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Hello everyone,

I am trying to set up my 40 idf carbs but I’m having no luck. I’m looking for a starting point for Venturi, Air correction, Main jet size. Here are my engine specs.


94mm Mahle pistons

69mm Scat crankshaft

1.25:1 Scat rockers

5.394 Scat connecting rods

ACN Road warrior L5 40x35mm cylinder heads

Weber 40 idf carburetor

Engle 110 camshaft

009 distributor with Petronix ignition

Compression ratio 8.9:1

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Standard air correctors are usually 1.80 for 44IDF. I don't know what normal size air correctors for 40 IDF. I'm guessing the venturi are 32mm in the 40 IDF, this seems correct for a 1915.

What sizes do you have? Do you also have a wideband O2 sensor and a suitable bung in your exhaust that will read all 4?

Idle jets could be anywhere between .50 and .60 size. There are 52.5, 55, and 57.5 in-between sizes available.

Mains could be 1.10 to 1.50, they are available in 0.05 increments.

It all depends on what the engine needs.

If it was me, I'd run it with the jetting already in the carbs to start, then change from that depending on O2 readings.


This is what I'm running on a pair of 40mm Dell DRLAs I got from CB back in the mid-1990's.  They’ve were set up to my engine specs (2,110cc, MOFOCO 044 heads, Engle 120 cam and I live about 500 ft. above sea level) by Dave at Blackline Racing in Utah with the following specs:

The jet sizes are:

.45 idles

1.30 mains

160 air correctors (to slow down the fuel through the main jets to overcome any hesitation - We went down from 180 air correctors)

.35 Accelerator pump jets

32mm Venturis  (This should work well on your 1915cc engine)

My E tubes are small (Del 02), but the numbers don't correlate to the Weber numbers so that won't help you so try the F7 as Danny suggested.

From a Pat Downs email  – Engine builder at CB Performance:  Jetting all depends on Venturi size and if the carbs have had any updates like the jet doctors or update kits. If they are bone stock 40mm DRLA carbs, the tried and true starting point is 180 a/c, 140 mains and .60 idles. Jet doctors can sometimes require a larger idle jet due to the direct air inlet from the idle jet holder.  A wide band a/f gauge is your best friend if you can get ahold of one and your exhaust type allows the installation of the 02 sensor.  Make sure your fuel pressure is correct for the Webers ( @DannyP - Isn't that 1.5 psi?)

Gordon, some good help and some not. Webers should be 3 to 3.5 psi, just like Dells. Solexes want 1.5 psi. Dell stuff(especially accelerator jet sizes and emulsion tubes) doesn't help this guy tune, he's got Webers.

I agree that 32 vents are the correct size for a 1915 with 40 x 35 heads. Before anything else, put the 32 vents in.

Then get the idle jets right. 80% of drivability is idle jets. Then get the mains correct. After that, you can play with air correctors and finally emulsion tubes. You can really fine tune with 1 mm of float height in transition too.

And BEFORE you do any of this: ice cold valve adjustment, clean plugs gapped correctly, and rest of ignition system checked and timing correct. Don't forget fuel pressure, no more than 3.5 psi running.

Just saying: big heads, big intake ports, .490 valve lift and like 260 degrees duration, 8.9:1 compression . . . this should be a stout motor with a lot of high-rev power, and I'd expect it to be a little soggy off idle. Assume you've got a set of 1.5-inch headers on it.

I'm thinking a lot of valve overlap too? And I'm recalling Danny seeing over-rich low-end drivability issues at some point with his engine because he'd switched to shorter velocity stacks and there was like a cloud of atomized fuel hovering above both stacks... something like that.

All of which to say: hang in there. There are a lot of moving parts involved in a tune.  You're absolutely sure you have the carbs synched? And the linkage is keeping them synched as you roll throttle?

My 2 cents (worth less than that, compared with Danny or someone else who knows something about carbs and tuning): 32 vents, 55 idles, 140 mains, F11s, 200 air correctors. Here's another guide, and another.

It does get easier as you get closer to perfect.

Last edited by edsnova

@JB356SR Ed is right, but honestly we're all guessing. I've seen HUGE motors with small idle jets, and small motors with .60 idle jets. It all depends on how your total combo works: it's about efficiency or lack thereof.

Synchronization at idle and of the linkage is very important, as is setting the idle speed and volume screws(engine must be warm). Do you have engine tin separating the top cool air from the hot exhaust air? Do you have the doghouse pieces so the driver's side carb isn't getting hot air blown on it?


That cloud of atomized fuel(reversion) was still there no matter which velocity stack was on there. I definitely recommend using stacks though vs. none. It made all the difference in the world for Joel(may he RIP). My problem was the short stacks allowed the reversion to interfere with the idle intake air. The CB jet doctors had the snorkels right at the edge of the short stacks. The reversion screwed up the mixture during transition, but I can't remember if it went lean or rich.

It would depend on how much room you have between the top of the carb and the inside surface of the air cleaner cover.  You need a minimum of 1-1/2” of space between the underside of the top cover and the top of the velocity stack for the carb to breathe without restriction.  

A velocity stack directs and accelerates the air flow into the carburetor throat so, yes, ANY carburetor will benefit from the addition of velocity stacks, especially a heavy breathing 1915 and larger engine from mid-range toward the top end of rpms.

My belief is that under 3K rpm you might not notice much difference in performance if you add velocity stacks, especially the short ones we can stuff inside our relatively small air cleaners.  BUT!   Stacks will add a bit of flow into your carb throats that might require a size or two larger main jets to match that new flow so you might see a performance boost from 3K to red line from them.

Generally, at higher rpms, the taller the stack the better.  That’s why high reving engines, like in dragsters, have really tall stacks.  Speedsters are limited by the vertical space available between the carb top and the underside of the body.  I can’t remember how tall mine are, but they’re probably under 3-1/2” max. and really only get effective over 3 grand.

BTW, it’s interesting that Thomlinson devotes over a page in the Dellorto book to velocity stacks, but zero words on them in his Weber book.  Don’t know what that means, just noting it.

Last edited by Gordon Nichols

Actually, idle jets can be partially active up to about 3500rpm depending on airflow of the motor.

I believe the stacks I have are 2.25 inch and that's what Webers usually come with. I would recommend a set, the 2.25 size fits under the air cleaner and work just fine.

I don't think the velocity stacks will alter your jetting much(especially short ones in a Speedster), but they are good for drivability.

I'm  going to be ordering velocity stacks tomorrow. I've been reading on the forum and just a little confused. Some member suggest to leave 1.5" from the top of the stack to the air cleaner top and some members suggest leaving 1.0" from the top of the stack to the air cleaner top. my question is, how critical is 1/2"?

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