Webers - issues transitioning from idle circuit to main

[deathpunk dan]

If you slowly/gradually bring the revs up to 1500 or so and then punch it, she revs out smooth, strong and clean.

However, if you goose it from idle, you get popping/spitting from carbs (cyl 5 & 6 only).

Like it's having issues transitioning from the idle circuit to main.

Otherwise, after much fiddling with timing (006E dizzy, with static timing advanced a few degrees), she runs the best she has since the 'rescue' process began.

Likely culprit?

[Steve@Rennsport]

Fuel pressure at 3.5 psi?

Float levels perfect?

Accelerator pump outputs the same?

[signature65]

ahh the dreded weber flat spot. A few things from my VW days.

idle Jets plugged. Number one cause.
accelerator pump is bad
jetting is way off.

[pitargue]

Are the fuel squirtters working?

Upside down secondary venturies?

Leaky manifold gaskets? Air leak somewhere?

[RWebb]

also -- some idiot "cleaned" the progression holes with a drill bit or other metal object

[deathpunk dan]

Slight air leak on the idle screw cylinder #5. No other leaks (bubble solution/shop vac test). Squirters definitely squirting. Float level checked and re-checked. New manifold and carb gaskets.

I used only q tips, carb cleaner, and compressed air when cleaning them out.

Will investigate idle jets and jetting in the next episode.

[JFairman]

If all the other things mentioned check out good and the reason you're having the problem is because the venturi's installed are a little big for the cams and compression ratio you're running, you can fix it to some dgree with slighty bigger idle jets with smaller air correction holes and maybe different emulsion tubes for the main jet and air correction jet.

Larger idle jet with smaller air correction hole will raise the rpm transition point of the idle circut. You compensate for the richer mixtur at idle with the idle mixture screws.
I played around with tuning and rebuilding Solex 40PHH and Weber 45DCOE carbs on BMW 2002's alot in the past.
I think they use the same emulsion tubes and jets as the Weber IDA downdfraft carbs.
I think I remember using 50f6 idle jets with 38mm venturi's on a 2002 with 300degree cam and 10:1 CR... With 125 main jet, and 170 or 180 air coreection jet mounted in F11 emusion tubes.
Had to have the rich idle jets to get rid of a big flat spot around 1500rpm.

[RWebb]

how long since this started happening

did you buy the carbs new??