FOULING ONE SPARK PLUG

I own a 1977 kz750b, newly restored, only 6000 miles.
I idles fine but starts to cut out and stumble when given throttle. Last night I put two new plugs in and let it run (idleing and revving the engine) for about ten minutes. I removed the spark plugs to find the left side plug completely fouled with black soot. The right side was just starting to turn the electrode white. Is this a carb or ignition problem?...also there was much more pressure (exhaust) coming out the left side muffler.....thanks...

I wouldn't think that it would be an ignition problem because both plugs fire at the same time. This bike has a lost spark ignition system. I would think that it would be the carb.

Yea, most likely carb adjustment or crud, though a gooberd up spark plug wire or boot might weaken the spark enough to do that.

I was tinkering with it last night and tried to adjust the carbs. I let it idle for a full ten minutes. During that time I tried different settings of the pilot screws. At one point I had them both screwed all the way in and seated. There was no change in idle speed. Is this normal? I tried one, two, even three turns out with no change in RPM'S. Would this suggest the pilot passage problem? I again pulled the plugs and found one normal and one completely black

bgulsvig wrote:

I was tinkering with it last night and tried to adjust the carbs. I let it idle for a full ten minutes. During that time I tried different settings of the pilot screws. At one point I had them both screwed all the way in and seated. There was no change in idle speed. Is this normal? I tried one, two, even three turns out with no change in RPM'S. Would this suggest the pilot passage problem? I again pulled the plugs and found one normal and one completely black

Definitely not normal, there is something very wrong with a carb that does that. Pilot circuit could be hosed up or maybe the choke circuit is putting in enough gas and the bike is running off that?

You should first set the fuel levels using clear tube.

The BS-38 carbs on the B models have the jets in the bowl. Drop the bowls (take out the drain screws) and soak them in cleaner and spray them out. If not done' drain and rinse the gas tank. Clean petcock and install filters in gas lines.


BTW: the pilot screws set at about 2 full turns out from bottomed for first adjustment on mine.

bountyhunter wrote:

bgulsvig wrote:

I was tinkering with it last night and tried to adjust the carbs. I let it idle for a full ten minutes. During that time I tried different settings of the pilot screws. At one point I had them both screwed all the way in and seated. There was no change in idle speed. Is this normal? I tried one, two, even three turns out with no change in RPM'S. Would this suggest the pilot passage problem? I again pulled the plugs and found one normal and one completely black

Definitely not normal, there is something very wrong with a carb that does that. Pilot circuit could be hosed up or maybe the choke circuit is putting in enough gas and the bike is running off that?

You should first set the fuel levels using clear tube.

The BS-38 carbs on the B models have the jets in the bowl. Drop the bowls (take out the drain screws) and soak them in cleaner and spray them out. If not done' drain and rinse the gas tank. Clean petcock and install filters in gas lines.


BTW: the pilot screws set at about 2 full turns out from bottomed for first adjustment on mine.

+1

Its not ignition related problem. If spark was weak on #1 plug it would cause identicle issues in the other #4 plug as spark travels through both plugs. If a connection problem exists it shows up on both plugs/cylinders/headerpipes.

Good Luck!