I have a question. While rebuilding these carb and buying a doner set for the throttle cable bracket, I noticed I have sets of slides with 1.5 and and 2.0 cutaways. Also the first set I bought were bored out to 34mm and had the intake side ramp modified and have 2.5 cutaways. The question is, what effect does the cutaways have in tuning. I put the 1.5’s in the rebuilt set in this post.
The stuff below is copied from a Kawasaki training manual I have on carbs.
"The amount of air entering the carburetor is determined by the throttle slide or throttle valve. The throttle slide rides in a closely fitting bore directly above the venturi in the carburetor body. The up or down movement of the slide is controlled by a cable (or lever) and spring. In the "fully closed" position, little or no space remains under the slide for the air to flow through the venturi. The higher the slide is lifted, the more air that can flow through the venturi and into the engine. When the slide is in the lower positions, the slide blocks part of the venturi in the carburetor body, and the area under the slide becomes sort of a venturi in a venturi. The size of the actual venturi or air passage changes with the position of the slide, so this type of carburetor is known, not surprisingly, as a variable venturi type. It is the most common type found on motorcycles and is used on all current Kawasakis.
At lower throttle positions, the cutaway of the slide affects mixture ratios. The effect of the cutaway is felt most from about 1/8 to about 1/3 throttle (1/8 to l/3 of the total rotation of the twistgrip and total travel of the slide). The higher the.cutaway, the leaner the mixture will be at a constant throttle position. This is because the higher cutaway offers less resistance to the in- coming air. Since the fuel supply remains relatively constant at any constant throttle position, the additional air results in a leaner mixture. Conversely, the lower the cutaway, the greater the air-flow resistance, and the richer the mixture.
The purpose of the slide cutaway is to provide for adjustment of the fuel/air ratio as the transition is made from idle to the full venturi action of the carburetor."
Further, on Matt's point about the bored out 34's...carburetors operate on a principle of physics that when air flows through a confined area (such as a carburetor) the air pressure will drop when air velocity increases. Air velocity will increase when the carb body narrows down, as in the throat compared to the intake. As the pressure drops in the neck, the pressure differential between this point and the fuel in the bowl (at atmospheric pressure) will cause fuel to be drawn up the jets and into the bowl. So if you make the throat area larger you are decreasing the pressure differential, which means less fuel drawn up the jets.
I have a couple of Z1B's restored, an '80 KZ1000LTD restored, a 1981 KZ550 restored and a 2008 KLR 650 for off road fun. My wife has a 2019 Suzuki DR 650 for on and off road.
The slide affects carburetion between 1/8 thru 1/2 throttle and especially between 1/8 and 1/4 and has a lesser affect up to 1/2.
The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be.
Mikuni throttle valves have numbers on them that explains how much the cutaway is and a slide with 1.5 stamped on it means a 1.5mm cutaway etc.
DOHC, thank you. That one’s gonna task my retention or lack of.
So more/faster air means less fuel from the main. I understand that, and the need for more air equals more fuel can be carried into the cylinder, equals more power. More/faster air requires larger main circuit jets, i.e. main, needle jet, and maybe a different shape/taper of the jet needle. All make sense. I’m having a hard time with the pilot circuit. A larger cutaway as installed in the bored out carbs I have means leaner bottom end. Why use a larger cutaway, seams a smaller cutaway would help fatten up the pilot circuit. Maybe I’m thinking wrong but the downstream edge of the slide is consistent regardless of cutaway size and as such only allow “X” amount of air in no matter how fat the cutaway is. Seams your only creating a lean bottom end, without gaining any additional air flow. Maybe, I don’t understand a high horsepower engine. Do some require a leaner bottom end to take advantage of the power up top.
I think it's really just one more knob to turn. The graph I posted earlier shows the cutaway effect is really focused on the 20% to say 40% throttle opening.
I think it's a way to fine tune the transition between the pilot circuit and the needle/needle jet circuit. The Keihin CR manual has a chart that shows how each of the different circuits and adjustments overlap with throttle position. The CR are very similar to the VM.
This shows that the effect of the pilot circuit (or "slow jet") tapers off around 1/4 throttle, and that the needle/needle jet circuit starts to pic up around 1/4. But at the transition between pilot and main, both of them have a much smaller impact than the cutaway. Also, the the pilot and main circuits work on slightly different principles, so being able to tune the the transition seems important. Otherwise you'd be force to mess with either the pilot or needle to try to fix the transition, and that might mess up some other part of the range.
As SWest pointed out the 1.5 is the recommended cutaway for the KZ1000. That's probably a good place to start.
'78 Z1-R in blue, '78 Z1-R in black, '78 Z1-R in pieces
'95 GPZ1100 (sold) , '00 ZRX1100
The following user(s) said Thank You: SWest, Mikaw