Weber 40 IDA 3C accelerator pump tuning

[p400]

I am looking to eliminate flat spots or hesitation with quick throttle application using Webers. I see .50 pump jets and .50 inlet/ bleeds are standard?
1) is there any technical, in any manual, about jet size and bleed size choices or alterations?
2) anyone using no bleed inlet valves?
thanks
Craig

[shbop]

Craig, I don't know the answer to your question, but have you tried bumping-up the pump volume with the actuator? Free bump anyway.

[p400]

Thanks Jon,
i am really just trying to understand the technical behind the inlet/bleed on a Weber. i understand kinda what it does, but how would you size bleed for a particular application? As you suggest, why not vary some other adjustment? linkage, cam shape, jet size? what size bleeds are you using? and why?
best regards
Craig

[shbop]

Craig, I'm heading out with the family, but I'll look at the size tomorrow and let you know. I can't answer "why", can only say that what I have works, for my engine. I wish someone else would chime-in with more knowledge.

[shbop]

Craig, I took one of my sets apart this morning. Same as yours: 50 I know this doesn't answer your question, but it got me qurious. The instructions describe the correct volume/stroke at .8 cc. It goes on to say that: "injection quantity can be varied by adjustiong the length of the stroke using the nuts on the threaded shaft "
I've never seen a different size pump jet, or delivery valve, but that doesn't mean much as I'm a relative novice, and have only the parts diagram to refer to.
Have you exhausted the possiblilities by adjusting the stroke?

[p400]

Hi Jon, thanks for the info. I did post this on the 911 tech forum, got a good lead at PMO on explanation of bleed -

4. Reducing Injection Quantity - We have found that race engines often need varying levels of injection quantity. Coming out of a turn, too much injection quantity can actually cause bogging. PMO has different float bowl check valves to adjust the injection quantity up or down. Rob King at S-Car-Go has cut the injection quantity down to almost nothing with excellent results. The normally installed float bowl check valves have a .5 mm side hole for bleed back. The larger the bleed back hole the less injection quantity. The valves come in increments of .1 mm up to 1 mm. We have 1.5 mm valves for people who want to almost eliminate injection quantity but still start the engine.

PMO, IMHO, is explaining the bleed as a way of controlling accelerator pump quantity. should you require more or less squirt, then you would fit appropriate bleed. What i would like to see is a Weber or period document that backs this explanation up. And/or discusses some limits on sizes?
Craig

[shbop]

So it becomes a matter of changing the size of the "inlet and discharge valve", in the float bowl. Interesting. I have several Weber documents, but nothing mentioning this. Again, perhaps some of the more experienced guys will chime-in. I appreciate the info Craig. Doesn't seem to be a day that goes by that I don't learn something here.

[rallye911]

The book you need is called 'Weber Carburettors - Theory, Tuning and Maintenance' by John Passini. Chapter 7 explains accelerator pumps.

Essentially, the size of the pump jet controls how much fuel is drawn through the jet by the intake depression, at constant throttle. The stroke and bleed between them determine how much is injected, and how quickly or slowly, when the throttle is pushed open.

Hope this helps.

Angus

[djdsc]

Think of it like this, there are two things going on with the accelerator pump and the spray jet, quantity and time. With a fixed quantity, a spray that lasts too long will go rich as the main jet system comes on. A too short of time will go lean as you move onto the mains. Too much or too little quantity along with too long or short of a length of a spray is two separate issues.

Think of what is happening here; what is controlling the quantity and what is controlling the time or length of the spray?

Pick a variable, set quantity by adjusting the accelerator pump rod, either adjustable or with the multi position and experimenting with the by-pass exhaust valve. In extreme conditions tuners have even re-ground the profile of the actuator cam. Then use the spray jet to adjust the length of time.

At constant throttle the accelerator pump system is not in use; the accelerator pump only pumps in coordination with depressing the throttle, it doesn't keep squirting.

by-pass exhaust valve

[p400]

Thank you djdsc, Angus, and Jon
I did some homework and find that all Porsche are pretty much set up with .50 jet and a zero bleed, which seems on the heavy side, but must work fine. Mine is setup with zero bleed, .50 jet.
As i look across all the the performance cars Alfa, Ferrari, Lambo, Aston, Maserati, Porsche...then consider the engine cc feed by a downdraft Weber ...look at venturi, mains, idle... the Porsche stills comes out the winner for pump volume in my opinion.
The lever cam profile looks to be an important factor as well, and now i would like to know about the variety used as well.
anyone have a 40 IDA S or T cam they can scan maybe 300 dpi and share a picture of ?
you must think i am nuts, i am!

[viejopatron]

I send a digi tomorrow of the cam. I too have a flat spot in my Weber-fed 2.2. I'll be back.

[kucharskimb]

craig
you might want to see about combining these threads. but i'll post here in the meantime, because some of you guys seem to have better literature...

i have a delayed reaction when depressing the accel pump. this occurs even when i manual actuate the lever on the accel pump. i just replaced the diaphrams, but have exprienced no improvement.

seems that the last couple places to check are all 8 check balls. i've cleaned all 8 and by either sucking or blowing on them (stay focused...) they hold air. another tuner has suggested that my reaction spring (i think usually #24 in parts diagrams) may be weak. does this spring also act to prevent back flow when the flat rubber diaphram presses against the 3 center holes on the back side of the accel pump body?

appreciate any assistance.
thanks

[p400]

Matt , i dont know if this picture will help, but here is close up of one side of the pump body, cover, big spring. little spring in the accel circuit.

[trekkor]

try a hotter spark plug.

That solved my problem, which I thought was carb cam related.

KT

[shbop]

If you guys are re-building, make sure that the opening on the secondary venturi faces the air correction jet. Grady Clay pointed this out a while back. When I checked mine, two were incorrectly placed.

While we're on the subject: Does anyone have a method for removing stuck chokes?. I've been soaking for days. Tried my heat gun. All to no avail. Any help greatly appreciated.

[kucharskimb]

not shown here is the metal disk and the inner rubber diaphram. starting at carb body, the small reaction spring is the first item installed. this spring rests against a cupped disk. the disk pushes up against the flat (plain) diaphram. the diaphram pushes up against the side of the accel pump shown in the pic.

so my questions are:

-if the spring is the wrong resistance, could this be a pressure threshold that the accel pump pressure must overcome before it will squirt up the tubes? my springs have been stretched out to increase effective tension. however, that was done to cure this same problem, and had no noticeable improvement. suggested by a tuner.

-when the reaction spring pushes up against the inner diaphram, and then against the three holes, does this prevent a back flow of fuel (a redundant check valve, if you will) to keep fuel up in the individual squirter tubes?

-what is the purpose of the reaction spring and this inner diaphram, if the upper and lower check balls are supposed to work like a fuel pump--alternating opening and closing? or alternately, what is the purpose of the upper check balls, if the diaphram is supposed to act like a check valve?

looking at the cam, i supposed you would want it to ramp up as quickly as possible to start the flow and build pressure. i'm not anywhere near the top of the cam lobe.

appreciate the help.

[viejopatron]

Maybe this will help. The X-section in the book isn't good quality, however I think you'll be able to see the check valve relationships. As far as a cam photo, none of them were any good but here's the best. Enjoy. This is a thread that has a pix of the book cover that the X-section came from; Weber carbs info

[kucharskimb]

[p400]

Hi Gary, Matt, Jon and all-
the inner diaphram does indeed work like a flapper valve - why you need both upper balls and diaphram must be sumpin the carb designer knows - we could theorize.
altering the spring dimensions i cant advise as to how this would work, but problem has to be something else. i think stretching spring to increase rate would be to overcome bad surface on three ports or poor diaphram material or ?

i am looking at cam and would like to establish some basic dimensions that we all could have as reference for a stock cam. tuners can redrill holes, grind off material. i want to know what is oem profile ......so here is mine, sombody verify it please. These dimensions were picked as an easy check, you can use some shop digital calipers to quickly get them all. abc i did not record the fractional because i feel just a quick ck is needed here to be sure the hole is in the stock place, the cam d dimension, you just use the back side of the cam as a flat for your calipers and wherever it hits the rounded cam is this dimension, i felt a more exact dimension was needed here.
i cant figure out how to edit image post, i will repost new image.

[p400]

new image
i believe my a and c dimensions are incorrect for Porsche.
i think a is 4mm making c a smaller dimension as well, but i need help from somebody with Porsche carb apart.