I have the factory service manual and the factory updates on the carbs from 76-79 for the gs750. The screws on the bottom were fir matching each carb to the cylinder volume. Yes the mains could be changed, but this bottom screw is a fine tune adjustment of the main. If you mark each bottom screw and can guarantee you put it back exactly as you took it out when changing the screw oring then you're golden. However, most orings are shot on these 45yr old bikes on these screws. A tiny fine adjustment of 1/8 turn or 1/16 turn made the difference in too rich or too lean or just right. If you change a piston or mill a head you'd need to adjust these screws as well.
That fuel screw is just for the idle, the main jet is under the slide needle. to change the mixture while the engine is at speed you need to change the size of the main jet, by buying new main jets that are a different size.
The screw on the bottom is not for the idle. It affects the main and the idle. It affects the vacuum on the bowel. The screw on the bottom is factory set to the carb it's attached to in order to give thar carb the correct volume of fuel mixture based on the cylinder's volume it's attached to. Basically you pick the main jet and needle jet and sync the carbs on a bench and adjust all the floats then set the screw underneath at 1 or 1 1/4 out. Then start the bike and use vacuummate and 4 colortunes at the same time to adjust all the carbs so they are running at the same vacuum at constant higher rpms by adjusting the bottom screws and screws on top of the carbs closest to the cylinders until you get a nice blue in the colortunes and the vacuumate shows them all equal. Then you can start to work on idle and the blips and transition from idle circuit to main circuit. It's an iterative process. There's a very good paper on it on gsresources website on the vm26. These carbs are not like most carbs. Yes lots were made in the 1970s but the smoothbore sliders were gone by the 80s and there are fewer and fewer people who can tune these. The pro tuners of the 70s are few and far between. I don't claim to be a pro. I just found lots of papers and read the factory manuals and talked to shops until I figured it out. My carbs are really well tuned and my bike pulls good and my pipes aren't blue and I don't smell gas in my exhaust.
Wait so the fuel screw, I though that was also part of the idle circuit. Is it not? Are you saying that the air screw affects the idle circuit and the fuel screw affects it when you open the carbs up? If so no wonder I can’t get mine tuned! I’ve been looking everywhere for information like this!
You are correct. This is a very advanced carb. Most bike shops are used to only 1 air screw or 1 fuel screw. This carb has 1 air, 1 fuel, and 1 main screw and then it has a slide lifter on each carb top under each lid and then a pilot needle size and then needle drop heights and a main jet. It was the Superbike of 1976-77 fir suzuki and highly tunable. It's old tech now, but if you know how to tune it it can be dialed in. It takes some doing and for sure understanding how the vm26 works to get it running right.
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I have the factory service manual and the factory updates on the carbs from 76-79 for the gs750. The screws on the bottom were fir matching each carb to the cylinder volume. Yes the mains could be changed, but this bottom screw is a fine tune adjustment of the main. If you mark each bottom screw and can guarantee you put it back exactly as you took it out when changing the screw oring then you're golden. However, most orings are shot on these 45yr old bikes on these screws. A tiny fine adjustment of 1/8 turn or 1/16 turn made the difference in too rich or too lean or just right. If you change a piston or mill a head you'd need to adjust these screws as well.
I just bought one today for my first bike 1979 I can't wait to enjoy it
That fuel screw is just for the idle, the main jet is under the slide needle. to change the mixture while the engine is at speed you need to change the size of the main jet, by buying new main jets that are a different size.
The screw on the bottom is not for the idle. It affects the main and the idle. It affects the vacuum on the bowel. The screw on the bottom is factory set to the carb it's attached to in order to give thar carb the correct volume of fuel mixture based on the cylinder's volume it's attached to. Basically you pick the main jet and needle jet and sync the carbs on a bench and adjust all the floats then set the screw underneath at 1 or 1 1/4 out. Then start the bike and use vacuummate and 4 colortunes at the same time to adjust all the carbs so they are running at the same vacuum at constant higher rpms by adjusting the bottom screws and screws on top of the carbs closest to the cylinders until you get a nice blue in the colortunes and the vacuumate shows them all equal. Then you can start to work on idle and the blips and transition from idle circuit to main circuit. It's an iterative process. There's a very good paper on it on gsresources website on the vm26. These carbs are not like most carbs. Yes lots were made in the 1970s but the smoothbore sliders were gone by the 80s and there are fewer and fewer people who can tune these. The pro tuners of the 70s are few and far between. I don't claim to be a pro. I just found lots of papers and read the factory manuals and talked to shops until I figured it out. My carbs are really well tuned and my bike pulls good and my pipes aren't blue and I don't smell gas in my exhaust.
The factory manual says not to turn them screws. Interesting.
Wait so the fuel screw, I though that was also part of the idle circuit. Is it not? Are you saying that the air screw affects the idle circuit and the fuel screw affects it when you open the carbs up? If so no wonder I can’t get mine tuned! I’ve been looking everywhere for information like this!
You are correct. This is a very advanced carb. Most bike shops are used to only 1 air screw or 1 fuel screw. This carb has 1 air, 1 fuel, and 1 main screw and then it has a slide lifter on each carb top under each lid and then a pilot needle size and then needle drop heights and a main jet. It was the Superbike of 1976-77 fir suzuki and highly tunable. It's old tech now, but if you know how to tune it it can be dialed in. It takes some doing and for sure understanding how the vm26 works to get it running right.