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MG Midget and Sprite Technical - Weber initial settings

Following my earlier query, I managed to pick up a nice second hand Weber 45 on a maniflow inlet. The chap told me it had come off a mini 1293 with fast road cam , ported head etc similar to what my car currently has. I have looked and it has 38mm chokes in which seem a bit large. I understand and will get the carb set up on a rolling road but would it be wise to downsize the chokes before if they are difficult to do on the car ?
Mike Fairclough

You might be better with a 36mm choke. I think I may have some spare chokes, but knowing my luck they’ll be 38s. I’ll have a look before Sunday.
Dave O'Neill 2

It'll take 38's ok -I'd be leaving it and see how it goes---What size mains has it got init--my guess would be 175 maybe 180
Mate ran 40 chokes in his 1310 with 190 mains and it was a rocket
IF you are changing chokes/discharge nozles, make sure you have the locking tabs on the retaining bolts---They are prone to coming loose without being locked
William Revit

Cheers Dave that would be great . For reference and I’m no expert on what all these parts are but it looks like it has
F2 Em tubes
170 Air corr
55 F8 Idle jet
50 Pump jet
165 Main jet
Mike Fairclough

Willy

I was thinking 36s because it’s a sprint car rather than a racer.

Mike

I’ve found a pair of 36s. For some reason, I’ve also got 4x 34s and a pair of 40s! Surprisingly, no 38s 😀
Dave O'Neill 2

Mike, your setup looks pretty close to what Vizard recommends so I would go with it as is for a start.

If you want to understand more of how they work I would recommend a book by John Passini called Weber carburettors Tuning Tips and Techniques published by Brooklands books. I think it was pretty cheap when I bought it several years ago and it covers more than just the sidedraughts.

Trev
T Mason

If you're going to dyno it I'd leave it alone and see what it does--It'd be a shame to stuff with it then find you need to go back to where it was. To me the mains are only just there but that depends a lot on the exhaust as well. If you're running a silencer then maybe they're pretty close but if it's a racer with a large/short exhaust the mains will need to be larger .
The dyno man will be able to read the power curve and advise on choke sizes if he/she knows their stuff.
Trev's idea of finding a weber book and having a good read is good, just so you get a grip of the basics of what/when each part does it's job.
There is a trap here as well-----------
If you're into the sizing charts and enter your engine size (1300) and get your pot size--325cc the chart advises what size venturi(choke) to use
That size will be for one venturi(barrell) per cylinder, probably 32-ish but I haven't seen a chart yet for siamese inlets with one barrel feeding two cylinders but it'll need to be way bigger than 32
An example is MGB's, you look up the capacity and the charts say 34-36 ish but thats one throat per cyl. Mgb's take 38's easy and most go quite well with 40's or larger if you run a larger 48 or 50 mm carby body
William Revit

Thanks for all the comments, I had looked at Vizards chart and Daniel’s book too but perhaps I need to read the book specifically to understand Webers as I’m not quite sure what all these parts parts do , how they interact and what you would replace to change performance. As Dave says the car is a fast road sprint, hill climb car and I understood that smaller chokes can improve torque and driveability . As the car is still legal for the road I could just try it and see if it even runs / drives and take it from there .
Mike Fairclough

What size are the Aux Venturi?

On my car I had the calibration perfectly set up (for maximum power) with 3.5 Aux Venturi and 185 main jets.

Then on advice from a very knowledgeable expert I switched to 5.0 Aux Venturi but 160 main jets but didn't notice any difference whatsoever and the calibration was still perfect.

So, what I am saying is that there is a specific relationship between the Aux Venturi size and main jet size and you cannot consider the main jet size in isolation of the Aux Venturi size.

I don't know what an F8 idle jet looks like and I've nearly always used F9s in various sizes.

The main jets look a little small for a 38mm main venturi.

It's worth finding out what size bleed back is fitted so you note that along with the pump jet sizes.

I've never understood why so few people use Full Radius ram pipes - they make the car so much better to drive and bog down a lot less.

I would stick with the 38mm main venturi (I have 40s on my road car) as smaller venturis just seem to make the engine work harder to rev (I have used 34s, 36s and 38s) though of course a lot will depend on what the engine needs which is where the rolling road comes in.
Daniel

Thanks Daniel, I will do a bit of dismantling and report back on the aux Venturi. At least it will be good to go to tuner with all these settings on paper so they haven’t got to start dismantling to find out.
Mike Fairclough

The F number on idle jets is the air bleed sizing for fine adjustment of the slow running/transition mixture.
As Daniel mentions F9 is the most common size and if swapping out idle jets to a different size like going from 45 to 50 it's good/best practice to stick to the same F number so you get a true indication of what the change has achieved
F8 is leaner that F9 "BUT BE AWARE" the F numbers don't go in numerical order, they're all over the place and need to be checked off against specs.
If your engine has 55F8 and correct light throttle mixture then leave it as is---If it tests that it's too rich but the next size down idle jet 50F8 is too big a step then 50F9 will give a slightly richer fuel supply than a 50F8 to give a middle ground
All good fun if you've got a bucket full of jets.
That's where the dyno man is good . he should have all the different jets to try and at the end you should only have to end up paying for one set.
It's a difficult position for the dyno operator because half the time he'll get presented with a car that some dude has found it amusing to drill jets out meaning what's it got isn't what's marked on the jets--very frustrating at times
William Revit

Thanks for the advice everyone, took the Aux vent out and they are 4.5 , not sure if that’s good or bad but at least when I go to the tuner I will have a list of all the specs unless they have been tampered with Will. I assume with how it is at the moment it will at least work and the car will run in some form or other. Just need to get the engine back in and try it for size especially the distance to the inner wing and the top mounted throttle linkage on the bonnet .
Mike Fairclough

This thread was discussed between 17/03/2026 and 20/03/2026

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