'78 Suzuki GS750 only running on 2 cylinders

[Dean85gt]

Trying to figure this problem out for a while now, it is my dads bike.

Anyways the two inner cylinders randomly kick in and out. We pull the plug wires off and they arent firing at all, then sometimes it will be fine.

Now thinking it was a bad coil we bolted on a new set of accel plugs and coils. Fire the bike back up and it still acts the same.

About 10 years the bike was converted to electric ignition as well.

The odd part is this, the trigger wire for both coils is the same wire...and it still seems like the right hand coil isnt firing, despite us changing it.


Any thoughts?

[Brian P]

Wiring from ignition ECU, connectors between ECU and coil, ECU itself.

[Dean85gt]

Got it narrowed down I think. There are 4 wires that come out of the electronic ignition. One of them that goes to the one coil (black) doesnt do anything to the bike when unplugged. If i unplug the other one it shuts the bike off...

So maybe this is why one coil isnt firing. Its a Dyna S ignition btw

[Erch]

heh that's funny. I'm just getting MY dad's '77 gs750 back on the road. No problems with the electrical on mine, mind you, so I wouldn't know how to help. Mine was running on two, as well, to start with but it just took a thorough carb clean, fresh gas, some playing with the idle mixture and just running it for a good while on 2 for them to kick back on again.


If you do rule out the ignition and you're still having problems, take a look at this guy's forums. He knows a ton about the old GS carbs and can help you find the info to rejet if needed. I'm not sure about the 78s but I know all three versions of the 77 had lean jets and now with either a 30-year-old busted filter or a new replacement like a K&N, you'll be running way too lean for the thing to idle or hit the midrange well without overheating. Might be something worth looking into.

[Mercaholic]

I'm well acquainted with the old GS bikes.

A few things to ask:

1. Have you confirmed no spark on this cylinders?

2. If a spark, have you had the carbs out and cleaned.

The carbs have many small port passages that don't stay open over extended periods so you really need to get into them.

There was little wrong with the original ignition systems so if you still have the parts around, try re-installing.

Here's a great forum source for info:

http://www.thegsresources.com/_forum/index.php

[Spyug]

+1 on the GS Resources board. all the knowledge you will ever need to keep these old bikes purring and a great bunch of folks to boot.

On the site do a search first as I'm sure these symptoms have popped up a time or two before. There are lots of folks runing the dyna ignitions so i'm sure an expert or two can help you out.

As mentioned, carbs and valve settings are big issues for poor running on these bikes. While nothing to do with your symptoms you should read up on these issues as well as you are likely to face them as well.

There are a few GS board members in the Toronto area as well so you might be able to entice someone out to help on site.

Check it out.

Good luck.
Spyug

[mihnea]

it's a 375 twin.

[B400]

I had the same thing happening when the 12v wires going to the coils were breaking. The tach would sometimes drop for a second and the come back. Eventually the wire broke and I was running on 2 cylinders on the highway. To fix the problem I cut the wires back and crimped on a new spade connectors.

[TwistedKestrel]

I had the same thing happening when the 12v wires going to the coils were breaking. The tach would sometimes drop for a second and the come back. Eventually the wire broke and I was running on 2 cylinders on the highway. To fix the problem I cut the wires back and crimped on a new spade connectors.

Just wanted to add that the tach symptom would not happen to the OP, as he likely has a mechanical tach.