Carb Circuits?

[Ohioan]

Can someone explain carb circuits to me? Is it throttle based or RPM based? (If I turn 1/2 throttle in first gear the RPMs are going to be a lot higher than 1/2 throttle in 5th gear.)

The basic bench sync and fuel needles handle the idle circuit correct?

Then the jet needle and slide control mixture at what?

Full throttle is main jet?

I'm just trying to get a better basic understanding here...

[jpaul]

For CV carbs, it's RPM based. As the engine speed increases, the airflow increases, increasing the vacuum that raises the slides, and consequently, the needles. for a better explanation then I can give, go to Basscliff's Website and check out the manual for the Mikuni BS32 carbs. It has diagrams and explanations on how the carbs function.

http://members.dslextreme.com/users/bikecliff/

[Nessism]

This is a perfect post for the new Carbs forum. How about asking the Admin. (Frank) to move this thread over there?

[Ohioan]

Paul, that makes sense.

Then the smoothbores would be throttle based?

Nessism, I couldn't find the new carb forum when I posted this...

[Ohioan]

I had a good book from the library that did a good job at describing how a carb works, what certain symptoms mean etc, but I didn't read the whole thing... it was kinda dry...

Carburetor basics, trouble-shooting, and rebuilding by Sandor D. Racz. 1974

[Nessism]

A little hard to find these new forums...

http://www.thegsresources.com/_forum/forumdisplay.php?f=129

[chef1366]

For CV carbs, it's RPM based. As the engine speed increases, the airflow increases, increasing the vacuum that raises the slides, and consequently, the needles. for a better explanation then I can give, go to Basscliff's Website and check out the manual for the Mikuni BS32 carbs. It has diagrams and explanations on how the carbs function.

http://members.dslextreme.com/users/bikecliff/

I disagree.
The air flow is created by the opening of the butterflies of the carbs. That is why I mark the throttle to do plug chops.
Throttle closed to 1/4 throttle = pilot into needle circit
1/4 throttle to 3/4 throttle = needle circuit that also feeds off the pilot and main
3/4 to full throttle = main jet ciruit the needle is out of the picture here but fuel is still being fed through the pilot circuit.
I agree that the RPMs (compression) do pull the air through the venturi that pulls the fuel out of the float bowl but the amount the butterflies are open determines how much.

[Ohioan]

opening the throttle butterflies increases the vacuum causing more air to be sucked through the airbox, into the carbs,and the air/gas mixture into the engine. That vacuum pull is what increases the RPMs. At full throttle the plates are horizontal, causing the airflow through the carbs to be at it's maximum.

So it all depends as to where the throttle butterflies are.. I see I see....

so when doing plug chops, it doesn't matter if you're in 1st or 5th gear?

[chef1366]

opening the throttle butterflies increases the vacuum causing more air to be sucked through the airbox, into the carbs,and the air/gas mixture into the engine. That vacuum pull is what increases the RPMs. At full throttle the plates are horizontal, causing the airflow through the carbs to be at it's maximum.

So it all depends as to where the throttle butterflies are.. I see I see....

so when doing plug chops, it doesn't matter if you're in 1st or 5th gear?

That's correct

[nastyjones]

I've never found a simpler explanation than this one.

http://home.everestkc.net/malsin/Motorcycle/carb/mc_carb_theory_101.htm

Paul

[jpaul]

I disagree.
The air flow is created by the opening of the butterflies of the carbs. That is why I mark the throttle to do plug chops.
Throttle closed to 1/4 throttle = pilot into needle circit
1/4 throttle to 3/4 throttle = needle circuit that also feeds off the pilot and main
3/4 to full throttle = main jet ciruit the needle is out of the picture here but fuel is still being fed through the pilot circuit.
I agree that the RPMs (compression) do pull the air through the venturi that pulls the fuel out of the float bowl but the amount the butterflies are open determines how much.

I may be way off base since carb tuning is definately not my thing. You're correct in that the throttle opening controls the airflow into the carbs. I was describing what controls which carb circuit is used from the view of the physics involved. If, for example, there is an air leak downstream of the carb, the throttle position needed to change the circuit used would shift because it would be harder for the engine to create the necessary vacuum in the carb to raise the slide. And in the case of doing plug chops, I believe that you take the bike up to speed at whatever throttle opening you're checking, and hold it at that opening long enough to stabilize, or for the carb to "catch up" with the butterfly opening, since there is a slight time lag between opening the butterfly and the vacuum increasing to raise the slide.

But then again, my eyes may be turing brown because I'm so full of chit.

[chef1366]

I may be way off base since carb tuning is definately not my thing. You're correct in that the throttle opening controls the airflow into the carbs. I was describing what controls which carb circuit is used from the view of the physics involved. If, for example, there is an air leak downstream of the carb, the throttle position needed to change the circuit used would shift because it would be harder for the engine to create the necessary vacuum in the carb to raise the slide. And in the case of doing plug chops, I believe that you take the bike up to speed at whatever throttle opening you're checking, and hold it at that opening long enough to stabilize, or for the carb to "catch up" with the butterfly opening, since there is a slight time lag between opening the butterfly and the vacuum increasing to raise the slide.

But then again, my eyes may be turing brown because I'm so full of chit.

I don't think your FOS and I respect your post. I agree the flow of air from the rate of the pull of compression (RPMs) will raise the slides in the CV carbs hence constant velocity carbs. It's just that the butterflies control how much air is allowed to enter.
CVs are smoother than VM carbs but not as responsive because of exactly what you said. The mechanical slides are ripped open with the throttle so without an accel pump you have a vacuum void for a split second until the fuel starts to flow. The CVs rely on the air flow to lift the slide so it's not instant so no void.

[Ohioan]

but I thought smoothbores were better for racers because there was not the delay between the time the butterfly opens, and the time the slides raise that comes with CV carbs?

[chef1366]

but I thought smoothbores were better for racers because there was not the delay between the time the butterfly opens, and the time the slides raise that comes with CV carbs?

But dragracers don't wack the throttle. They start at full throttle with a rev limiter.

[jpaul]

Smoothbores would be a better choice for roadracing, because of the quicker response. For dragracing, if your revs drop to where you are off the main jets, you've probably lost already.