One Carb - Quick Question

Bike XJ900F (1985)
Carbs and engine were in working condition before being dismantled from the bike cleaned and refurbished.

Having stripped down and cleaned and refurbished the carbs, wet and dry set the float levels, sync'd the carbs, cleaned out pilot and main jets etc (but not splitting the rack), I've found one carb (#4) to be running very rich to the point of having black sooty deposit on the new spark plug after only a short time, and predominantly at lower throttle openings.

Note: Cylinders 1, 2 and 3 are running fine and leaving the plugs a tan colour.

No 4 enrichener plunger looks OK from what I can see externally and from under the rubber diaphragm (can't extract it from the Mikunis without splitting the rack).

Note, the engine and carbs as a unit was cleaned by soda blasting some time ago. The carb throats were blocked off at the time but not from memory the two air jet holes on either side of the edge of each carb.

Quick questions is: Having considered everything else... Could a blocked pilot AIR circuit cause that #4 carb/cylinder to run rich?

And yes...all valve shims have been checked and set within spec.

Thanks for any knowledgeable opinions.

Cheers

 

From the logic of the situation .. all other carbs are good, problem at lower throttle, suspected grot in air circuit, possibly in the enrichment circuit in general ... Yes, not enough air with fuel = rich.

 

It's hard to block an air-jet passage, they are (relatively) huge...............always a possibility, but more likely caused by something else.

Also, remember that sooty plugs are an indication of incomplete combustion, not necessarily due to a "rich" fuel mixture:

Why are my plugs carbon-fouled (dry, sooty black deposits)?:

- Carbon fouling is the result of incomplete combustion----for any reason. It is most often associated with an overly rich fuel mixture (whatever the cause), but can also be caused by an overly lean fuel mixture (or poor spark, etc.) In a lean-mixture condition, most of the un-burnt mixture gets pumped out the tailpipe, but some fuel droplets remain in the cylinder and add themselves to the next intake charge. That's not a very precise way of metering the mixture, so when it's finally rich enough for a spark to ignite, that particular charge may be too rich, resulting in incomplete combustion and plug fouling. So carbon-fouled plugs can be due to rich or lean conditions.......your Colortune spark plug will tell you for sure. If you have a light blue or white-ish flame, intermittent flame, and/or intermittent flashes of yellow within an otherwise white-ish flame, then your fuel mixture is too lean.

And by the way.........if the spark that happens to ignite this overly-rich mixture is the "wasted spark" (which occurs in each cylinder at the top of the exhaust stroke, it will occur JUST AS THE INTAKE VALVE IS OPENING ---- so the backfire pressure wave may be directed back up the intake tract!

 

Thanks for the replies.

And Chacal, it seems counter intuitive that a lean mixture can create a black sooty spark plug but your explanation makes it a very real option.

Time to extract the pilot jet (again) and see if anything has gotten into that tiny hole.

Cheers!