I run triple 40's.

When I'm WOT it take's a moment to clear the excess fuel and then the motor comes alive.

How do I reduce the volume of fuel moving through the jets.

I understand there is an adjustment on the sides involving an accuating cam?

Pics would be a bonus...


thank you


KT

Before adjusting the pump, try to find one of those little measuring vials that is used to measure the amount of gas that is actually squirted out. You'll have to measure each barrel to make sure they all squirt the same and then check your handy Weber manual to see how it compares.

The lever for the pump diaphram actuation rides on a little cam on the throttle shaft. On a two barrel 40, this is betwwen the barrels. The fulcrum of this lever arm can be moved (there are two alternate locations) to influence the length of throw transferred to the pump. Longer lever (from fulcrum to pump diaphram) = more fuel discharged. Also be sure the little metal wheel that rides the cam rotates freely and does not touch the cam in steady state conditons or fuel will dribble out irregularely.

SGB does a great job at describing early Weber IDF carbs. The fulcm pivot point can be adjusted.
Late model IDF's have a spring on an arm with a tiny lock nut that can adjust the amount of stroke applied to the diaphram.
No idea how IDA's work but I suppose it is similar.

This where a A/F meter comes in handy so you can tell what is happening when. I can watch all the circuits actuate from idle through to progression ports Acceleration enrichment, main jets kicking in and what WOT looks like You know under load foot pasted to the floor @ 5 grand or better.

Pardon the hijack, but what's "SGB"?

Thx,

Skip

SGB is the dude who posted #3




KT

Whoops. I thought it was a reference to some standard Weber carb tome...

Thx,

Skip

Carefull, a stumble can come from being lean too (not enough accel pump). The best way to tune is w/ a WBO2. Lift at high revs for several seconds, then floor it again. You can then adjust both the volume and rate of the accel pump to smooth out AFR.

Send me your email and I'll scan you the pages from my weber book tonight.

Eg:
1) goes rich, then lean, then flattens out. Volume is right, but is delivered too fast.
2) stays perfect, then rich, then perfect. The rate is perfect, but is being delivered too long. Reduce the volume.
3) goes lean immediatly. Not enough rate.
4) etc.

SMD

Tighten this nut for more volume, loosen for less.


Click to view attachment

It's a real good idea to measure the volume with the carbs off the car first. There's no other way to ensure that you're getting the same volume into all barrels, and different volumes will also cause a hesitation.

great!!

KT

Measure volume for each barrel as stated above. Can be done on the car (with engine off!). Haynes and others will have a spec for 911 Webers in cc's (usually the amount for one full stroke of the throttlle, operated by hand at the carb).
If you don't have a calibrated vial, I have made my own as follows:
Buy some disposable plastic syringes (used for insulin) at the drugstore. Most are calibrated in cc's. Pull the needle out and cut the top of the syringe off a bit above the line indicating the desired volume.
Then wrap a wire tightly around the tube and bend it up a couple of inches to act as a holder to use in lowering the tube into the barrel and locating it under the discharge tube. Squirt one full stroke of gas into the tube, remove and see how much is there. Adjust accordingly.

It's been a while, but as I recall there is only one adjustment for each carb (only one pump) so I usually attempt to get the middle barrel on target. If one of the others is way off, it means it is plugged and a cleaning/overhaul is in order.

Very helpful.

thanks


KT

Just took a look at my syringe supply. They are 1.0cc/100 unit capacity.
I think the spec for Webers is between .4 and .6 cc's per stroke. That translates to between 40 and 60 units on the tube.
I am only guessing on that spec as I could only find it in my Haynes 911 book for Zenith's (70-71 2.2 911) but I think that is about right for Webers as well.

My accelerator pump does not have that adjuster like in the picture.

The accel jets do push the same volume of fuel, however.
I hung a small plastic tube and took note.

Will I have to make my adjustment with a smaller jet size?


KT

You said "triple Weber 40's". I assume that to mean the normal Weber 3 barrels that came on pre-FI 911's, 40IDA-3C for example. They have a pump adjustment as per the picture posted above by Glen Stazak.

Yes, sir!!

From what I can see mine are without adjustment externally.

Forgive the picture quality.


KT

Picture quality!?

Trek, First thing I'd do is clean and lube that linkage. Yuk...

This is the same carb
Look Here...What is this connected to?
Click to view attachment
Click to view attachment

If there is no connection to the butterfly shaft then you have nothing to actuate the accelerator pump