Can anybody explain how and when the transition occurs.

Is it based on throttle position?

It usually happens when you step on the gas pedal.

You experience may differ.

There are one or two ports in the carburetor body. As I remember, one is below the butterfly and one above. At closed throttle (butterflies in rest position), fuel is drawn from the lower port for idle. As the butterfly moves off it's stop, the second port will be exposed adding more fuel and at the same time airflow increases. As the throttle is moved farther, the vacuum in the venturi increases and the fuel drawn is shifted to the main jet spray bar. To avoid a "bog" in the engine when the throttle is opened quickly (meaning lots of air flow, no gas), the accelerator pump squirts a shot of fuel in to avoid the potential for this tremendously lean condition.

Zooooo ... yes and no to the throttle position. It is also the velocity of air thru the venturi that causes transition from idle ports to run ports.


Hey .... I haven't done carbs in lotsa years and this is what I remember. I can send you a whole shitpile of information on Webers or you could just buy the book and read.

I'm gonna do that.

Thanks for the info.

I assume you're talking about IDA3Cs.....IDTs & IDSs are the same I this regard.

I'll give this a shot.

Yes, throttle position is the main factor, but there are more.

On the outside of each barrel there is a slotted head screw about 1/2 dia. Behind those is a fuel passage with 3 small holes which open into the barrel right at the butterflies.
The amount of fuel these deliver is controlled by the mixture screws and the position of the butterflies adjacent to them. As the butterflies open the high speed/turbulent air passing by the butterflies draws air out of the holes progressively(?) or at a greater rate....or both. This is the idle circuit. It is imperative that these holes are clean....they do crud up. They are delicate (did I mention they are tiny?)and should be cleaned only with a soft
pointy tool.....a sharpened toothpicks work as does a copper wire.

I'll leave out the accelerator pump for now.....

As the engine speeds up, the fuel ports in the Aux venturi get enough vacuum to begin to draw fuel into the barrel (main circuit). Exactly when this happens varies. Cam overlap has a major effect. The more overlap, the later the transition, but you can figure it's somewhere around 3-4K rpm.

While all this is going on, the accelerator pump is dumping fuel into the barrels....but not at constant throttle settings. One "stroke" of the throttle should produce a certain amount of fuel from each port....and the amount escapes me at the moment.

Getting all this to work in concert is the trick. I have found that the idle circuit needs to be rich to over come a "flat spot" (lean) during transition. This is problematic as it then becomes too rich on the idle circuit and plugs foul. To overcome this I use a multi spark ignition like MSD along with a hot coil & large gapped plug to "burn through". This really doesn't cure the plug fouling but makes it manageable.

I'm reasonably sure I missed a couple things.....but this is as close to understanding the SOBs as I get.

The two old geezers here remember better than I do apparently. Sounds pretty close to what I remember. I'd have to dig the book and other reference out. Speaking of which, buy the Weber book. It's cheap.

Geezers rule......when they can remember how.

emulsion tubes

Jeff speaks......but not much.....which is unfortunate cause he's forgotten more about tuning than I know.

I believe the idle jets continue to add fuel through the whole range
the transition is adding the main jet to the total, but I don't believe the idles ever stop being a part of that total....

Excellent information.

I've been running with what I believe is a clogged idle jet or 2 and kind of feel how when the mains kick in it smooths out.

So... clean 'em? The idle jet holders screw right out. Give 'em a shot of compressed air and screw them back in. Now you could have an issue with an idle circuit and not just a jet, but it couldn't hurt.

If you've got a clogged idle jet, you'll know it. It runs for . It effects idle, transition, and you'll feel it cruising on the fwy. I've had an idle jet get clogged a couple times at an AX (last year at the WCR for example) and drove it home with it clogged. A few minutes to clean the idles and it runs great again. You'd expect an "idle" jet to effect idle only...not so.

Most of the racers are advised to run larger than street idles specifically to avoid clogs.

clogs are very common
we've been picking them up at the track a couple of times a day
they take about 30 seconds to fix and even less to find

screw the holders in with the motor idling and a quiet enough enviornment to "hear" the motor.

each one you screw in will cause that cylinder to start getting rough (as your screwing up the balance) The moment you hear the motor get rough, screw that one back out right where it was before you touched it.
ie: a half turn in, and a half turn back out.

when you screw one that does not cause a change in the pitch or smoothness of the idling motor, you know that one is plugged.
take it out and blow it with compressed air, or carb cleaner in a can with a straw.

then go back and re-balance them
brant

And better yet .... use Fuel Injection .....

and quit practicing the black arts....?
never....

(here is a fun one, I just figured out how to make the IDS vacuum drawn auxillary enrichment tubes work in combo with other jetting.... talking about diving into the deep end. but no more top end lean condition)

What is this "holder" of which you speak?