Kawasaki VN750 Forum banner

1 - 14 of 14 Posts

·
Registered
Joined
·
62 Posts
Discussion Starter #1
Whenever I crank the engine, the front plugs fire at the wrong time and the engine kicks back/blows air out the carb intakes. (valves/compression/timing has already been tested). I got a new ignition coil, and while it got slightly better, it still happens while cranking/ low RPMs.

After installing the new ignition coil and still having the issue when cranking I got super pissed. I started the bike on just the rear cylinder, revved it up, and slammed the spark boots on for the front cylinder.

Surprisingly, the bike started to "run" in the front cylinder. It sounded like it was running, but it was choppy, was backfiring, and cutting in/out. nothing came out of the carbs until the RPMs fell to around idle, and then I got blowback.

The whole time, the tachometer was jumping around, never reading above "1500". I know it's set up to the front coil, which leads me to believe there is an issue with the wiring to the coil since the coil is a known good one. Where should I start?

I saw there was a mod to bypass the harness and use a relay, should I try that? How is the coil "timed"? The wires leading into it are a mess and have chunks of electrical tape connecting them, so I'm pretty confident this is a wiring issue.

Thanks for any help, I'm really trying to salvage this bike.
 

·
Registered
Joined
·
7,631 Posts
Have you checked out the pickup coils behind the stator cover? The wires are known to break, and the pickup coils themselves can go bad.

The coil relay mod involves getting full battery voltage to the primary side of the ignition coils. The spark gets triggered through the CDi box via the pickup coils, separate from the 12v primary supply.
 

·
Registered
Joined
·
62 Posts
Discussion Starter #3
Have you checked out the pickup coils behind the stator cover? The wires are known to break, and the pickup coils themselves can go bad.

The coil relay mod involves getting full battery voltage to the primary side of the ignition coils. The spark gets triggered through the CDi box via the pickup coils, separate from the 12v primary supply.
I'll give the pickup coils a check. Do I need to drain the engine oil to check those?

So by doing the mod, the battery goes straight to the pickup coils, which then go to the CDI box, and then the ignition coils, correct? or does the mod bypass the CDI box?

One more thing, I took the plugs out and they all had a strong spark. I set all 4 next to each other and took a slow-motion video, and saw that all 4 fired at the exact same time. Is that how they're supposed to fire?

Thanks a lot for your help!
 

·
Registered
Joined
·
1,248 Posts
The coil relay mod leaves the pickup coils untouched, but supplies battery voltage directly to the v+ terminal of the ignition coils (shared common red wire), bypassing the ignition switch. This is done just to provide as much voltage to the spark as possible when the coil gets the 'fire' signal. The pickup coils still go to the CDI, and the CDI still provides the timing signal to the ignition coils.

Noooo, they aren't supposed to fire together. If the wiring has been hacked up, it's possible there's a crossed wire and one pickup coil is triggering both cylinders.
 

·
Registered
Joined
·
7,631 Posts
I'll give the pickup coils a check. Do I need to drain the engine oil to check those?
There could be a small amount of oil that would drain when the cover is removed, but it shouldn't be a gusher. Some people lean the bike to the right to pull the cover, you'll at least want it on the centerstand.

All plugs firing at the same time is probably the reason it was popping back through the carb, ignition timing being off. I would look at the pickup coils, wiring, then the CDI.
 

·
Registered
Joined
·
62 Posts
Discussion Starter #6
The coil relay mod leaves the pickup coils untouched, but supplies battery voltage directly to the v+ terminal of the ignition coils (shared common red wire), bypassing the ignition switch. This is done just to provide as much voltage to the spark as possible when the coil gets the 'fire' signal. The pickup coils still go to the CDI, and the CDI still provides the timing signal to the ignition coils.

Noooo, they aren't supposed to fire together. If the wiring has been hacked up, it's possible there's a crossed wire and one pickup coil is triggering both cylinders.
I'll give the pickup coils a check. Do I need to drain the engine oil to check those?
There could be a small amount of oil that would drain when the cover is removed, but it shouldn't be a gusher. Some people lean the bike to the right to pull the cover, you'll at least want it on the centerstand.

All plugs firing at the same time is probably the reason it was popping back through the carb, ignition timing being off. I would look at the pickup coils, wiring, then the CDI.

I'll give the wires a check, thanks a lot guys, I'm really looking forward to getting this beast running for summer!
 

·
Registered
Joined
·
62 Posts
Discussion Starter #7
There could be a small amount of oil that would drain when the cover is removed, but it shouldn't be a gusher. Some people lean the bike to the right to pull the cover, you'll at least want it on the centerstand.

All plugs firing at the same time is probably the reason it was popping back through the carb, ignition timing being off. I would look at the pickup coils, wiring, then the CDI.
So I found out that the previous owner spliced the green wire to the rear coil and also sent it to the front coil, which would explain why they would fire at the same time.

Now, my question is with the stock wiring harness, where does the correct wire and terminal for the front coil separate from the harness? It's the black wire that goes in pin #4 on the ic ignitor. I'm tracing the wire back through the harness, but unwrapping the electrical tape is taking ages, and I want to be able to find wherever the proper connection is/whatever happened to it to make the PO do this boneheaded move.

Thanks.
 

·
Registered
Joined
·
1,248 Posts
Chances are low that anyone is going to know where any splices inside the harness are off-hand. It's probably going to be easiest to run a jumper wire along the harness from the black wire at the connector of the CDI to the black wire at the coil. Tach will probably be messed up, but ... maybe do a test? If you splice in to both ends of the harness without cutting the original wiring, you might just provide the tach with a strong signal via the new connection at the coil end of the harness if there's a dodgy spot in the existing black wire between the CDI and the T that splits to the coil and the tach.
 

·
Registered
Joined
·
7,631 Posts
Go with Thorn's last post, run another wire from pin 4, alongside the original pin 4 wire. Assuming that's correct trigger wire for the front coil. (didn't consult the diagram myself) Looks like this should get it going.

Maybe a coil relay mod gone wrong? It felt like it ran on the front cylinder because there was still some combustion when the intake valve finally closed. Firing way too early on the front cylinder.

I wanted to say, cut the jumper wire and start it, but it looks like the original was disconnected when they did the jumper? Maybe they thought it was shorted to ground?
 

·
Registered
Joined
·
62 Posts
Discussion Starter #10
Go with Thorn's last post, run another wire from pin 4, alongside the original pin 4 wire. Assuming that's correct trigger wire for the front coil. (didn't consult the diagram myself) Looks like this should get it going.

Maybe a coil relay mod gone wrong? It felt like it ran on the front cylinder because there was still some combustion when the intake valve finally closed. Firing way too early on the front cylinder.

I wanted to say, cut the jumper wire and start it, but it looks like the original was disconnected when they did the jumper? Maybe they thought it was shorted to ground?
Excuse my ignorance, but what is the jumper wire? Thanks.
 

·
Registered
Joined
·
7,631 Posts
Excuse my ignorance, but what is the jumper wire? Thanks.
....previous owner spliced the green wire to the rear coil and also sent it to the front coil...
That's what I usually call a jumper wire. Thinking if the original wire to the front was still intact, removing the jumper might let it start up.
 

·
Registered
Joined
·
62 Posts
Discussion Starter #12
Excuse my ignorance, but what is the jumper wire? Thanks.
....previous owner spliced the green wire to the rear coil and also sent it to the front coil...
That's what I usually call a jumper wire. Thinking if the original wire to the front was still intact, removing the jumper might let it start up.
Ah, ok, thank you!
 

·
I'm a rider
Joined
·
964 Posts
So I found out that the previous owner spliced the green wire to the rear coil and also sent it to the front coil, which would explain why they would fire at the same time.


Thanks.
That’s how they are from the factory. Green is ground. Red is trigger. At least on two of my bikes, haven’t checked the colors on the rest.
 

·
Registered
Joined
·
62 Posts
Discussion Starter #14
That’s how they are from the factory. Green is ground. Red is trigger. At least on two of my bikes, haven’t checked the colors on the rest.
Yes, but the green was spliced and also went to the trigger. After rewiring the trigger wire to go to pin #5 on the CDI box and splicing it into the tach, everything works as it should.
 
1 - 14 of 14 Posts
Top