Cold Cylinder

[CaliGS]

I bought a 79 gs750 a few months ago to cafe and just got it road worthy a few weeks ago. I have been riding it work (11 miles one way) the past couple weeks (it has been running fine) but found something funny today. I noticed, when looking at the chrome on the headers, that #2-4 look they are getting hotter than #1 (chrome slightly changing color). I started the bike and noticed that #2-4 get hot in 15-20 sec but #1 is staying a lot cooler (cool enough for me to leave my hand on it).

I decided to test the spark. I first pulled #4, started the bike, and looked for spark. When I took #4 and grounded the threaded part of the base (not the electode) I would get a blue spark at the electrode. But when I grounded #1 the same way, I would not get a spark at the electrode. However, when I ground the electrode to the head I would get a blue spark at the electrode and from the electrode to the head.

When I pulled all 4 plugs #2-4 looked normal and #1 looked fouled. Could this just be a bad plug or do I have a problem with a coil/wire/cap? When I did a comp test, all 4 came out at 120 (even #1)

I wounder if I could have been riding on 3 cycliners this whole time? All of my time fixing the bike has been cosmetic so far because it has always seemed to run good.

Here is a pic of the bike:

[almarconi]

You could have a number of thing happening. Could be a bad plug wire on the cold cylinder, dirty carbs, gas not getting to carb on the cold cylinder. When was the last time you replaced the plugs/wires? Have you cleaned the carbs recently?? Your service manual should have test procedure to test the plug wires and coils. I had a similar problem on my KZ550 after I rebuild the carbs, #2 cylinder wasn't getting hot. Turned out that the K&L rebuild kit had the wrong jets in it. Once I put the right parts in the carb all 4 cylinder get hot.

[Steve]

Quick check...swap the plug wires for cylinders 1&4. They come from the same coil and fire at the same time,
so there should be no ignition problems.

If #1 is still cold, it's a carb issue. If #4 is now cold, it's the plug wire or cap.


.

[CaliGS]

Steve-
Thanks for that suggestion! I didn't realize that you could switch the wires like that, but it makes sense.

I had a little time this morning so I switched the wires like you had suggested and 1 stayed cool while 4 hot hot real fast. So, I thought to myself "the #1 wire must be fine, it has to be the carb". Then, after I have the tank off, airfilters off, and carbs in my hand...I decide that I should swap plugs before tearing into the carbs. I put the carbs back on and primed them real quick and sure enough all it was, was that stupid spark plug. Thank goodness - that just made things a lot easier for me!

thanks again

[Dave8338]

That was nice and simple...

Great looking bike, by the way!

[CaliGS]

Well, I have to say I kind of feel like a jackass. I've been working on my GS a few months and have been trying to ride it pretty regurally for the past couple weeks...I've thought it was running good. Then I find out I have a cold cylinder (as described above) and that I have been riding the thing on 3 cylinders because of a stupid spark plug!! I changed the spark plug out today and - holy crap! The bike is a lot more responsive now. I didn't really think that it was unresponsive before...until now.

You know, I really shouldn't be posting this. I should just let the post die in the abyss of forgotten posts and hope that not to many people read it.....but, I'm excited that its fixxed!



ok....now it can die. 8-[

[Greg O.]

I recomend you change the oil now as you may have pumped a lot of raw gas into the engine instead of burning it. This can dilute the oil.

Greg O.