82 GS650g - black sparkplug #3

[Tino]

Hello all,
I have just purchased a 1982 GS650G and going through the tune-up process. The bike has everything stock. I have encountered a situation which puzzles me and couldn't find any forum threads that could shine some lite on it.

The bike runs fine but still doesn't have too much power in higher gears.
I went to replace the sparkplugs and found that the #3 sparkplug is black. All the other ones 1,2,4 are looking fine. This tells me that I am running rich on cylinder 3 but here is what puzzles me. When I pull the plug wire on cylinger 3, while the bike is in idle, nothing happens, the motor sound the same. However if I pull any of the other plug wires from 1,2,4 sparkplug, the engine almost dies. Even more weird is the fact that with the #3 plug wire pulled I can rev the engine and the motor sounds the same just as if the #3 plug wire would be connected to the sparkplug. The plug wire looks ok, no cracks, and it seems to conduct electricity since I can feel it.
Also have replaced al sparkplugs and nothing changed.

Any advice would be very much appreciated,
Thanks in advance,
tino

[tkent02]

Sounds like #3 isn't running at all, or maybe it only runs at higher throttle openings?

Does it's exhaust pipe get hot?

Does it have spark in that cylinder?
The coil is good if #2 is firing, but your #3 spark plug wire may not be.

Is that carb getting any fuel?
If the float system isn't letting any fuel in that cylinder won't run at all.
Pull a drain plug out of that carb, does any gas come out?

If the pilot fuel passages are plugged it may run when you open the throttle but not at idle or low power...

[Tino]

Hello, and thanks for the reply.

The exhaust pipe from 3rd cylinder is hot and the #3 spark plug wire when I disconnect it, I feel electricity going through it. If I approach it to the engine body, it will spark. So could it still be the plug wire? I will try, when I get home, to pull the drain plug from the 3rd carb to see if it gets gas.

I am not too clear on this sentence: “If the pilot fuel passages are plugged it may run when you open the throttle but not at idle or low power...”

The motor sounds the same at idle or if I rev the engine, with the pulled #3 plug wire.

Thanks,
Tino

[flatlands_ninja]

i had the same sorta thing with my 750
i changed coils,wires, and plugs (weak spark on #3)
cleaned the carbs

nothing changed

will be looking at it again soon though

shawn

[Zook]

The wire could be bad and not firing the plug under load.

Is the plug black and sooty (fuel) or black and wet (oil)?

If fuel it could be the fuel valve vacuum line. IIRC it goes to that cylinder (someone correct me if I'm wrong).

Might be low compression on that cylinder, too.

[tkent02]

Hello, and thanks for the reply.

The exhaust pipe from 3rd cylinder is hot and the #3 spark plug wire when I disconnect it, I feel electricity going through it. If I approach it to the engine body, it will spark. So could it still be the plug wire? I will try, when I get home, to pull the drain plug from the 3rd carb to see if it gets gas.

I am not too clear on this sentence: “If the pilot fuel passages are plugged it may run when you open the throttle but not at idle or low power...”

The motor sounds the same at idle or if I rev the engine, with the pulled #3 plug wire.

Thanks,
Tino

You shouldn't be able to feel anything, unless you are getting the shock from the spark itself going through you. This you would know about!

Pilot circuit supplies fuel for idle and for the smallest throttle openings only, and has very tiny passages that can clog up easily if the bike sits too long.

The other fuel passages are used for bigger throttle openings, are much bigger and don't tend to clog as easily, so the bike may not idle or run at low power, but could run correctly when you open the throttle.

[hikermikem]

I would bet the carbs need cleaning - especially the pilot fuel passages as tkent has suggested.

Find the links here that describe the carb cleaning process & replace the o-rings as well. Raise the jet needles .040" when you reassemble the carbs - I have the exact same bike & raising the needles gave the bike back its zip!

Adjust the valves as well, unless you know them to be OK.

Good luck

[hikermikem]

If fuel it could be the fuel valve vacuum line. IIRC it goes to that cylinder (someone correct me if I'm wrong).

The vaccum line from the petcock goes to carb #2.

[Zook]

Thanks, hiker!
I've owned my GS lss than a week, and only read the book once. This will help me remember.
That and rebuilding my carbs next week!

[Tino]

You guys are awesome. I cannot believe how fast I got so many answers.

The plug #3 looked dry and only a little bit of oil on the first/second thread of it. So I think the darn thing fires but why the motor wouldn’t stall or at least have the smallest change in sound, when I pull the wire plug, like it does with the other cylinders.

I don’t know anything about adjusting the valves but I will try to find some info on how to adjust them.

[maro]

Just chimin in here, as I have had the same issue with my bike "81". The pipe might feel warm, but check to see if it's as hot as the rest. This is not easy as you are subjecting your skin to hot metal pipe. My pipe gets warm too, but i can feel that it doesn't get as hot. As mentioned, the pilot jet is prob clogged. and fuel isn't pullin though in the lower range. BTW, the jet is located in the float bowl under the black cap. Really easy to get to (unless the screw top atop the jet is stripped, see my post on drilling this out), unscrew it, hold it up to the light. You'll see it's clogged. Put into carb cleaner for an hour, the spray it with cleaner, and air. Once it's clear, put it back.


hey hikerman, how do you re-set the main jet. I have my carbs off, and I should just do this, no?

[Tino]

Hello Maro, thanks for the reply

I will try to get my hands on a laser thermometer to measure the temp. of all 4 pipes.
Did you think I could get to the pilot jet on carb3 through the bottom without taking the entire carburetor out?

[hikermikem]

hey hikerman, how do you re-set the main jet. I have my carbs off, and I should just do this, no?

Hmmmm.... if you mean the needle jets, then I don't know if you should or shouldn't raise them. My GS650GZ had very little power in the 3-5K RPM range, with plenty of pull above 5K to red line.

I'm no carb professor, but as I understand it, different passages & jets within the carb control the different mixtures & amounts of fuel flowing throught the internal carb circuits.

The gurus here basically talk about 3 different throttle positions, each controlled by overlapping jets & circuits:

1/4 throttle
1/2 throttle
WOT

The jet needles are those long pins that hang from the bottom of the slides. They are tapered, & move up & down by vaccum within the jet needle seats (long perforated brass "tubes", with the main jet screwed in at the bowl end) controlling fuel flow during 1/2 throttle. By raising these jets (long pins) slightly, you can essentially "start" the fuel flow at a high level, thus resulting in better acceleration & power.

Of course, everything has its point of "diminishing returns" so too high or too low isn't good either.

Check the link in the forum here showing how to disasemble & clean the carbs - it will also show how to take the jet needles apart. Replace the small plastic spacer with a few small washers, but not to the thickness of the spacer - stack the washers so they are about .040" LESS thick than the plastic spacer; that will RAISE the jet needle.

Good luck

[maro]

Hello Maro, thanks for the reply

I will try to get my hands on a laser thermometer to measure the temp. of all 4 pipes.
Did you think I could get to the pilot jet on carb3 through the bottom without taking the entire carburetor out?

I would seriously doubt you can get to it properly without taking the carb rack off. Taking the rack off isn't all that difficult. It will be a bit of a PITA if you have never done it before though. I took my rack off last night, took me 15 minutes...because it was my 5th time doing it. Let me know and I can give you a quick step by step on how to do it somewhat painlessly. Getting it back on is another story. One thing at a time though.

[maro]

Replace the small plastic spacer with a few small washers, but not to the thickness of the spacer - stack the washers so they are about .040" LESS thick than the plastic spacer; that will RAISE the jet needle.

Good luck

Thanks! That's what I was looking for.


and

Forget it. I think I 'm good for now without having to fuss with the needle height. Maybe this winter I will mess with it. That way when I eff something else up in the process, I won't feel bad wasting time fixing it!