1981 gs450t spark plug continuously firing when kill switch is on run

[Guest]

I have a 1981 gs450t I found in a barn. It is complete except for the seat, and I only paid $200 for it. I've set the timing on it, cleaned the carbs and put all new gaskets on everything, however I've only got it to fire up once for about ten seconds on one cylinder. Turns out both spark plugs were continuously firing, the mosfets were occillating, but only on one cylinder. It was like there wasn't enough power being supplied to fire both spark plugs, but when I unplugged the coil feeding the plug that was "stuck" on the other spark plug would start firing continuously. I don't think it's the pickup coils because the problem persists regardless if the pickup coils are connected or not. So my question is is this a common type of failure of the igniter box, or is this a telltale sign of another problem?

[JTGS850GL]

The 450 uses a waste spark system. Both cylinders fire at the same time. One cylinder is on the compression stroke while the other is 180 degrees out at the end of the exhaust stroke. Only the cylinder on its compression stroke will fire air/fuel. The other cylinder will still spark but the spark does nothing. Thus the "waste spark" nomenclature.

[coreyspare]

I figured it would do something like that by looking at the pickup coils and the lobes that trigger them, however when I turn the key and flip the switch from off to run I hear a faint humm from the cylinder. I pull the spark plug and it's got an arc going across it and it won't stop. I know the coil has to be occillating pretty fast to generate a continuous spark non stop at the plug. The strange thing is only one plug is stuck on the other gets a small spark every four seconds or so, but when I disconnect the coil attached to the spark plug that's stuck on the one that was firing a small spark every four seconds is now stuck on. I've got a working used igniter box ordered and a new regulator too, I'm just hoping the newer igniter won't fail because of something on the bike.

[John Park]

The 450 uses a waste spark system. Both cylinders fire at the same time. One cylinder is on the compression stroke while the other is 180 degrees out at the end of the exhaust stroke. Only the cylinder on its compression stroke will fire air/fuel. The other cylinder will still spark but the spark does nothing. Thus the "waste spark" nomenclature.

Both cylinders do NOT fire at the same time. That's why there are two coils and two triggers. They do, however, fire on the off stroke as well, as if it were a two stroke.

On a four, two cylinders do fire at the same time, but this motor has only one of each pair. Essentially, it is the same ignition system expect that the coils have only one lead each and two cylinders are missing.

Sometimes I miss those other two cylinders, but mostly not….

[John Park]

I figured it would do something like that by looking at the pickup coils and the lobes that trigger them, however when I turn the key and flip the switch from off to run I hear a faint humm from the cylinder. I pull the spark plug and it's got an arc going across it and it won't stop. I know the coil has to be occillating pretty fast to generate a continuous spark non stop at the plug. The strange thing is only one plug is stuck on the other gets a small spark every four seconds or so, but when I disconnect the coil attached to the spark plug that's stuck on the one that was firing a small spark every four seconds is now stuck on. I've got a working used igniter box ordered and a new regulator too, I'm just hoping the newer igniter won't fail because of something on the bike.

Before you destruction test another igniter, you may want to just check that the stator and regulator pass basic specs. Otherwise, the voltage supplied could be who knows what or maybe even unrectified AC. ?? I'd be tempted to get it running with the alternator disconnected and test the voltage output and such before you fry something. Your ignitor sounds pretty kooky to me.

Ignitors don't like to have a coil HT lead disconnected with nowhere for the spark to go. These ones seem to tolerate it pretty well but some Suzuki dirt bikes would fry the trigger really easily if you did that. I Always have a plug resting on the block when running the ignition; I've never had a trigger failure.

It isn't just Suzukis or bikes; I've had a 'mechanic' fry the complete ignition system out of my truck by leaving the wires loose while doing a compression test. I'd warned him not to, and his boss paid to replace it, but I'm not sure he learned anything.

[coreyspare]

I'll go ahead and try to get it started and running off the battery when the new igniter comes in. I also ordered a new rectifier/regulator just in case. Do you have any idea what each of the 3 wires from the generator should be putting out in ac?

[John Park]

I'll go ahead and try to get it started and running off the battery when the new igniter comes in. I also ordered a new rectifier/regulator just in case. Do you have any idea what each of the 3 wires from the generator should be putting out in ac?

The three wires are floating from ground. IOW, there should be no continuity to ground from any of the three. Running at 5000 rpm, a figure of 75 volts between any two leads is usual; a little less or more could indicate a non stock stator, but more than 10 volts either way would be suspect. There is a test sequence outlined on this site if you look for it.

The stock regulator/rectifier is what they had at the time, but there's been a lot of progress since then. It is what is called a shunt regulator, which disposes of the excess voltage back into the stator which creates a lot of heat and tends to burn out the stator windings. The sooner you convert to a series type which more elegantly trims the excess voltage off, the more reliable the system will be. Both the stator and R/R will run a lot cooler and last seemingly indefinitely, plus the voltage regulation is a lot more stable.

The usual R/R used is the Shindengen SH 775 which is found on a number of other bikes, but the usual best price supplier is using one from a Polaris ATV. There's many a thread here on the swap. I posted a thread a few months back on putting one on an '82 450T showing the bracket I made which moves it to under the right sidecover where the tool roll goes. Some put it under the battery box but I'm not sure there's enough room on a twin. It's considerably larger than the stock unit.

You'll also be wise to check that the grounds to the frame and engine are clean, as sitting for a long time will probably corrode them somewhat.

[coreyspare]

How will i go about identifying a series regulator from a shunt regulator? Is it safe to assume any new generic one I get for the bike will be the newer series type?

[Steve]

How will i go about identifying a series regulator from a shunt regulator? Is it safe to assume any new generic one I get for the bike will be the newer series type?

A series regulator does not have the word "series" on it, you have to know the model number. The SH775 that was mentioned is one of the better ones that is readily available.

You can assume that "any new generic one" will NOT be the newer series type, you have to look specifically for one. And when you do go looking for one, it's probably not going to be in the description.
A search on eBay for "series regulator" might not find anything. "SH775 regulator" might find something. "Polaris 800 regulator" will find a bunch, then you have to weed through them to find the SH775 regulators, while watching out for the ones that say "fits Polaris Razr" or "replaces SH775", etc. You have to see the SH775 in a picture to verify that is what it is. It is clearly printed on the fin that is opposite the terminals, and is not shown in all the offerings.

.

[JTGS850GL]

Both cylinders do NOT fire at the same time. That's why there are two coils and two triggers. They do, however, fire on the off stroke as well, as if it were a two stroke.

On a four, two cylinders do fire at the same time, but this motor has only one of each pair. Essentially, it is the same ignition system expect that the coils have only one lead each and two cylinders are missing.

Sometimes I miss those other two cylinders, but mostly not….

According to the 450 wiring diagram, both coils are triggered by the same wire from the igniter. No way the coils could not fire at the same time. Yes, there are two signal generator pickups but both coils would still fire at each interval. This is still defined as a waste spark system since both cylinders fire the spark plug even when it's not on it's compression stroke.

[John Park]

According to the 450 wiring diagram, both coils are triggered by the same wire from the igniter. No way the coils could not fire at the same time. Yes, there are two signal generator pickups but both coils would still fire at each interval. This is still defined as a waste spark system since both cylinders fire the spark plug even when it's not on it's compression stroke.

I think you have two coils which are supplied continuously by one power lead +. The grounding to negative of each is separately controlled by the igniter. You could substitute a set of points and connect the + side of the coils to the battery with a single lead and still trigger them to ground sequentially with points. The ignitor just does that by a remote signal from the trigger.

If they did what you are describing, there would only need to be two wires to the coils, or even more simply, one coil with two HT leads.

It's the same system as the fours, but with two cylinders missing and single HT leads.

[coreyspare]

If the igniter box is that simple I could just make my own out of some mosfets, resistors, and a couple of diodes. That would save me a bunch of trouble.

[JTGS850GL]

I think you have two coils which are supplied continuously by one power lead +. The grounding to negative of each is separately controlled by the igniter. You could substitute a set of points and connect the + side of the coils to the battery with a single lead and still trigger them to ground sequentially with points. The ignitor just does that by a remote signal from the trigger.

If they did what you are describing, there would only need to be two wires to the coils, or even more simply, one coil with two HT leads.

It's the same system as the fours, but with two cylinders missing and single HT leads.

Actually you are correct. When I looked at the schematics I didn't see the white wire. The only wire I saw coming from the igniter was the black/yellow. Sorry for the confusion. 2 cylinders are not my strong suit.

[coreyspare]

Well I got a working igniter in the mail today but I still can't get the bike to start, it just kinda coughs and has a little white smoke coming from the exhaust. Now in thinking I have to fix something with the carbs.