Smaller jet or larger exhaust?

[Caprimaniac]

Hi everyone. Guess it's quiet in the US at the moment, as it is here while this country is coverd in snow.

But I'm doing my homework watchin "GT Racer" at one of those cable channels and planning the steps to take as soon the ice melts.

My webers run quite well now; and I'm considering the problem with an over- rich blend at very small throttle opening followed by gently og hard acceleration, to be related to the accelerator jet- system only. I have very rich blend; AF about 10-11 and black smoke when accelerating from 1500- 2000 rpm. These are 48 IDF's.

An old carb- guru over here told that the way to cure this was to get a smaller accelrator pump jet. I have now a 50 jet. Redline once told me to go with a 40. OK; that will reduce the rate of fuel from the pump.

But what about getting the exhaust (bleed) larger? It is now closed, size 000. Open it up will give a smaller total amount og fuel expelled in one stroke.

I am tempted to say that reducing the jet is the way to go: Instant volume will be reduced. The stroke will last longer, also giving a little extra while opening up at higher revs?


Anyone wants to say something about this?

[Gaz64]

Rune,

I'd say you're whole combo : capacity, compression, camshaft, choke, emulsion tube, main, and air corrector produces a really strong signal at the booster and your e-tube/choke combo can get away with less accelerator pump volume/duration.

Ideally, this is what we would all like to be chasing, to have a carb that could respond well enough to not need accelerator pumps.

Alas, this will never happen due to the difference in mass between air and fuel.

I'd be fitting 35 bypasses as a start.
If you have the later IDFs with the adjustment nut on the acc pump rod, wind the nuts out progressively to reduce the volume.

35 is the smallest offered by Weber, you can then drill them out to 50 then 60 etc if needed.

[Caprimaniac]

Originally Posted by Gaz64

Rune,
Ideally, this is what we would all like to be chasing, to have a carb that could respond well enough to not need accelerator pumps.

Hello, Gaz.

Actually; that was what came to my mind while falling asleep to one of my Weber books yesterday... would have been great; UhU?

I checked my dealer; no 40 acc. jets available, but 35 exhaust OK. And, yes- you are probably correct about adjusting the rod nut out. Although; after drilling into the valve covers to make room for the rods, I cut some off the rods as well (1/8-1/4" I'd guess).... But; I'll turn them as much outwards as possible. (Maybe I tried this already; should have written it all down- too lazy, you know.)

Thanks
RS

[Rick Parker]

You can also epoxy the accelerator pump jet holes closed and redrill your own size. Works well.

[Caprimaniac]

epoxy is not beeing broken down by gasoline. This is tru, then? Better get some small drills, then...

Thanks
RS

[Gaz64]

I'd be buying the smallest pump jets you can find and work up from there with a drill set.

Number drill set 61-80, or preferably 0.3mm to 1.0mm set.