1992 Gsxr running rich (Black Smoke)

I have a 1992 GSXR that I had a Vance & Hines Exhaust system installed
and a factory Jet kit installed about a year ago. I also removed the
stock air box and installed K&N individual clamp on filters. Recently I
have noticed puffs of black smoke. Also the engine is stuttering
around 4k RMP. Also it struggles above 7k rpm. I removed the Spark
plugs and they were black. I installed NGK Iridium Spark Plugs hoping
this would help, but I can't see a change. I'm considering installing
an MSD ignition. The bike is finicky. Some times it runs awful, at
other times it flies like a bat out of HELL. I need to correct this
running rich issue. ANY advice will be helpful. If I can't get this
corrected I will be forced to purchase Hayabusa.

 

1. Clean your air filters.
2. Do *NOT* over-oil your filters.
3. Drain your fuel bowls to get dirt and water out of them.
4. Replace your fuel filter (assuming you have one), this can cause
stuttering.
5. Make sure your choke lever is disengaging, i.e. that your choke (idle
enricher) is not stuck in the "on" position.
6. If all else fails, remove the carburetors, disassemble them, and
clean them out with carburetor cleaner, making sure to shoot the carb
cleaner into all ports and orifices. Also at this time check the
condition of the diaphragms and make sure that they are not holed.

This is not an ignition problem, this is a carburetion problem. Either
your carbs are not getting enough air (dirty/over-oiled air filters),
the idle enricher is stuck on, or there's dirt in the carbs. Diaphragm
problems can also cause issues. All is fixable, it just requires work
and close reading of the shop manual for your bike. (you *DO* have the
shop manual for your bike, right? Right?).

 

Don't bother with the MSD ignition, it's a carburetor tuning problem.
It's not the main jets, the carbs only run on the main jets from about
7/8th's throttle to full throttle.

You have opened the idle mixture screws too far. This is a common
mistake made by inexperienced mechanics. When you turn the idle mixture
screws counterclockwise expecting the idle RPM to increase, the idle
mixture gets richer and the engine RPM slows down. So the inexperienced
mechanic
will turn the idle RPM knob to increase the RPM. This uncovers the
transition ports just downstream of the throttle butterflies. And then
the idle mixture screw has very little effect on the idle mixture.

So, turn all the idle mixture screws clockwise, until they seat
lightly. Turn them back out half a turn and see how the engine runs.
Adjust the idle RPM knob to the specified RPM and turn the idle mixture
screws clockwise. The engine RPM should increase and you should have to
turn the idle knob down again. You may end up with the idle mixture
screws open only 1/4 of a turn.

You will know that the idle mixture is correct when the engine starts
with just the choke on all the way, you won't need to turn the throttle
at all. The engine will warm up quickly and won't seem "cold-blooded".
It will have good throttle response from closed throttle.

If the idle mixture is too rich, you will experience the stuttering you
mentioned and the idle RPM will hang up too high when the throttle is
opened and suddenly closed. The engine RPM should normally rise up and
then return to the specified RPM in the manual. An engine with rich
idle mixture will have a dull, thudding exhaust sound.

If the idle mixture is too lean, the engine will not start as easily
from cold. It will take a long time to warm up. The sound of the
exhaust will be "piffle-piffle-SNAP" as you roll the throttle off. The
engine may stall as you slow down. The engine will run too hot.