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Posted by: Boothy Jun 21 2022, 07:10 AM

Morning All,

I've searched but haven't located the info on venturi sizes for Weber 40s. I have a 1.7 with a big bore and CB carb cam. The Weber I have came with a bag of venturis so I'm trying to determine the right size.

Thanks in advance!

Posted by: jaredmcginness Jun 21 2022, 08:12 AM

Somewhere in the neighborhood of 28-32mm vents would be my guess for your application. Depends on the exact specs of the big bore and cam.

Someone else here knows more than I.

Posted by: dr914@autoatlanta.com Jun 21 2022, 08:14 AM

Richard Parr at PMO is the authority. Nice guy, call him for recommendations

Posted by: Shivers Jun 21 2022, 08:18 AM

When I was running a 1911, I had 28's in my 40's. Ran well

Posted by: GregAmy Jun 21 2022, 08:32 AM

Halfway down.

http://www.914world.com/bbs2/index.php?showtopic=358140

Posted by: Al Meredith Jun 21 2022, 08:43 AM

I have the Weber manual and it has graphs that give a quick way to figure vent size .500 CC single cylinder per carb barrel indicates at 5000 RPM max power about a 32MM to 34MM vent . Go to "www.aircooled.net" and you will find all kinds of formulas and "rules of thumb" for picking vent size and jet size.

Posted by: sportlicherFahrer Jun 21 2022, 08:49 AM

Another helpful post I used for figuring sizes for my 2056.

http://www.914world.com/bbs2/index.php?s=&showtopic=330691&view=findpost&p=2620183

Posted by: Eric_Shea Jun 21 2022, 08:59 AM

My spreadsheet coincides with @GregAmy's graph in his link as well. 31 is what it calculates to. For best drivability (where you will spend "most" of your time), I'd agree with 28-30 being your sweet spot. With your altitude and displacement, I'd also shoot for:

150 Mains
175-195 Air Correction
45-50 Idle jets (45 may be too small)

Posted by: nathanxnathan Jun 21 2022, 10:57 AM

1.7 with a big bore isn't a 1.7 anymore — I think knowing your displacement is important here. I found 28's to work well for my 1.7 but I like the advantage that a tighter vacuum signature offers at lower rpm's vs tuning for max power at higher rpm's. Maybe 30's I'd try for your situation, but I'd go to the trouble of getting 28's 30's, and 32's and see how it feels with all of them — not that hard swapping them out really.

Posted by: nditiz1 Jun 21 2022, 11:05 AM

28's will be fine. I have run them up to a 2056 and were still fine. The main thing that it will do is move your power band. If when you are running up the tach you get to a part where your RPM increases, but you don't feel like you have any more power "flat" then you would want to increase the vents.

Posted by: 930cabman Jun 21 2022, 11:33 AM

start with 28's and go from there. What condition is the engine in? compression #'s and/or leakdown #'s

Posted by: SirAndy Jun 21 2022, 01:00 PM

I had a 2056 with 40 weber carbs and i had to choke it down to 32mm venturis to get rid of a very noticeable flat spot coming off the idle jets onto the main jets.

Remember, the smaller the venturi, the higher the air velocity, the better fuel/air mix.
Also worth noting, get the tallest intake runners you can find to add a bit more low end torque.

Posted by: Boothy Jun 21 2022, 02:03 PM

As always, you guys are the greatest!

Thanks for the help! Shooting to have it all in and hooked up by the 4th.

Posted by: Boothy Jun 21 2022, 02:07 PM

Right on velocity. Bernoulli's principle. An increase in velocity results in a decrease in pressure.

When I was 19 in aircraft school the men's room had a sign "For those of you with short stacks and low manifold pressure, taxi up close."

Posted by: r_towle Jun 21 2022, 06:49 PM

28