Topic: One Sooty Plug - All The Rest Good

[Wingnut74]

74 CB750 - rebuilt engine over the winter.  All stock with airbox, except added Cyclex sidewinder pipes.  Runs almost great with great pull (never rode bigger than a 400 before so I am easily impressed).  Cylinders 1,3,4 plugs have just right tan/beige (if anything a tiny touch lean), but at times I do hear a misfire in mix at 3k on up to 5k. I am still in first 150 miles so don't go much above that at all. Plug 2 is very black dry sooty - super rich.  I read a lot of posts and have suffered through endless interations to success tuning before, but I'd apppreciate any thoughts on what else to check before pulling the carbs off. Here is what I know:
- Cyl 2 runs ok mostly, pipe seems to be same temp as the rest - runs just real rich
- Really good spark from new coils on all 4 plugs. Everything is new/replaced, coils, wires, caps - not the spark
- Since 3 of 4 are good, it cant be airbox, or airflow?
- I checked and retightend all the carb/airbox/intake clamps - were not great fit- retightened - still sooty #2
- Possible remaining causes:
 -main jet fell into bowl - will check before pulling, but doubting #2  would run if that was it?
- Float too high - could have sworn this was good - made a gauge that worked.
- I just noticed from a thread that I forgot to add the carb bowl vent hoses? COULD that be it? Will put back
- Plugged airhorn vent - I don't get how this could richen things up - IS there a way to clean this while its on the bike?
- anything else to check before I pull them? thanks

[flybox1]

Clear tube test that bowl.
What is its fuel level?

[pmanning]

I had a main jet back out of the emulsion tube on a carb one time.  I don't remember if the cylinder was firing at all but it was a relief to find the jet nestled in the float bowl rather than something more catastrophic.  Also make sure the choke plate for that carburetor hasn't come loose from the lever arm.

[Wingnut74]

OK thanks. Will check it out. Anyone know if you can adjust the cyl 2 float height with the carbs on a cb750k4? Looks tough.

[evanphi]

All you need is a long flatblade screwdriver and a set of loooooong-nose pliers.

Like this:

Pro tip: If you want to raise the float height without removing the float (and losing your needle) each time, you can reach in there and give the tang a pinch (to lower it) with some long-nose needle pliers. If you need to lower your fuel level, raise the tang up with a flat screwdriver. The hose for mine is made with an old drain plug that I cut the head off, and jammed onto a piece of 1/8" fuel line.

[WV_750]

I may just be hamfisted, but I had a close call with breaking a float post doing it that way. As far as taking the float out to adjust, it's frustrating if you don't get the adjustment right the first or second try, but worth the lessened risk IMO. Also, getting a correct measurement is sometimes tricky at that angle, so I hold the needle seated (not by the spring of course) before lifting the float to check height. It gets a little cramped on those inner carbs.

[flybox1]

Check and adjust all on the bench. Setup an aux fuel source and a stand to hold your carbs.
Get it right before you put the carbs back on.

[DaveBarbier]

I had a main jet back out of the emulsion tube on a carb one time.  I don't remember if the cylinder was firing at all but it was a relief to find the jet nestled in the float bowl rather than something more catastrophic.  Also make sure the choke plate for that carburetor hasn't come loose from the lever arm.

Me too, last year, haha

[Wingnut74]

Found it! Many thanks for the suggestions - checked all those things and they were all OK. Had airbox off and decided to ream and flush the airhorn vent tubes, and was poking around with light, then noticed a little sludge in the hole for the cyl #2 idle air screw, removed the screw and blasted gumout into the screw hole onto a paper towel, crap came out. I must have forgotten to clean that port and jammed a new screw into the sludge. So add one more to the cause o sooty plugs --> clogged  air screw. I would not have thought that was a factor in the 3k-5k range but I guess it contributes a share of airflow in that range still. New Plug is now good color, thanks!