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Hello,

I am a local bike shop owner (Small business) and we are having some very odd issues with a 95 GSXR 750.

the bike came in with a laundry list of issues. To help bring the bike back to life we did the following

UltraSonic clean the carbs
Carb Jet Kit (122.5 Main Jets + Needle work)
4 rebuild kits for the lower end of the carbs
All new OEM Wires + ignition
K&N Air Filter kit


When the bike came in it it was DOA. After the carb work we got the bike back up and running with a dead spot around 2.5K. After riding it for a while it was getting worst. We took the carbs back apart to find that the O-Rings we used for the Internal Choke system were a little to big and they would not allow the tubes to sit correctly. We got a set of rebuild kits for the lower end. This got the bike to run and run very well. The issue is the 122.5 Jets are a little to rich and now the bike is killing spark plugs. We down Jetted the bike and now it will idle (Pilot Jets seem great) but when we crack the throttle it kicks out a bunch of black smoke and wants to bog/die.

When the carbs came off last time Carb#3 had the return slide spring smashed up in the top of the cap (Bike was still running VERY strong). So we repaired that issue and down jetted to

Carb1: 112.5
Carb2: 110
Carb3: 110
Carb4: 112.5

(Before Jetting Change)
Plug 1: Rich
Plug 2: Perfect
Plug 3: Rich
Plug 4: Rich


My question is...

Has anyone on this forum had a run around with these Mikuni carbs? My thoughts are to up jet it just a little more (117.5 all around) and bump the needle a little richer (from 3 to 4).

We have over 20 years of experience with carbs and normally do not have much issue at all but for some reason this bike seems to have some ghosts.

Either way. Any war stories or opinions would be much appreciated. Looking for educated opinions.

Thank you in Advanced
 

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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.
 

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My thoughts are to up jet it just a little more (117.5 all around) and bump the needle a little richer (from 3 to 4).
The last thing you want to do is raise the needle, you should be going leaner when you get a rich bog, and bringing fuel in later, and less fuel. Lower your float height, lower your needle position, find a larger needle or or one with less taper, lean your fuel screw, increase the size of your IDLE AIR Jet......

Start with lowering your float height and making sure they are all the same height, and then lower your needle next and see if there is improvement....
 

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I am running 115 MJs and 35 PJs on all 3 of my bikes (a 92, 94 and 95) Float height, correct pilot and mixture setting are key to the bike running strong. Sounds as though you are too rich (indicated by getting worse as it got hotter), and the flat spot between 2-3k is a float level issue likely. You need to raise your float height 1 mm and retry. Goes without saying that you want your mixture screws set the same and you have balanced the carbs prior to this.
 

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After 3 yrs of screwing around, the best setup I've found so far on my 93 750 is 122.5 mains, stock pilots (135?), mixture screws out 2 turns, and the clip in the middle. New orifice tubes and needles were like magic and made a huge difference.
Tried swinging for the fences with 136's, lol ran like stink and sucked the fuel back so fast i thought there was a hole in the tank. Lasted about 10 minutes, but it was fun.
 
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