Join Date: Aug 2018
Location: South west Missouri
Motorcycle: 2018 GsxR 1000R
Re: 1995 GSXR 750. Need some tips/Tricks for Tuning out the carbs
So jetting these carbs is fairly simple if you go about it in the right sequence. Think about what the carb is doing dynamically as you ride the bike........ If you are in a corner, the throttle is off, then you crack it open slowly, and then pick the bike up a few degrees, and increase the rate you open the throttle...... With this in mind........and assuming you have good cylinders and valves and they are each pulling similar vacuum........
You have three different circuits that you must tune and then blend into the next circuit...... If you understand the dynamic order or sequence of how these three circuits get put into play, it becomes much simpler to get the right answer.....
Your three circuits are...... the idle circuit.......the slide/needle circuit......... the main circuit.........
Your idle circuit should be set first, it is influenced by the size of the idle fuel jet, the size of the idle air jet, the float height (translates to fuel height in the bowl), and the fuel screw position. The range of influence on AFR for the idle circuit is from idle to 1/8 open throttle.
Once you have this right, then you can work on your fuel mixture from 1/8 throttle to about 3/4 throttle. This circuit is influenced by the needle taper, the float height, the needle height. The needle height influences the timing of how soon the fuel increase begins in relation to the butterfly or slide opening, the needle taper controls the rate of fuel increase. The float height affects how much vacuum is needed to get the fuel moving out of the bowl. Once you have the transition from the idle to the needle circuit blended well, you can make your final main jet adjustment.
When adjusting the main jet circuit, don't forget you have a main fuel jet, and a main air jet in most systems to control AFR.....
Now on to your specific stated problem....... " but when we crack the throttle it kicks out a bunch of black smoke and wants to bog/die."
This likely has nothing to do with your main jet, you have not reached a throttle position where the main circuit is the dominant influence on AFR (5/8 to full open).......... It sounds like your float height may be too high ( too much fuel in the bowl), and or your needle position is too high, and or your needle shape is too sharp a taper from the major diameter ( it narrows too quickly from the large diameter). Also if your idle air jets or your main air jets are clogged, it could cause a rich condition just off idle when opening the throttle.
Last edited by S1ON1000R; 08-24-2018 at 12:33 AM.