larger updraft carbs?

[JonE]

Are there any traditional choices for specials tuners, for those wanting to fiddle with gravity feed but wanting the performance of at least a Ruby sidedraft and ideally a 19mm choke downdraft? Nothing as lary as anything which was used on a factory Sports.
I've been looking at the Zenith 30 VEA but all the old carb data sites on the web seem to have gone now. I suspect it is too large a choke.
Thoughts? It would also need to be reliable... hence wondering whether there was an updraft equivalent of the 26VA.

[Howard Wright]

Hi Jon

You can bore out the 22FZ Venturi and the original updraft manifold. Thinning down the butterfly will also help.

I’ve done this on a carb ready for my RK engine build (just back from having a rebore and rods white metalled). But I don’t know how it performs in practice. Hopefully a useful increase and no visible modifications externally.

Cheers

Howard

[Henry Harris]

I would use a modified SU H1 or HS2 on an upturned Super Accessories inlet manifold.

26mm updraft Solex is another, though probably difficult to find, possibility.

    1936 BMW

   

[Bob Culver]

The strange thing is contemporary road tests of the 1st lwb seem to get much same performance as the RP, and RN  users on here claim at least comparable performance. Not my territory but I understand it was/is fashionable to bore the vertical run of manifold on some. Not much point if the horizontal is then smaller.
Even when a car is used everyday over a standard test route can fritter hours fiddling with jets, esp if optimum economy also sought.
Without extensive experiemnt i found a 26VA bored on two tapers to 19mm gave more power, but not as much as 1 1/8 SU.

[Parazine]

The Zenith 24T-2 might be worth considering. I find that they give better tick over, better progression on opening the throttle and may well be more power.
You can read more in the article I wrote, which was widely distributed and appears on the BA7C website.

Others find that the carb is not "vintage" enough for them - it's a personal choice thing.....

[JonE]

Thanks for the Zenith thoughts. I think as I don't need to stay standard looking for the Gould body, I also need the ease but want to get to at least 18mm choke which I don't think the 24T will do? If I bore out, I have to think about jetting...
I've already got Howard's Special images where he has created a bespoke inlet from a damaged upended downdraft and a new alloy stub for a 30 degree SU... but i'm just trying to think simplicity (and no alloy welding).
Henry - I've got a horizontal OM 1" ready to use, but would you adopt Howard's route, or have you made a secondary converter for such? I'll have a search on the Solex... thanks.

[Parazine]

I also need the ease but want to get to at least 18mm choke which I don't think the 24T will do?

24T-2 carbs were fitted to a wide range of engines and various choke tube sizes were used from 12mm up to 20mm in 1mm increments. Similarly, a wide range of jets was available, although now I fear, most would have to be reproduced by a competent lathe operator. Some people have experimented with the variable jet fitted to the Ferguson and other versions, with mixed degrees of success.

You see a wide range of offerings on eBay, mostly Ferguson or JAP versions which yield a range of jets and choke tubes. My 24T-2 spares box is full of esoteric parts from many dismantled carbs and a range of different size choke tubes.

[Ian Williams]

Jon, as this is a Ulsteroid special I would personally go with Henrys suggestion, there are alloy inlet manifolds on the market that are right for the job. The ALR for a semi downdraft SU would be perfect, turn it upside down, Fit a 1 1/4" SU, get a spacer turned up to lower the float chamber about an 1'', fit a blue spring and number 5 or 6 needle (dependant on engine tune) you will be very happy with performance.

[JonE]

Ian - agree, but I just think the 1 1/4 looks too modern even though it's the easiest route. And I've got a finally-restored OM unit sitting here. I spoke to John Barlow today who has some steel flanges so even if I have to make an additional 1" one up myself, it shouldnt be too difficult to weld up to add to the log manifold. (Howard has done the entire thing in aluminium as a pleasing single bespoke manifold, but I can't find the wall thickness he used - I don't want a carb hanging on 1.6mm aluminium tube).
Henry's updraft Solex idea - are around but expensive so not worth pursuing.

[Howard Wright]

Ian - agree, but I just think the 1 1/4 looks too modern even though it's the easiest route. And I've got a finally-restored OM unit sitting here. I spoke to John Barlow today who has some steel flanges so even if I have to make an additional 1" one up myself, it shouldnt be too difficult to weld up to add to the log manifold. (Howard has done the entire thing in aluminium as a pleasing single bespoke manifold, but I can't find the wall thickness he used  - I don't want a carb hanging on 1.6mm aluminium tube).
Henry's updraft Solex idea - are around but expensive so not worth pursuing.

Hi Jon

From memory the aluminium tube I used was 2mm. I paid around £30 to get it TIG welded. The carb is a 1 1/4” H2. Currently running a GG needle and blue spring.

Cheers

Howard