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Thu Aug 25, 2011 10:54 am

I am going to try some experiments regarding venturies and performance. I have the stock supplied 28s. After reviewing Aircooled's catalogue, I noticed they also have a 36mm vent that will slip in. Will this hurt all around performance more than the 32 vents? Is bigger necessary? John's description states that the 36s essentially turn your 40 IDF into a 44.

(this is for the 1904cc with 40x35 valves-port/polish work has been done)

Thu Aug 25, 2011 11:26 am

I think 36 vents for 40idfs are too big for your combo personally. I wouldnt go bigger than 32. Remember you need air speed to make power.

Quote: John's description states that the 36s essentially turn your 40 IDF into a 44.

A 40 carb is a 40 carb no matter what you do to it. Application and combo. You gotta have the right engine combo to efficiently make the 36s work in a 40mm sized carb. Id consider running the 36s in a 2110 or larger with the right ported heads and cam combo. Sorry not yours. My two bits.

Thu Aug 25, 2011 11:47 am

its all good. thanks for the clarity. Racer Dave suggested I try the 32s a while back, as well. Is there a rule of thumb on when you need to go from 28-30 or 32 vents? According to a Weber sales rep, the 28s are fine for my engine. According to the engine Jedi's, the 32s along with bigger jetting is what I need.

Thu Aug 25, 2011 11:54 am

28s to 32s is a huge change, I'd try that first.

Thu Aug 25, 2011 3:12 pm

thanks John! what jets do you usually have people run with the 32 vents, here in Utah? Dave suggested I try 130 mains and 190 air instead of the 200. Do these sound right? also, is it you that has the jet and exchange option?

Thu Aug 25, 2011 3:34 pm

yes we have the jet exchange for our customers. I'll even do a venturi exchange for a regular. Don't expect it for a first timer though. We support those that support us.

E-mail me for jetting recommendations.

Thu Aug 25, 2011 4:46 pm

The 36 vents are really big, if you need 36 vents they ought to be in larger carbs too.
for the 32 vents you will probably need 200-220 air and 135-140 main

Thu Aug 25, 2011 5:24 pm

32 vents, 130/200 at sea level.....Utah? What elevation?

RC

Thu Aug 25, 2011 7:51 pm

You need smaller venturis at elevation than the same engine at sea level. Be conservative at first.

Thu Aug 25, 2011 10:47 pm

Fri Aug 26, 2011 9:46 am

Paul- Im about 5000 ft or so. Definitely not anything close to sea level. Im trying to get this jetting real close the first time, so I dont have to spend multiple Saturdays and wasted money doing jets multiple times. I assume the mains should be 130 or 125, and the air Im not sure. Some say 200, some say bigger or smaller.

Fri Aug 26, 2011 9:51 am

I'd start with a 210-215 air.

Fri Aug 26, 2011 9:59 am

whoa John, that is different than what I was told. Makes me wonder if the mains and idles I was thinking about are even close to what I need. Its hard to get a good idea on what I need from people that are not familiar with our elevation.

Fri Aug 26, 2011 10:03 am

[email protected] wrote: You need smaller venturis at elevation than the same engine at sea level. Be conservative at first. I would of figured with less air you would need biger vents to get more in easyer.so much fer my thinken I live at the ocean of mexiaco.I do see where a smaller will speed up what you are getting.never realy thought about it much. racing at bristol we just brought the 2nd stage in a lot sooner.(non vw with two dominators&2 fogers with lenco)I guess with small cc vw a smaller vent would help with torque at that alt.and or go with a better less volume,longer runner intake manifolds to speed up things.

Fri Aug 26, 2011 10:14 am

RockCrusher wrote: 32 vents, 130/200 at sea level.....Utah? What elevation?

RC

5000' with 32 vents
125main/210-215air/45-47idle

Fri Aug 26, 2011 10:59 am

with less air you have less air pressure, so your signal to bring the mains in is lessened. You recover that with either more compression, or a smaller venturi.

Fri Aug 26, 2011 11:05 am

may be a dumb question, but I must know...

what are some symptoms that you NEED larger vents? Ive been tuning my 40s for so long that there is a good indication they need mods. Jetting seems good for their current condition (115 mains, 200 air, 50 idles, 28 vents). Here is the thing= the more I tune/experiment on this motor, I am finding that the power is mostly noticeable in the hi revving ranges- where the carbs probably start to limit further power/intake air. Larger vents might help, but I am trying to figure out how to know if they are necessary or not. anyone have any good indicators to figure out if they are needed?

Fri Aug 26, 2011 11:08 am

I go up until I have not increased power. If the venturis are too big, you CAN tune it out but it's not for novices to do, it's not easy. I have no venturis at all in my 48 IDFs here at 4400', and it runs sweet. But it took a lot of work to get there.

Fri Aug 26, 2011 11:10 am

On that note, if you are running high rpms, I can see going to larger if you can, vents...

Thing is this, are you running dual valve springs and all were as you can benifit from larger vents...

See, to have the larger vents to breath, just make sure you have a high rpm motor, or you will thrash it....

Mine, is a Single Valve Spring 2110, so it is set up for all the torque I can get up to my Max RedLine, which is like 5,500 RPMs....

Dyno-Balancing is also one of those things that are nice for high Revs...RB

Fri Aug 26, 2011 11:16 am

Rail- I only have single HD springs for now. Am I screwed with them? I cant put the duals on for another couple months. this engine feels similar to a nice 1600 in the lower revs and putting around, until you put your foot into it. I still need to buy a tach, in order to have the numbers to know where exactly the hell I am.