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MG MGB Technical - Weber DCOE size

I was running a 45 DCO on my race B. I have been told to change to a 48. They aren't exactly common or cheap, but i notice on E bay they sell for $499. But they also list 50 mm size...why not i thought,they are still a fraction smaller than 2 inch SU's that the B's ran at le Mans.
Does anyone have any thoughts ??
You will probably want to know the details of the rebuild...valve sizes are 1.683 inlet and 1.374 ex. and the cam is Kent 719 SP I've got roller rockers arriving and the block is 1950 with MGC followers.I may at a later stage run another more modern cam and get more head work done. Any comments would be welcomed. thankyou Renton Murray
R F Murray

Hi Renton. Some years ago I held agency"s for Dellorto and Weber etc and the best reults achieved on the my dyno were with Dellorto DHLA48"s. Apart from the best power outputs the drivability was much better than with Webers and for some reason they did not seem as suseptible as the Webers to bias the flow, i,e more equal cylinder filling tho I can"t explain why. The steps in the jetting are 1/2 that of Weber so they take a bit more to set up accurately but the pay back is better running . The larger 48"s used with 36 or 38 mm chokes gave better results than 45"s with the same size chokes. I believe that it is still possible to find New Dellortos in the U.K. but there certaily are not many around..


jim soutar

Renton the size of the weber as stated is not the size of the venturi. A 45 weber may have say 36mm venturis installed and is then a 36mm carb.

Tuning a weber for performance you need to change these venturis. If for example you have 36mm in at the moment perhaps if all you are interested in is top end power then you can fit larger venturis into your 45 and will therefore have a bigger diameter carb. Obviously the 45 will require rejetting as a result but so would any change of carb.
What size venturis have you at present?
Robert (Bob) Midget Turbo

At present it is running 40 chokes.
R F Murray

I had a pair of 2" SU's on a full house 1970cc engine and it ran real strong through to 7000 but couldn't get it to perform cleanly below 4000-- swapped over to a 45Weber with 40mm chokes as you are running and it made it fantastic way down in the rev range and a lot more driveable and easier to start cold but although the mixture was spot on it lost it's guts from 6000ish on It still reved right out and was strong but just didn't feel AS strong. I think the 40mm chokes are probably the correct size but a 48mm body just to get a bit more venturi effect going is probably the answer. Therfore I agree with whoever told you to get a 48 but as you say and I agree a 50 should be good as well and that would give you the option to experriment with maybee a tad larger venturi. After all a good A series engine with a 45 can hack 40mm chokes without a problem
Them valves of yours are a tad small for your engine as well----- Cheers Willy
WilliamRevit TasmaniaAustralia


Refering to this chart ( venturi size and power it can be seen that in a 2000 engine 40mm venturis are pretty good to 7000 RPM and no more. So it is no surprise that William experiences drop of power above 6000RPM.

I dont agreee that a 48 weber gives any benefit over a 45 except it can take bigger Venturis/chokes.

Is your engine an over bored to 1950+ and what is your rev range? This will determine the venturi size which will then determine carb size and jetting

Robert (Bob) Midget Turbo

Good chart that i've been searching for a listing on f numbers for idle jets over the weekend and there it is thanks and yeah you are probably right about the 45/48 body not being any advantage apart from being able to fit the larger chokes if they are needed-- As you say it all depends on the rev range used. Cheers Willy
Williamrevit TasmaniaAustralia

Hi Willy I have always found rule of thumb numbers for idle jets to be a little on the small size. So I generally begin a number up on the idle, if it is hesitant on a trailing throttle (Classic weak mixture) then I go even richer.
Robert (Bob) Midget Turbo

Bob Yeah, the actual idle jet size isn't the problem I'm having it's the transition mixture just above idle and I had forgotten which order the F numbers went in to select a pair of idle jets to adjust this. It would have been nice if they went in number order but will sort it out using that chart Thanks again Willy
WilliamRevit TasmaniaAustralia

The F16 emulsion tube seems to be the most commonally used one. I experimented with quite a few before settling on it. RAY

This thread was discussed between 20/06/2009 and 25/06/2009

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