Carburetor theory ?????

[theboomanchu]

Who's right? Me or them? Sorry for the long post!

I bought and installed dual 34mm single throat solex's for my 1641. This is a year round driver, so I wanted the chokes. Live at around 3450' above sea level. Overall, the carbs seem to run slightly rich throughout the rpm range. (idle on up)

Carb theory dictates that as air passes through the veturi, the velocity increases and the air pressure decreases, atomizing and pulling the fuel through the jets into the airflow. Logic therefore tells me that if the venturi size is increased, the velocity will slow down, in turn increasing pressure, resulting in a leaner air/fuel mixture. To simplify, a larger veturi will allow more air through, but no more fuel as the jet size is the same, resulting in a leaner mixture.

Now, when I talked to the supplier of the carbs, the person I spoke with was adamant that increasing the venturi size from 24mm to 28 mm would have no effect on the mixture, but that the larger veturi was simply designed for use on bigger engines than 1641s, with no explanation as to why. This got me second guessing myself, since they are in the business, but the more I think about it, the more I think they are full of ***it!

There are a couple of smaller main jets available, but the installed low speed jet is as small as they come. It would be simpler to just replace the venturi than buy several main jets and dial them in while not being able to do anthing with the low speed circuit, if my theory is correct.

Any points of view on this would be appreciated.

Theboomanchu

[Marc]

Your reasoning is sound.
In fact, the rule of thumb for main jet selection on many carbs is to multiply the venturi size by ~4 (maybe a bit more for aircooleds).
Increasing the chokes from 24 to 28 should have roughly the same effect on high-speed mixture as decreasing the main jet size proportionally, say from 135 to 115.
However, it won't make much difference at idle or during the transition phase off-idle - the lower pressure that causes fuel to enter those circuits comes from manifold vacuum and the venturi-like effect of air racing around the edge of the partially-closed throttle plate, not from the main venturi.
I don't have any experience at jetting above sea level, but if a stock 1585cc 1967 Type III with 32PDSITs could run 27mm venturii I don't see why 28s shouldn't be OK for you...you may still be fat at low speeds however.
It's normal for cylinders 1 & 3 to run a bit richer at idle than 2 & 4 with any dual 1-bbl setup so you need to strike a compromise there to have all four firing right at idle.
Do these carbs take the same idle jets as stock Solex 1-bbls? 55 and 50 are pretty common, and you can find 45s in some old Type III carbs.

[theboomanchu]

Do these carbs take the same idle jets as stock Solex 1-bbls? 55 and 50 are pretty common, and you can find 45s in some old Type III carbs.

I believe they do, and they have 50s in them now. I guess I wasn't thinking it through fully until you mentioned the low speed circuit, and how the fuel port is basically on the manifold side of the throttle plate, and therefore would not be afffected by veturi size.

I still think I will go with the bigger venturis, which will help get more air volume in the bit thinner air, and to take advantage of the 044 heads I have. It just doesn't run quite to what I expected with the heads and the w100 cam. Thanks for the reply. I guess I shouldn't let someones opinion cause me to second guess myself like that, but it has been many years since I've done this type of thing, and I am a bit rusty!

[mightymanx]

Do you have a balance tube between the carbs conecting the intakes? The 3 people I know that run 34 solexes, and my self that run 34 ICT's (same carb minus chokes) have found they will not tune for anything, unless there is a balance tube between them. I don't fully understand the reason for this but I have seen it for myself that it works.

[theboomanchu]

Yes, I installed the fittings and hose that came with the kit. I'm not sure I understand the reason for it either, but figured it was tested "as is", and I'd better use it the way it was designed!