I've tried all sorts of choke tubes and jets on my Speedy, including 19, 21, 22 and 25 mm choke tubes. The best setting so far is the standard Austin settings. Interesting comments in Geoff Roe's book on Bert Hadley about setting up the Speedy carb settings. Here are some of my trials and comments:

MJB Speedy Choke main comp
22 100 50 OK
22 90 50 Better
22 90 55 Better
22 90 65 seemed to miss a little
22 90 55 not so good
22 85 65 looked better on colour tune

21 85 50 good but not top speed
19 70 65 acc poor
19 70 75 better acc/hills?



Pigsty engine 25 120 40 good
21 85 55 white plugs 2011
21 90 55 better, white plugs 10-04-11
21 100 55 YE9 valve burning? July 2013

Some other settings:
J Moon Feb 1948 750 MC Special Builders Guide
Choke main comp
Nippy 21 85 65 slight flat spot
Nippy 21 110 45 max power
Nippy 21 80 75 OK
Nippy 21 75 65 Economy
Std engine 21 85 50 Good
Nippy manifold/head

W H Wright from Zenith 1955 from letter Bob Wyatt A7CA 2005D slow 45, prog 100, needle 1.5
Choke main comp
Nippy 19 70 60

Martin Eyre
Grasshopper 23 92 65

Hope this helps.

Location: Beautiful North Hampshire

what a fabulous list, Martin!

Can I just clarify, when you say the best have been the standard Austin settings, do you mean 25mm, 120, 40, 60, 2, 1.5, 90 as per the March 35 Handbook?
And presumably reducing mpg being the linear progression from 19mm upwards?

When you say choke tubes, does that mean things which insert into the 25mm bore to step it down? Or are they just entirely different carb bodies?

I will try and find Geoff Roes' comments... is it still in copyright or can it be reproduced now?

I confirm I mean 25mm, 120, 40, 60, 2, 1.5, 90 as per the March 35 Handbook.

The 30 VEI carburettor has interchangeable choke tubes, as do many other 30mm bore Zeniths. The choke tube has little if any effect on consumption, it is all about getting the flow rate and hence venturi effect correct. Worth reading up in the 750 MC Special Builders Guide, Design for Competition and Austin Seven Handbook.

I have used 30 VIG without accelerator pump, with a 19mm choke tube on a tuned Ruby. My current Pearl, with Nippy manifold, large inlet valves, Paul Bonewell cam and followers and skimmed head, has a 30mm Zenith with 21, 100, 50 (as Martin Eyre's Simplicity), which goes very well and returns 40 mpg.

Good luck

Location: Beautiful North Hampshire

Jon - I agree Martin's list is incredible. One thing I didn't mention before is that whilst awaiting my Speedy carb. reconditioning, I fitted either a VM4 or a VM6 (could have even tried both) and quite frankly couldn't detect much if any difference in performance. Maybe not much to be worrying about? Cheers, Bill in Oz

Location: Euroa,Victoria

Bill - I'm not worried, just trying to get to the bottom of the Austin route of pursuing such a venturi over and above the equations! Using the swept volume of one cylinder and 5000rpm, I get 19.3mm as the golden figure for venturi. Jack French discusses "22mm"; (1954.. p232 of the 750 green book) as"theoretically desirable"... (based on what?) contrasting it against 19mm for Simplicity's (then) engine state.

No-one has yet given any comments on such things, nor I suppose may they be able to but it is fun to try and tease it out! Would Austin have just started out with the golden figure, tried other choke tubes because they were available, and selected those results just because they gave the physical results just like with Martin's trials above?

I've quote a few of you in drawing together a few bits on Zeniths here for ready reference. Hope that's ok.

Still interested in people's thoughts on the theoretical equation as refererred to in previous post. Was Jack French considering higher revs and - perhaps - max bored out engine? And why does the Speedy thrive, as Martin's experiments show, in excess of even the 22mm?

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Martin, were the Speedy Jets settings on on a Zenith 30FEVI ? I came back from a run yesterday with signs of fuel starvation and very white plugs with 100 main jet...am going to put in a 115 or 120 as recommended somewhere (can't remember where !). James

Location: Ferring, West Sussex

Apologies everyone, I seem to have put an 'F' moment regarding downdraught carburettors and inserted an F in front of the VEI in error as Jon has quite rightly pointed out. The Carburettor reference number should be 30 VEI..a senior moment I'm afraid ! James

Location: Ferring, West Sussex