CV Carb Brain Teaser - Main Jets & Gas Mileage

1982 KZ650 with Mikuni BS32 CV carbs. Everything I read says that the Main Jets do not come into play until about 3/4 throttle opening and above, and below that, the jet needles / needle jets dictate the amount of fuel flow. On these carburetors, there is no mechanical linkage between the butterfly throttle plates and the round slides. Only vacuum raises the slides, off-setting spring pressure above the diaphragms.

With the original 107.5 main jets, and running a steady 75 MPH speed on the highway for a whole tank of gas, I consistently got gas mileage of 51 to 55 MPG. To gain some wide open throttle power, I changed the main jets up 2 sizes richer to 112.5, and yes the power is better. But when using the throttle very gently, and running the same 75 MPH speed, I now get about 45 MPG. Not terrible mileage, but why should it be any lower than before? NOTHING was changed but the main jets. I put the original jets back in, and mileage went back to up to previous levels.

To run 75 MPH, my throttle is open about 25%. My theory is, that perhaps the vacuum is strong enough, even at this opening, to raise the round slides to the point that the main jets become the restricting factor. This would explain the decrease in gas mileage. My solution would be to use stronger springs, above the vacuum diaphragms to prevent this early raising of the slides. (I wish there was a way to look into the back of the carbs from the air box, while running down the highway, to see when the slides start to raise).

Is it possible that the slides are raising prematurely? Anybody care to comment?

On the Mikunis I run (BS-38), the main jets "feed" all the other carb jet systems. That's why the setup procedures advise you do wide open runs to get the right main jet size before you dial in the needles and pilots. I believe the mains do have a small but measurable effect on overall running even at 1/4 throttle opening as your data would indicate. I think your carbs are working correctly.

You could always drop the needle 1 position leaner and see if it still runs properly in the midrange.

The jet needles only have one groove, although I have experimented with adding spacers to go richer. All the above discussion is with the needles at the stock (leanest) setting. The bike ran best with about 1 mm. richer spacers, but mileage was even worse with that setting.

I have located a gizmo that I can borrow. It is a flexible tubular device that has a lighted "eye" on the end. The other end has an eyepiece like a microscope. I plan on removing the air filter, re-installing the air box top nozzle and feeding the device down into the air box and aimed at one of the carb inlet areas, I should be able to see the slide raise. Should be interesting - If I can do so without crashing ! I'll let you know how it works out. Thanks for the input guys.

WABBMW wrote:

Is it possible that the slides are raising prematurely? Anybody care to comment?

Well, you could add spacers to the spring side and increase its load.

Capt:

You are reading my mind !

Capt America wrote:

WABBMW wrote:

Is it possible that the slides are raising prematurely? Anybody care to comment?

Well, you could add spacers to the spring side and increase its load.

That will change the piston position across the whole range, not just at light throttle opening.

I suspect if you delay piston rise, you will get a balky throttle response at light throttle openings at lower RPM range.

bountyhunter wrote:

On the Mikunis I run (BS-38), the main jets "feed" all the other carb jet systems. That's why the setup procedures advise you do wide open runs to get the right main jet size before you dial in the needles and pilots. I believe the mains do have a small but measurable effect on overall running even at 1/4 throttle opening as your data would indicate. I think your carbs are working correctly.

I agree with this. Although the needles limit the flow through the main jet between 1/4 and 3/4, they are blocking a larger opening when you increase the jet size. IMO, this would lead to a larger fuel flow rate at pretty much any throttle position. Your mileage findings seem to confirm this. It would seem to me that you could limit the flow rate back down to where it was by changing the taper on the needles.

Are there different needle profiles available for these carbs from the factory? I suppose that a custom set of needles could be machined, but who knows what the correct taper should be?

Hey guys - I did my experiment with the special "eye" stuck into the intake runner on one of the carbs while running down the road at 70 MPH. I could watch the slide raise as I opened the throttle and/or the RPM and load increased. Here is how the slides responded:
1. At a steady 70 MPH, the slide was raised about 1/3 of the way.
2. At wide open throttle in any gear and the RPM at 6,000 or above, the slides were fully open.
3. At wide open throttle in 5th gear but only 2,000 RPM, the slides remained in their lowest position.
4. Under very light load, such as when allowing the bike to gradually increase RPM in 1st gear, by the time you reach about 5,000 RPM, the slides start to raise.

So in summary, the slides on these CV carbs are raised based upon how much vacuum there is. The vacuum is triggered by air flow into the carbs. The air flow is based on a combination of throttle opening, engine RPM, and engine load.

I believe that testerossa and bounty hunter are right on target with their comments. My motorcycle runs well, and I probably should not worry about the gas mileage being down a little. Thanks again for the info.