down the throats of the carbs and couldn't get a pop.

 

Yep sounds like you got a problem or two. With all that electrical bugaboo you might want to look at/clean all the connectors behind headlite. Have you looked in to the "kill" switch circuit? How about the connectors at the boxes. Have you clean the contacts. If all of those are good, then I'd might expect a bad box. Even if one box was bad the eingine should fire and run on two cylinders. That's why I don't suspect the boxes first.

 

This is getting weirder. I started checking voltages at a few points. The ignition switch is switching everything like it should.

Noticed the tail light wasn't working either. At the plug for the tail light control box I have a good continuity from green to frame but no 12V on brown.
Checked the headlight plug, no voltage.

Now this seems really odd: With the headlight fuse out and the plug off the bulb I have 12V on either side of the fuse holder!!!
Looks like the start switch cuts out the headlight, so maybe something is dead shorted in the start button. I guess I'll look there next.

 

I think you are on the right track with the spark box(es). You might also check the pulse generators as well. Good luck!

 

The starter button is fine, it is cutting voltage to the headlight when pressed but supplying when not.

I'm really scratching my head here. I traced voltage through the start button to the left control pod, and at the headlight plug I have voltage from white to frame and blue to fraem as I should, depending on which position the hi/low headlight switch is in. However, I don't have voltage across the plug itself (white to green or blue to green). Something is messed up, I think, on the ground side of the headlight plug. When the key is on, I actually have 12V on the green wire @ the headlight plug to the frame. That sure can't be right.. as there is then no voltage from white to green or blue to green. I'm haven't traced that green all the way to ground yet but something has to be crossed up somewhere.

Side note, the wiring harness doesn't appear to have been molested by anyone.

 

Man, I hate electrical problems!

 

HMMM... a little more poking around and the common theme on everything that isn't working is that when the key is ON there is 12V between the green wire and frame. Same at the head light connector, tail light connector, etc. Now I'm suspicious this "floating" ground is causing the whacky tachy and also the reason its not running.. who knows what the cdi units are doing. Or maybe they've been cooked.

Just to be sure I checked a green wire to frame on my other gen 1 and there was no voltage, which only makes sense..

 

Does anyone know where the Green wire actually is grounded? I'm sure I have a bad connection there. I see it on the electrical but haven't found it on the bike.

Looking at the schematic, the two things that do work like they should, the neutral light and oil pressure light, don't use the green/ground wire but are grounded directly to earth/frame ground. It all makes sense now.

Thanks!

 

The start switch could be the cause of the problem.
The tach could be the problem and the workshop manual says it must be replaced if the engine will not start. Try disconnecting the yellow wire on the tach and see if it starts.

The black boxes are pure electronic and almost never fail.

Switches like the start button are electrical/mechanical and have a high failure rate.

The space shuttle is grounded right now (as we speak) because of a faulty switch.

 

Good photos of that lame switch.
Next time mine breaks, I am going to have a separate start switch and separate lights
on switch. End of story.

BTW... I hate plastic JUNK.

 

Pretty sure the switch is fine, at least it tests fine. I just need to find where the harness grounds and I bet I'm back in action. I hope

 

ok sounds like you have the same problem i had for many months and it turned out to be a stupid mistake...i was getting 12v to everything until i hit the start switch..she'd crank but no fire..id let off the start switch and get one spark...turned out it was a ground problem...the kid helping me work on the bike hooked the main ground wire to the starter relay making it HOT..crazy i know but with the rubber flap covering the post it just went unnoticed...the main ground for the wiring harness bolts to the center of the frame under the seat..the crossover piece..its flat with a bolt hole dead center...my bike had been sitting for 15yrs...hooked the ground to where it was suppose to be and she fired right up...

 

Yep, that ground wire was disconnected and tucked up under the frame rail. Hooked it up, and it fired right up. Lights and tach all working too.
Horn and front turn signals not, but that will be easy enough to fix!!

Thanks a bunch everyone!

 

Yes make sure your ground is good ,and make sure you have a ground from the battery to the engine, as well as a ground from the harness to the frame.

Remember you have copper wires going to a steel bolt holding the copper wire against the aluminum engine. Bad news. Copper and aluminum is the worst corrosion potential.

 

I have found that application of an anti corrosion grease such as NO-OX-ID helps greatly in keeping the corrosion in check on dissimilar metal connections.

 

ok sounds weird, have you checked the to see if the fuel pump is pumping? by disconnecting the fuel line from the fuel pump tube to the carbs, and see if it is actually pumping fuel? if you have not changed the timing and it was working before, the timing should not have changed enough to stop the engine from getting ignition, if you know you are getting spark, and you know you are getting air, then it must be problem with fuel. However this may be a cascading set of problems, that is what I am dealing with right now.

 

Great!...thanks for reporting back.

 

That is because we cannot see electrons we can only measure them.
One exception. Turn off all the lights in the garage and start your car.
For the cars that still have plug wires, if they are bad you can see the spark arc to ground on the engine.