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MG Midget and Sprite Technical - Weber advice ?

I am just trial fitting a second hand Weber 45 on my old engine before fitting a new sprint and hillclimb one. Thankfully doesn’t appear to be leaking from anywhere and the SU fuel pump stopped pumping so assume full of fuel. I tried pushing the throttle lever fully a couple of times and I expected to see some fuel being squirted looking via the trumpet but couldn’t see anything. Is that normal, it’s not connected by a throttle cable yet . Haven’t tried a start yet . Ta
Mike Fairclough

It’s a long time since I’ve played with a Weber, but IIRC you should be able to see the pump jets squirting.
Dave O'Neill 2

Have a look on this page https://240260280.com/Tech/Carbs/Weber/DCOE%20Theory%20Operation%20and%20Tuning.html (that'll need copying and pasting due to the embedded spaces) as it has various videos and covers many parts of the Weber operation including the accelerator pump.

I not used a Weber but I have used a Dellorto and they have an external accelerator pump IIRC underneath and you can see it operating externally and see the fuel being injected into the carb throat, I think on a mates Weber DCOEs you could see the same.
David Billington

The acc. pump squirter is a long way in from looking in the front , it's way down near the butterfly, it could be squirting but you can't see it.
There are a pair of brass plugs on the top of the carb that hold the squirter in, if you remove the plugs and operate the throttle fuel should come out there,, if yes then the squirters 'might' be blocked, just pull them out and clean/check them---
William Revit

Thanks all , yes I was expecting to see fuel squirting but I didn’t realise it would be that far in , thanks William. I was looking with a mirror as the trumpets are so close to the inner wing guess I could miss that. I might try a start up and see how it goes
Mike Fairclough

You might want to check the specific type number of your 45 DCOE as they do vary. The difference is in the progression hole drillings though how big a deal that is I've no idea and perhaps neither has anyone else.

The other point about 45 DCOE is that some were made in Italy and later ones in Spain. Early ones have brass floats that are soldered to the float mechanism while later models have nylon floats. It seems that petrol with Ethanol in it will over time, damage the soldered brass floats and they will need to be replaced (I'm advised). However, as far as I know all the Spanish produced 45 DCOEs have nylon floats and only late model Italian produced 45 DCOEs have nylon floats. I'm also advised that the body is specific to either brass or nylon floats.
Daniel

Not so sure about the brass floats having their solder gobbled up by ethanol, maybe it's a problem, but haven't struck it personally and have friends running their cars on E85 for a few years now---don't know but I do know E85 eats into aluminium so maybe that's where the different weber bodies come into play and the plastic floats go with the different bodies as a precaution--??

As far as the progression holes go, yeah most later bodies have an extra hole but with the early ones with just the two it isn't an issue--If there's a lean mixrure issue in the progression circuit a size smaller secondry venturi usually cleans that up.

Webers are more of a sports/racer carby where a delorto is a bit more happy for lower throttle activity, but a properly tuned weber will drive quite well without having to drill extra transition holes-it's all in the tuning to suit the particular engine's needs---
William Revit

Just an update on this and more questions please. I am managing to start the car and it will run but only if I hold the “choke” lever 1/2 open which suggests it is running lean? If I let go it dies and won’t idle. I have got the idle mixture screws 1.5 turns out and the air bleed screws lightly seated. It is a Spanish 45 152 model. I have had to use a used manifold gasket as I am just trying to see if the carb works at this point before I swap over engines but doesn’t appear to be leaking. It’s on a maniflow inlet and MISAB plates.
Mike Fairclough

What size idle jets do you have in it?
Daniel

Hi Daniel 55 F8 if I’m looking at the right thing
Mike Fairclough

Mike,

It's worth checking the ends of the idle needles are intact. It's not unheard of for them to get jammed in and the ends break off, a mate traced problems with his 2nd hand Weber 45 to that.
David Billington

I had to google the difference between F8 and F9 idle jets and it seems that the F8 lets in more air than the F9. So, it might be that 55F9s might work out better for the engine or 50F9s or 60F9s etc.

The other choices for idle jets are F2s (which I have used) and F11 which I have not.

Having said all that I wouldn't have thought the difference was big enough for the engine not to idle. I would try opening the idle screws a bit more.

Whereabouts in the West Midlands are you?
Daniel

Thanks David I whizzed them out and they seem fine. I will try turning them out a couple more turns Daniel and see if that helps. I do plan to have it all set up on a RR when the new engine is in but thought if at least I could get it to run it would be a start. I plan to get the new engine started and bedded in on the SU’s that I know work. I’m just south of Birmingham.
Mike Fairclough

55 should be ok, I'd suggest you run some carby cleaner through it---just run it till the carb is empty, take the round wingnut cover off the top and pour the carb cleaner in and leave it for a day or two. If the carb has been sitting around it's possibly gunked up in there somewhere and a good soak'll sort that.

With idle jets, I don't have much luck with f8's and it's a good idea if changing sizes to stick to the same f number untill you get into finer tuning--
55f9 would be my choice but it should idle perfectly with what you have.---It's a waste of time and money changing jets etc untill it's been on a dyno.

55 should be plenty for your engine and the f number doesn't really effect idle mixture, it only comes into play really during the light transition period a f9 will give a slightly stronger mixture there and go out marginally further into the main jet circuit, it just depends on the venturi choice as to when that transition happens

If the idle jets are clean you could try removing the mixture screws and the wingnut cap and giving the idle circuit a blow with a airhose from the mixture screw end back into the carb

Both air bleed screws should be seated as you have them then when checking airflow speed at idle ONE of them can be opened slightly if needed to balance airflow between barrels --maybe if they were previously open then you closed them it might just need bumping up on the idle speed screw
William Revit

Thanks for all the input. I tried winding the idle mixture screws out 3.5 turns and it started and idled better but spat a bit when opening the throttle then when it was warmed adjusted them equally till it stopped spitting back. That will do for now . Just got to take it all off again and swap engines. Wish me luck
Mike Fairclough

Couple of things from that Mike---When it's warm and adjusted, if you still have the screws out over 2 turns for best idle speed then yes it needs larger idle jets---the sweet spot should end up around 1 1/4---1 1/2 turns at best idle-
60f9 would be next step IF you're over say 1 3/4 turns out.
Also Weber carbs, because there is no vac advance, they tend to like as much initial timing as you can poke at it, specially with a modded camshaft--worth a try at 16-18 btdc and that spitting and coughing will disappear.
William Revit

This thread was discussed between 17/07/2026 and 04/08/2026

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