Topic: Carb Richness query

[johnyvilla]

So I was plug chopping this lovely morning and at WOT found 3 good looking plugs with one burnt. Is it common to run different main jets in carbs, or should I be looking for the reason why this lone cylinder is running rich? Jets and needle placement are identical (CB750k6 by the way) valves, timing and points spot on. Carbs synced, and I'm not aware of any compression discrepancies. For 36k she pulls strong. I feel like a leaner jet in that cylinder would be more of a band-aid than addressing the real problem. O yea, last time I checked, the choke valves were all functioning properly. Any ideas before i start mixing and matching jets? thanks, you guys have been a lot of help.

[333]

When you say burnt, is it black?  If so, it's not burnt, it actually means too rich, or more likely not enough spark.  If the carbs are as you say- clean, synced, ..., then spark(or lack therof) may be the problem.  Switch the plugs around.  If it moves to a different cylinder, it's bad plugs.  If it stays at the same cylinder, then it may be the cap or connection to the wire.  The cap connects to the wire by a "screw" into the wire.  Unscrew it and inspect for corrosion.  Clean cap if needed. Clip the wire back about 1/4 of an inch.  "Screw" the cxap back on.  If this doesn't help, it may be "back to the carbs" to check the air screws.

[johnyvilla]

Good idea. Ill try that tomorrow. Ive seen carbs need different needle positions to run uniformly but never different main jets. Any other ideas? The plugs have around 300miles on them, but they have been in and out 15 or 20 odd times. 

[Bodi]

Make sure the cylinder is firing properly, as 333 says a missing cylinder will darken the plug. WOT creates a different environment for the plug to spark in, and it's not unusual to have a problem only show up at high throttle and the resulting high cylinder pressure at ignition.
The jets are nrmally the same for all carbs - the CBX had different settings for the centre 2 carbs for a richer and cooler firing mixture but that's an oddity.
Check the float levels, the idea of draining the carbs one by one into a cup and comparing the amount of gas coming out sound good. A higher fuel level makes the mixture richer.
The carb's main jet emulsion tube may be clogged up, all the tiny cross-drilled holes have to be open. This also richens up especially at WOT.

[Deltarider]

the idea of draining the carbs one by one into a cup and comparing the amount of gas coming out sound good.

I'm afraid it isn't. If you drain one floatchamber, the gas in the fuel line above it comes along with it. Remember that one fuel line feeds two carbs? So when you drain the second (adjacent)floatchamber, you'll have less fuel in your cup. Moreover, on most bikes the fuel lines that feed carb 1 & 2 and carb 3 & 4 respectively differ in length.

[OldSchool_IsCool]

Another theory:  Maybe when cleaning, the orifice size of one of the jets was accidentally altered.  If you don't find the cause before opening your carbs, maybe swap the mains from one carb to another and see if the dirty plug chop follows.

[HondaMan]

Has the bike been sitting a while, like a year? There are a couple of passages that can partially block themselves under certain circumstances and cause an isolated mixing problem, assuming the floats are all OK and the spark is good on the cylinder.

If you get to the point where you want to talk more about this, drop me a line...

[johnyvilla]

Yea, she sat for a year or two in the cold streets Honda Man. I dunked the carbs, floats are all happy, and blew out all the passages with low compressed air(cigarette lighter air pump!). I haven't had time this weekend to swap plug wires. This sounds like the likely culprit to me.