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PostPosted: Tue Jan 09, 2018 10:42 pm 
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I'm sorting the Italian Weber 40 IDF carbs on my new to me '65 356C and had a couple of questions.

1. I've got a serious flat spot when I "get on the throttle" and even in less aggressive driving especially from a standing from start (seems to be a common issue). I'm still sorting through what size jets and emulsion tubes I've got as well as accelerator pump volume. I'm also replacing the distributor which is an 022 "reproduction" (really a modified 009 as several have told me) with a proper 022 so I'm betting a lot of this will help. If that doesn't solve it I was going to look at adjusting accelerator pump volume. My carbs use a cam vice the threaded rod to regulate flow and I read somewhere here (can't seem to find it anymore) that it was possible to adjust the volume without having to actually file / re-profile the accelerator pump cams. Does anybody know if that's true and, if so, how do I do it?

2. I know Webers need a heavier return spring or in some cases I've seen 2 springs per carb. I have two of the "factory" round springs that fit over the throttle shafts and would like to install them both on the rear of the carb (as viewed from the engine compartment). It's a simple install with the right side carb as it's set up just for the arm / spring but I'd also like to install a spring on the left carb next to the throttle arm. I looked for a left side (mirror image) spring with no luck. I was also trying to see if there's a way to modify the spring and potentially notch the left arm to make it work. Has anybody found a solution to doing this and gotten it to work? Are there round springs that are left and right that somebody has come across?

https://imgur.com/a/hipDY

Sorry for the dumb questions but thanks in advance for any help.

Greg


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PostPosted: Wed Jan 10, 2018 10:45 am 
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Greg,
If these are the old Italian Webers with the cam actuated accelerator pump your hesitation upon acceleration may be from too much squirt causing a rich stumble as opposed to a lean spot at say 2,200 rpm. The original cams are WAY to aggressive, something like .8cc per stroke when they should be about .15cc per stroke. I spent one winter in the basement grinding the cams until I achieved the proper squirt. This involves dismantling the carb, removing the throttle shaft and cam and by trial and error regrind the cam profile, then reassemble and measure the squirt at the nozzle.
The picture shows the reground cam on the left which works. The cam on the right already has the "hook" ground off the lobe but it still gave way too much squirt.
I think someone sells a conversion kit to modify these carbs to the threaded rod style, though I found this out after the fact. You can get them to work, it just takes time and tinkering.


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PostPosted: Wed Jan 10, 2018 3:40 pm 
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After some additional research, it appears I can buy some version of this to help solve the accelerator pump volume issue:

https://www.pegasusautoracing.com/produ ... duct=79701

My question now becomes:

Can I change out the accelerator pump discharge jet / nozzle in the Weber 40 IDF to regulate the volume of fuel that is actually delivered on the old Italian Webers?

From reading, it appears the jets come in different sizes but it appears changing these only adjusts the duration of the accelerator pump discharge and not the actual volume. I would think that putting a smaller jet in and adding a larger accelerator pump discharge valve (as linked above) would regulate the volume / flow rate enough to prevent stumbles and running issues. Can somebody confirm that?

As a side thought, if the different sized accelerator pump jest do regulate volume and not just duration of the accelerator pump discharge, what are the volumes of these jets (published specs)?

All of this leads to my last question:

What is the factory specified volume that should come out of an accelerator pump jet in an Italian Weber? I know a broad question but I wonder if Weber ever published a document that specified the volume each nozzle size would produce when the throttle was opened for a given carb type.

In the end, it appears I need a volume of .15 - .20 cc per accelerator pump jet when I press the throttle down so finding the correct jet nozzles and bypass valve combination would, theoretically, get my carbs to the right value.

Sorry for the long winded post but thanks for the help.

Greg


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PostPosted: Wed Jan 10, 2018 5:08 pm 
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Location: Lafayette, NJ
Greg,
I doubt you can solve the aggressive cam output by changing jets. I didn't try the changing of jets solution so maybe someone with more experience than me can comment.
I've never seen any published discharge volumes for the jets, sorry.
Possibly call Pegasus and ask?


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PostPosted: Thu Jan 11, 2018 9:56 am 
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Greg Carter wrote:
I'm sorting the Italian Weber 40 IDF carbs on my new to me '65 356C and had a couple of questions.

Are there round springs that are left and right that somebody has come across?

Sorry for the dumb questions but thanks in advance for any help.

Greg

I've purchased jets and emulsion tubes from this vendor and am happy with his service. He offers R/L springs here: https://www.ebay.com/itm/WEBER-IDF-SHAF ... 43ed6ddce7 Note that he does say that these fit "only the Alfa Romeo type of Webers" but at the same time says they will fit the IDF. Also, what emulsion tubes are you using? When I had Webers, I used the F7 and eliminated 99% of flat spots.

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PostPosted: Thu Jan 11, 2018 12:34 pm 
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Instead of focusing on the accelerator pump volume, I would recommend following John Willhoit's suggestions with respect to jets, etc. Both of my 356C coupes have Weber IDF40s (but not with the cam-adjust accelerator pumps).
I have the stock 200 air correction jets instead of Willhoit's recommended 175s, but I also upped the idle jets to 55 instead of 50.
Both cars run just fine, whether I use the Bosch 050 distributor or the 123 ignition (electronic distributor). Your Bosch 022 should also work well.
Here's his setup from 2003:
[Subject: Re: To all you weber experts
From: John Willhoit <jbwillhoit@yahoo.com>
Date: Sun, 20 Jul 2003 16:09:55 -0700 (PDT)

First step in adjusting would be to screw the idle
bypass valves closed (the little screw with a nut on
it next to the idle mixture screw). Next make sure the
throttle linkage is balanced (back off the idle stops
until the butterflys just close and check that the
adjustment from side to side on the throttle rods is
even). Open the idle stops about 1/2 turn, the idle
mixture screws 1.5 turns and start the car. Adjust the
idle equally on both sides to get it at about 1K rpm.
Screw the idle mixture screws in individually until
the idle starts to slow down then back them out about
1/2 turn.

That should adjust the carbs correctly. Before doing
the adjustment, I would recommend changing to the
following setup with Weber 40s: 28mm venturis, 115-120
mains, 175 ac, F7 e-tubes, 50 idle. If you have an
050, set the max timing to 36 degrees (about 10
degrees at idle). We have spent a lot of dyno time
with Webers and this will produce the best running
engine, especially with a stock exhaust With the
smaller venturis you will give away about 2 hp above
5200 but will gain torque and drivability up to that
point. If you still have a hesitation it could be a
characteristic of the cam. Norris cams use older
profiles which have a slower ramp and a lot of seat to
seat duration. You can help this by opening your
valve adjustment to .008/.0010".

Try it. If you're not happy I'll eat a bug.

John Willhoit]

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PostPosted: Thu Jan 11, 2018 1:27 pm 
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Location: Lafayette, NJ
Dave,
Part of Greg's problem is that the cam and roller accelerator pump set up is not adjustable. As it comes from the factory the squirt is .8cc or more, that's for 1 squirt! I doubt its fixable with jet changes to the accelerator circuit.


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PostPosted: Fri Jan 12, 2018 12:46 pm 
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Im not sure anyone has answered your question. Here is my 2 cents.
The throttle shaft end nuts SHOULD NOT BE TORQUED TIGHT. They are held in place with little locking tabs, which are bent over the nut flat.
Loosen both nuts. Test to make sure shaft TURNS FREELY.
Carefully tighten nuts just enough that shaft still TURNS FREELY.
Lock nut in place with locking tab.
Test to make sure shaft returns to shut position with circular spring only.
Should not need additional springs.

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PostPosted: Sun Sep 30, 2018 1:44 pm 
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Re: Air correction jet size and effects.

OK, I get it, going to larger sized air correction LOWERS the RPM at which the main jet comes in.

But, my question is: how does this relate to the ever so famous “off idle flat spot”?

Here is what I think is going on.
As the throttle is depressed - the vacuum surrounding the emulsion tube is lessened, so LESS gas is sucked in - causing the fuel mixture to go lean at that point - causing a hesitation.

What I don’t understand is HOW the larger air correction jet solves the lessened vacuum situation.

If you can remember vacuum windshield wipers, you may start to understand.

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PostPosted: Sun Sep 30, 2018 2:21 pm 
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If you have a flat spot between 2,200 and 2,700 rpm, using a F-11 emulsion tube solder up the top 2 holes. Make sure no solder gets into the center bore, if it does a 3mm (or.125") drill will remove it. Carefully file the excess solder on the outside of the tube so it is flush with the O.D. of the tube. Re-install and give it a try.
My engine is a 1720 with stock "C" cam, 55 idle jet, 135 main jet, 205 air correction BUT I'm using 30mm
venturis. If you have the stock 28mm vents your jetting may be different. No flat spot.


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PostPosted: Sun Sep 30, 2018 5:51 pm 
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Roger Shapiro wrote:
Re: Air correction jet size and effects.

OK, I get it, going to larger sized air correction LOWERS the RPM at which the main jet comes in.

But, my question is: how does this relate to the ever so famous “off idle flat spot”?
Not every flat spot is caused by the carburetors but if they are the cause the reason is usually because of the timing of when the idle circuit turns off vs when the main circuit starts up. Both circuits could be off causing lean conditions or both circuits could be on to the point of being very rich.

To move the point that the main circuit becomes active can be done with the air jet size. The air jet changes the amount of air pressure that pushes down on the fuel as well as the volume of air. Larger air jets allow the atmosphere to apply more pressure which makes the fuel start moving sooner. Smaller air jets reduce the pressure and delay the timing of when the main circuit begins to move.

Usually a Weber with a flat spot has a timing gap from one circuit to the other which is a lean spot which can be filled by going to larger air jets which will make the main circuit start sooner. The standard air jets that come with IDFs are too small for our Porsches so try some sizes around 200 but you may have to adjust your main jet size to suit if the mixture gets too lean at high rpm WOT.

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PostPosted: Sun Sep 30, 2018 6:09 pm 
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Roger Shapiro wrote:
As the throttle is depressed - the vacuum surrounding the emulsion tube is lessened, so LESS gas is sucked in - causing the fuel mixture to go lean at that point - causing a hesitation.

What I don’t understand is HOW the larger air correction jet solves the lessened vacuum situation.
The carburetor creates its own vacuum. The manifold vacuum below the throttle plate(where your wiper vacuum source is) is reduced when the throttle is opened but the venturi affect in the carburetor increases vacuum as the air flow increases and air flow increases as the throttle is opened farther.

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PostPosted: Mon Oct 01, 2018 11:18 pm 
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I love my Solex 40PII-4...


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PostPosted: Tue Oct 02, 2018 4:11 pm 
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I love my Dellorto's.

(I couldn't let that pass, Cliff :D )

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PostPosted: Wed Oct 03, 2018 11:08 am 
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.... and his "C" would probably love its original Zeniths. Mine does.

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