Downdraft Zenith carburettors

The Bill Williams book on Austin 7 Specials notes: “for simple general purpose layout with adequate performance, there is very little to beat the single downdraft carburettor. Many well know Austin Specials have performed most creditably with a downdraft Zenith carburettor as fitted to the Sports engines”.

The Nippy and Speedy utilised the 30VE1 (note its a ‘1’ not an ‘I’) which is difficult to find and expensive. The following gives some of the variants used successfully with some adaption to the jets to provide the same settings. It should be noted that the Nippy settings were over-choked in relation to the engine size, so some sort of magic occurred. Can anyone throw light on this? Jack French refers to Simplicity racing on a 19mm choke; conversely he also refers to 22mm being a theoretical perfect figure.

If Venturi size = 20 x Square root of (swept volume of one cylinder/1000 x required revs/1000), that yields 20 x root(186.75/1000 x 5000/1000) = 19.325mm. The choke on the Nippy was 21mm; on the Speedy 25mm.

new Zenith carb MS June33


from (S.R. is Slow Running jet)

Variants of the 30VE1

An official Zenith Carburetter Data Sheet states the following:
Austin 7 Sports “Nippy” 1933 to 36 – Carb Type 30VE1 C.S. No. 527
Austin 10 (Cast Iron Head) late 1936 – Carb Type 30VE1 C.S. No. 698
Austin 10 Sports 1933 to 36 – Carb Type 30VE1 C.S. No. 528
Choke, Main and Comp. jets all differ across the above three 30VE1 type carbs; the important fact to establish is the C.S. Number of any 30VE1 offered for sale.
(quote from Jeff Taylor, Austin Friends forum)

Zenith made a vast range of carburetters with a wide range of variants of each model to meet the demands of engine manufacturers. There were over a dozen different versions of the 30VE1. Each with different Choke and Jet sizes and even internal drilled passages. The best way to identify the correct carb. is by checking the ‘Contract Sheet’ number usually stamped on the casting (Fuel supply lug).

Graham Medcroft added that – in addition to those listed above – the 30VE1 number for the 1936-38 Standard 10hp A10 was F1200. See also his 1955 Conversion Oufit sheet (cream) below for further VE1 variants that may be over 23mm choke size.

He also identified that the very late Nippy was fitted with a 30VM5 number C855; this Zenith sheet from 1961 shows that the model was supplied for replacements, presumably after the VE1 was discontinued. It is also interesting that this model has a choke of 19mm, much nearer the ideal dimension – presumably to cater for the later Nippies with both sports and standard engines.

Zenith 1961 sheet


some other observations;  NOTE that the differing state of engine tune means these aren’t directly comparable, but give a good range to help understanding

The 30VM4 also came with the Big 7, as did the 30VM6, but I could never tell where any difference lay. Either worked very satisfactorily on a Speedy engine until the original d/d carb was readied. (Bill Sheehan, Australia)

I have VM4, VM5 and VM6 in my collection of bits, I can not find any size difference between the 4 & 5 other than the cold starting choke mechanism being slightly different. I chose the VM6 as it has a larger Venturi, which is much closer in size to the Nippy VE1 than the other two and it was also in NOS condition. Jet sizes 85 main, 75 compensating, nice clean pick up, good top speed and almost perfect plug readings. Once the new engine freed up further tweeks -f inal setting 95main,65comp which gives good overall performance. However I am not entirely happy with low RPM torque, which may just be down to the Nippy cam, although I used the same type of cam in one of my special engines with reasonable bottom end. I have just fitted a VM5 to the Nippy and we will see if the slightly smaller Venturi makes any differences. Oct 2016 update: 30VM6: 23mm choke 95main 65comp versus 30VM5: 19mm choke 75main 50comp. Big drop in jet size so better fuel consumption and no apparent loss in performance. Based on these results I would suggest both std Nippys and Speedys were fitted with over sized carbs, which is surprising. Easier starting from cold with the 19mm, with the 23mm I would have to flood it first. This suggests to me that at cranking speed air flow through the Venturi was too slow for effective atomization of the fuel, whilst when it has been running we have the hot spot to assist. I would also like to try a 21mm choke but don’t currently have one to play with. (Ian Williams, New Zealand)

I have been using a Zenith downdraught 30VM-4 marked “8HP” on the body casting which carries the choke cam lever. This has a 19mm choke and a bore of 30mm.This works very well on my pretend Ulster, using a John Barlow aluminium inlet manifold and correct Ulster exhaust manifold/silencer and long original type tailpipe. It pulls cleanly from low speed up to 5500rev/min (and more I suppose.) Looking at my old notebooks from 1996 I tried 16 different main/comp/slow run jets and found the best was 85main/55comp/60 S.R. which is as a standard Nippy I think. (Dave Wortley)

Martin Baker observations for Speedy [Choke (mm), main, and comp settings respectively]

25, 120, 40 Best [+ 60, 2, 1.5, 90…. as per the March 35 Austin Handbook]
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
21 100 55 YE9 valve burning?

some others:
19 70 60 Nippy; W H Wright from Zenith 1955 from letter Bob Wyatt A7CA 2005D (slow 45, prog 100, needle 1.5)
23 92 65 (Martin Eyre, Grasshopper)

Nippy manifold, large inlet valves, Paul Bonewell cam and followers and skimmed head, 30mm Zenith:
21, 100, 50 (as Martin Eyre’s Simplicity) goes very well and returns 40 mpg.

This article from J Moon (Feb 1948) 750 MC Special Builders Guide is also very informative:

Zenith – The A7 side draught or Ford 8 downdraught were popular carbs on tuned engines. The 17mm fixed choke in the side draught is too small for a higher revving engine, the 19mm of the Ford 8 works well with a combination of bottom end traction without restricting revs. Smaller venturi chokes can be machined out to the size you want. Its best to re-machine the venturi shape back in but they do work if just drilled through parallel. Beware that you’ve got the right size carb, the Austin 10 or the Ford 10HP engine had similar looking carbs with a bigger choke, the venturi size is cast or stamped into the top of it. If you’re not sure—measure it, try various sized sockets or other round things down the hole till one just slides through then measure its diameter in millimetres. Austin carbs usually have A7 or A10 stamped onto the mounting flange as well.

For tuned engines, main jets around 70 or 75 and compensating jets around 65 or 70 will not be far out. The main jet affects the mixture at full throttle and at revs, the compensating adjusts the mixture at part throttle running, pick up etc. Generally if you go up on main jet size you can reduce the compensator size and vice versa. A range of jet sizes are available from Zenith specialists, standard sizes are in multiples of ‘5’. You can get in between sizes from carbs for production runs of a standard model. The number relates to the metric size, 75 is 0.75mm, with the range of modellers drills available you may be able to drill out your own – make sure you re-stamp the sizes!

All this applies if the venturi stays the same size. Obviously if you try a different venturi size all the jet settings will change. We’ve always just muddled around with the few alternatives we’ve got in the jets tin but if you wanted to be technical you could do your pie R squared bit, work out the difference in diameter and do the same for the jets. Some Zeniths have removable venturis, the VE1s from the Speedy and Nippy certainly do and I believe the first sidedraught ones did as well. These are the easiest to re-machine. While I’m on about the VE1 venturis, I don’t know how Austins got the 21mm and 25mm sizes to work on the Nippy and Speedy. 19mm seems to be about optimum for a tuned A7 engine. As another aside, if you can’t find a genuine VE1 for your sports engine the Austin A30/35 Zenith looks about right and works quite well. (Dave Armstrong’s Speedex site)

If it’s running OK leave well alone. Clean up with thinners to degrease. Then use the correct size screw drivers and spanners only and try leaving the carb to soak in paraffin for a week. Try warming the carb up by pouring hot water over it. Most of the early carbs nuts/bolts are BSW.

Setting the float levels on early Zeniths can be difficult but it does pay to get it right. Bottom feed float chambers with the needle and toggles, these are the usual areas for concern. Try turning the toggles over to the unworn side. It won’t help if more washers are put under the needle seating in the hope that this will stop flooding because too large or too many washers will effect the float level causing weak mixture or flooding. Bear in mind to that the heat from the engine will sink in to the carb and expand the fuel, causing it to flood even when the engine is turned off .

Jets are used to meter the amount of fuel to other parts of the carburetter. Don’t put wire or needles through the hole to clear it. Compressed air is needed. Later Zeniths have a square head on one of the float chamber bolts which is inserted into the jet to unscrew it .If the carb has not been apart before try boiling a kettle, let it stand for a bit, then pour it over. Later type Zeniths are made from alloys which have a low melting point so – care! Once the carb is apart clean all the internal parts carefully before inspecting them for wear. Throttle spindles should be checked and also the throttle body. Any play here will cause bad running, wasted fuel or stalling. Check the body for distortions especially the flange. Get a sheet of fine grade wet and dry put it on a surface plate or even a sheet of glass and wet it with paraffin; holding the carb gently move the carb in circular motions keeping the carb straight at all times .You will see the high and low spots. The spindles will need to be replaced if there’s too much play also the butterfly. This will mean the body will very likely need to be reamed out for either new bushes or over sized spindles.When I have new spindles manufactured I always have them made over size; this makes it easier when reconditioning. (from MTB Carb Specialists, summarised)

Zenith Carburettors Service Bulletin: click on pics for full size

Choke sizes on various Zenith downdrafts:

19mm (note, these may not all suit the layout of the 7):
26VF3 c.853
24U902 905
26VM3 c.935
26VA c.541
30 UH-2
30 UJ-2
VM5 c.855A (noted elsewhere as being fitted to late Nippies…)
VM4 c.855
VM7 c.1450

26VME c.1511

26VME 20,21 and 22mm

(on 30 series)
VE1 ;21 (Nippy) 23 and 24mm
VM; 27mm – CARE!
VM-2; 23 or 25mm
VM-4; 19, 22 or 23mm
VM-5; 19, 22 or 23mm
VM-6; 23mm
VM-7; 19 and 21mm
VM-8; 22 and 25mm

See these below for determining the C.S numbers needed.

Here is a diagram with generic part descriptions; note main jets are smaller than the compensating jet.

VM4 cs865VM4 cs865a

Burlen Ltd. 01722 412 500; they are the Zenith Carb. Co and the best source of spares.

Zenith 1929 Carburettor instruction manual (missing page 4 but its also on Burlen; though not as pretty) here

Any corrections  or suggested additions – please make contact.

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