1977 KZ750 Twin AGAIN!

Heres the problem....The bike has been running decent for a long time but now the thing won't start and my biggest concern is the bike has a huge backfire EVERYTIME AND ONLY when I turn the key(power) off. I'll try and start the thing for 5-10 seconds at a time...it'll turn over but it won't start...even with a 1-2 second shot of starting fluid. And everytime I go to shut the key off....BLAM! A big backfire. Whats causing this? I've got the choke on, gas on, fully charged battery...but I get nothing. ????
smithgpz put in an electronic ignition and I got the carbs from WG. Things *were* fine.
I'm 2 seconds away from selling this damn thing and putting the money into my 69 Camaro. At least it'll start.

Post edited by: shawntmartin, at: 2005/12/22 18:16

Post edited by: shawntmartin, at: 2005/12/28 18:03

Shawn, do you have the lobe switched 180 degrees? It seems everytime I take the timing off it goes back on reversed. I'm thinking that when you kill the power with the key it discharges the coils and with all that gas in the cylinder it pops it once.

That or if it was running fine with with it after you put in the elec ign., maybe the reg/rec is going out. Do you have all lights and guage lights running?

do you have an orig coil? maybe the elec ign somehow is burning it out.:pinch:

All the lights work properly. I don't know if the coil was changed when the ignition went in. This sure does seem electrical. I'd have to talk to smithgpz but I can't seem to get a hold of him. I haven't touched the timing since it was installed. It ran good..then. I've left smithgpz a few U2Us but he hasn't replied. i don't know... I'm pretty bumbed out about this. Anyone else around central Ohio work on bikes for a good price?

Pull your spark plugs and see if you have a strong regular spark as you crank it over. Remember to ground the spark plugs, you do not want the coil to attempt to fire a plug that is not grounded.

Lets look at your bike's symptom / firing when you switch off the key. Turning off the key stops the current flow thru your coil's primary circuit. When the current thru the primary is interrupted the magnetic field in the coil crashes. This causes a surge of current in the coils secondary circuit. The surge of current in the secondary causes the spark plug to fire.

Old Suzuki Triple Two Strokes with leaky carbs had a problem when the owner would turn on the bike's ignition in the spring. Turning on the ignition caused a rapid rise in current in the primary resulting in a current surge in the secondary resulting in a spark igniting a pool of gas on the piston resulting in a 2 foot flame out of the muffler.

Back to your bike - I suspect the electronic ignition is not working. Normally a capacitive discharge electronic ignition causes a surge of current in the primary and this results in a surge of current in the secondary. It is pretty neat that the same coil designed for use with points that interrupt current flow in the coils primary circuit can be used with a capacitive discharge electronic ignition that instead, causes a rapid rise in current in the primary circuit.

The key to understanding the way it works is to recognize that in both cases a rapid current change in the primary coil circuit causes the secondary to fire the plug.

Now back to your problem.
- If you were to go back to points it should run if the coil is still working.
- If your Timing Pickups on engine failed then it may be an adjustment problem. Like a gap or it may be that they drifted, or it may be that the advance is stuck open.
- If your Capacitive Discharge Box (Electronic Brain Box)failed then you likely have good pickups and a failed brain box.
- You may have fried the coil. You could take the coil off your Camaro and try that but a car coil is made to run with a resistor to drop the voltage to something like 6 to 8 volts. Anyway the bike coil failed it would run with a car coil for a while and you will have determined that your bike coil has failed.
- Since the coil fires when you switch off the ignition, I am betting that the coil is still good.

I agree that a 69 Camaro is easier to work on but the ignition system that you have on your bike is not that difficult to figure out. It is winter outside, you have time to figure it out.

Spock

You'll have to forgive my ignorance about the electrical side of this bike and about the question I'm about to ask... (I'm decent with the Camaro) but this bike's ignition is still a little foreign:laugh: to me. Anyway, what is the best way to test the electronic ignition. I'm suspecting this is the culprit sense I had to send this one back because it overheated when smithgpz installed it because the wires in the box where reversed. Instead of giving me a new one...they "fixed" this overheated one and sent it back. I want to find the easiest way to test it and then give Newtronic a call if its defective. This thing is less than year old...if I remember right.

I am not experienced with tracing thru an electronic ignition. You may want to contact the manufacturer to see if they have a trouble shooting procedure.

I found the following explaining the automotive coil though I think that your coil is likely OK.

Hope the image displays, you can find it at:
hyperphysics.phy-astr.gsu.edu/hbase/magnetic/igcoil.html

Spock


The primary winding of the ignition coil is wound with a small number of turns and has a small resistance. Applying the battery to this coil causes a sizable DC current to flow (Ohm's law) even though it has a significant inductance which slows the increase of that current to its maximum value. The secondary coil has a much larger number of turns and therefore acts as a step-up transformer. But instead of operating on AC voltages, this coil is designed to produce a large voltage spike when the current in the primary coil is interrupted. Since the induced secondary voltage is proportional to the rate of change of the magnetic field through it, opening a switch quickly in the primary circuit to drop the current to zero will generate a large voltage in the secondary coil according to Faraday's Law. The large voltage causes a spark across the gap of the sparkplug to ignite the fuel mixture.
The current in the primary coil is caused to drop to zero quickly be the opening of a fast transistor switch. This causes the magnetic field in the primary coil to drop rapidly, generating a large voltage in the secondary coil. The generated voltage is proportional to the rate of change of the magnetic field according to Faraday's Law, and the amount of change is multiplied by the effect of the iron core in the coil.

Post edited by: Spock, at: 2005/12/22 19:46

Hey Shawn,
Could it be that you have a stuck valve? Exhaust, possibly? Just guessing but it would allow the final spark to torch off whatever you've managed to pump into the exhaust system.

Any Chance?
KZCSI

I think Spock has nailed it
According to your desciption the only time you have spark is when you cut the power to the coil, causing it to discharge.

That would mean the negative side of the coil is constantly grounded (just like points stuck in the closed condition), so turning the key off is like the points opening and firing the coil.

So either the wire is shorted to the frame, or the pickup/ignitor is bad.

I'm not to far away (I think you are in Ohio), so you could just give it to me :evil: :evil:

Post edited by: steell, at: 2005/12/23 00:16

My 78 did that I had fouled plugs.

This all started with my bike surging or stuttering while I was riding. So, I limped it home and found my B8ES's were disgustingly black. So I put in 2 fresh hotter B6ES's and I've never been able to start it since. So its not the plugs.

Steell,
Yea, I'm getting spark to both plugs but if my eyes aren't deceiving me..it seems that the left plug has a bigger spark. And like I said..if my eyes aren't fooling me, I don't think the right is getting a spark as often as the left. This would be a lot easier if my garage wasn't 5 below zero. Maybe Santa will bring a propane heater.

I just e-mailed Shaun at Newtronic and this is his reply... (ok...I'm I reading this right? Is he sending me a whole new sustem for free?.. Damn nice if he is!)

("Hi Shawn,
I'm really sorry you are still having problems! I have tested and kitted up a whole new system for you and it is in the post today to the address below.

The things I would check in the mean time - make sure both the coil and yellow/red wire to the box have +12 volts still going to them when the ignition key is turned on. If not the problem is somewhere in the bike's power system.


Also, check the coil is still in good order (measuring the resistance between the two low voltage wires, should be no less than 3 Ohms, and no more than about 5 ohms.

With the plugs out on the cylinder head and blue wire disconnected from the loom to the black box, shorting and un-shorting the blue wire from the coil (coil negative) to earth rapidly (kind of flicking it if you will), should produce sparks at the plugs albeit weak ones, if the coil is in good order.

If either of these coil tests fail, replace the coil.

If all seems to be good with the above, I would first replace the 2 wire Newtronic loom and the black box and see if this solves the problem (even if it appears to I would run the bike for a good while before deciding it is definitely cured), if not, then replace the lamp assembly also.

I hope this helps, and again - sorry for the troubles.

Shaun R - Wishing you a Merry Christmas.

Wow, they sound like stand up folks. Invite them onto the board! Sounds like they know their stuff...