Venturi on Weber 40s?, Venturi advice |
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Venturi on Weber 40s?, Venturi advice |
Boothy |
Jun 21 2022, 07:10 AM
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#1
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Member Group: Members Posts: 88 Joined: 17-January 19 From: Abington, MA Member No.: 22,805 Region Association: North East States |
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! |
jaredmcginness |
Jun 21 2022, 08:12 AM
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#2
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... Group: Members Posts: 502 Joined: 12-June 19 From: Baltimore Member No.: 23,209 Region Association: MidAtlantic Region |
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. |
dr914@autoatlanta.com |
Jun 21 2022, 08:14 AM
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#3
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914 Guru Group: Members Posts: 7,833 Joined: 3-January 07 From: atlanta georgia Member No.: 7,418 Region Association: None |
Richard Parr at PMO is the authority. Nice guy, call him for recommendations
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! |
Shivers |
Jun 21 2022, 08:18 AM
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#4
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Senior Member Group: Members Posts: 2,368 Joined: 19-October 20 From: La Quinta, CA Member No.: 24,781 Region Association: Southern California |
When I was running a 1911, I had 28's in my 40's. Ran well
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GregAmy |
Jun 21 2022, 08:32 AM
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#5
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Advanced Member Group: Members Posts: 2,286 Joined: 22-February 13 From: Middletown CT Member No.: 15,565 Region Association: North East States |
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Al Meredith |
Jun 21 2022, 08:43 AM
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#6
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Senior Member Group: Members Posts: 960 Joined: 4-November 04 From: Atlanta, ga Member No.: 3,061 |
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. |
sportlicherFahrer |
Jun 21 2022, 08:49 AM
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#7
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Nothing to see here. Group: Members Posts: 1,078 Joined: 18-April 05 From: Tacoma, WA Member No.: 3,945 Region Association: Pacific Northwest |
Another helpful post I used for figuring sizes for my 2056.
http://www.914world.com/bbs2/index.php?s=&...t&p=2620183 |
Eric_Shea |
Jun 21 2022, 08:59 AM
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#8
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PMB Performance Group: Admin Posts: 19,274 Joined: 3-September 03 From: Salt Lake City, UT Member No.: 1,110 Region Association: Rocky Mountains |
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) |
nathanxnathan |
Jun 21 2022, 10:57 AM
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#9
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Member Group: Members Posts: 274 Joined: 16-February 18 From: Laguna Beach, CA Member No.: 21,899 Region Association: Southern California |
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.
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nditiz1 |
Jun 21 2022, 11:05 AM
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#10
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Senior Member Group: Members Posts: 1,180 Joined: 26-May 15 From: Mount Airy, Maryland Member No.: 18,763 Region Association: MidAtlantic Region |
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.
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930cabman |
Jun 21 2022, 11:33 AM
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#11
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Advanced Member Group: Members Posts: 3,009 Joined: 12-November 20 From: Buffalo Member No.: 24,877 Region Association: North East States |
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. (IMG:style_emoticons/default/agree.gif) start with 28's and go from there. What condition is the engine in? compression #'s and/or leakdown #'s |
SirAndy |
Jun 21 2022, 01:00 PM
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#12
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Resident German Group: Admin Posts: 41,623 Joined: 21-January 03 From: Oakland, Kalifornia Member No.: 179 Region Association: Northern California |
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. 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. (IMG:style_emoticons/default/driving.gif) |
Boothy |
Jun 21 2022, 02:03 PM
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#13
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Member Group: Members Posts: 88 Joined: 17-January 19 From: Abington, MA Member No.: 22,805 Region Association: North East States |
As always, you guys are the greatest!
Thanks for the help! Shooting to have it all in and hooked up by the 4th. |
Boothy |
Jun 21 2022, 02:07 PM
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#14
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Member Group: Members Posts: 88 Joined: 17-January 19 From: Abington, MA Member No.: 22,805 Region Association: North East States |
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." |
r_towle |
Jun 21 2022, 06:49 PM
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#15
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Custom Member Group: Members Posts: 24,574 Joined: 9-January 03 From: Taxachusetts Member No.: 124 Region Association: North East States |
28
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