Weber Carb Jets

Technical MGB discussion
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Jeffrey Lister
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Weber Carb Jets

Post by Jeffrey Lister » Wed Mar 01, 2017 7:00 pm

I've recently fitted a Weber 45DCOE carb to my 1950cc stage 2 engine with a kent 724 cam, does anyone have experience of this setup and able to advise on suitable jets prior to a full rolling road tune.
The engine runs fine after 3000 rpm but up to 3000rpm it is a bit hit and miss, mild flat spot when accelerating.
Thanks
Jeff Lister.

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Peter Cresswell
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Re: Weber Carb Jets

Post by Peter Cresswell » Thu Mar 02, 2017 9:40 am

Hi Jeff
Every engine will be different so your aim to get the car on a rolling road is right. Your problem might be something simple such as incorrect float levels, so check these first.
To get some idea of a better setting at lower rpm, try a call to Webcon (Weber Concessionaires) and ask for their technical department (see http://www.webcon.co.uk ). The only problem is the main jets, air correctors, emulsion tubes, etc., may need changing and some of which are quite expensive (and you need 2 of everything). Even when you have fitted the recommended jets etc., when on the rolling road they will probably be changed again! Remember to take the jets you already have out and note their sizes to save you buying the same as you already have! The sizes are etched on each (Main jets will be a 3 digit number like 170, air correctors will also be a number like 180, emulsion tubes will F + a number e.g. F2, and the idling jets will be a number + a letter + a number e.g. 45F9)
Also, you haven't mentioned the distributor. This could also be over/under advancing at lower rpm and give the symptoms you describe and the exhaust system can have a dramatic effect on low rpm running.

Unless you have to drive a long way and in traffic to get to the rolling road, it might be better to put up with the poor running and leave it to them to sort out all the fuelling and ignition settings in one go. Bear in mind that it might take some time to optimise the settings (most rolling roads say a minimum of 2 hours). It is all trial and error where they make a change, do a power run, analyse what effect the change has had, try to improve it further etc., etc. The good news is once done, a Weber will hold those setting for ever!
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Jeffrey Lister
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Re: Weber Carb Jets

Post by Jeffrey Lister » Thu Mar 02, 2017 4:45 pm

Thanks Peter,
You've confirmed what I've been putting off that is to put it on a rolling road for a comprehensive tune up.

Cheers
Jeff

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Peter Cresswell
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Re: Weber Carb Jets

Post by Peter Cresswell » Sat Mar 04, 2017 1:29 am

I've experienced only good results from rolling road sessions! Firstly my racing MG TC, which I extensively developed in the 1970s, went from about 60BHP at the wheels to 90BHP with a 4.5hour rolling road session. This equates to around 125BHP at the engine from a 1930s design that gave 54BHP as standard. Not only that but the Horses were all Shires rather than Dartmoor ponies!
More visual is the curves for the Mini Cooper I had tested a few years ago. The print outs for the Power and Torque curves show a dramatic improvement before (in black) and after (in bright blue).
[attachment=1]power002.jpg[/attachment]
[attachment=0]torque002.jpg[/attachment]
Notice how the power and the torque disappear as the revs increase, despite the car feeling quite driveable on the road! The rolling road session improved the car enormously by plugging the holes I didn't even feel!
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Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Ian Fozzard
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Re: Weber Carb Jets

Post by Ian Fozzard » Sat Mar 04, 2017 1:08 pm

That's very impressive Peter, and I would agree with your views on the desirability of a good rolling road set-up. However it must be a place which is used to "our" cars! I took my 1954 Triumph TR2 (used in Hillclimbs) to a rolling road in Scotland and it was an expensive disaster. The performance was not improved and the novices running the place didn't have a clue - my distributor points were loosely fitted and I only just got home before putting things back how they were before the "tuning".

So, take care who you go to - we need some recommendations Peter!!

IanF
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Peter Cresswell
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Re: Weber Carb Jets

Post by Peter Cresswell » Sat Mar 04, 2017 2:36 pm

Hi Ian
You are absolutely right about the 'standard' being variable! Usually the best recommendations come from friends or other competitors who have used a particular rolling road and can give a recommendation based on their experience. Also you're dead right that they have to understand carburettors and distributors rather how to remap ECUs, and they also need to have stocks of jets for Webers (and Dellortos) or needles for SUs and Solex.
It also can be expensive as the best seem to charge around £100 per hour to do a set up. The session I referred to with my MG TC (4.5 hours) involved setting up the jetting (I used a single 45DCOE), including the chokes and trumpets; the timing including modifying the distributor and tuning the exhaust length, so not cheap because it is all labour, but it turned the car into a race winner. Not that is of a concern on a race car, a slight benefit as a side effect was it improved the fuel consumption from 2mpg to 5mpg!!
I think it would be wrong to recommend (or condemn) one business over another on an open forum such as this, but I will mention the Mini Cooper was done by Mikeanics in Congelton (contact details can be read in top right of graphs) and people bring historic rally cars from up to 200 miles away, so you have to be willing to travel some distance to get the best. I am though happy to help with some recommendations on a 1 to 1 basis by email or PM. I only know about the Midlands though!
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Ian Fozzard
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Re: Weber Carb Jets

Post by Ian Fozzard » Sat Mar 04, 2017 3:43 pm

Thanks for that Peter, I'm sure your advice would be very helpful to anyone who as not got specific experience of using a rolling road. I won't ask for recommendations though, as I'm up in Fife! I have considered coming down to see Peter Burgess or the folk at Aldon, but I never have, so can't provide any personal recommendations. However I have had lots of good advice and parts from Peter Burgess who has always "gone the extra mile" to be of assistance whilst I've been building my MGB and midget engines.

best regards

IanF
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

nickh6511
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Re: Weber Carb Jets

Post by nickh6511 » Tue Apr 16, 2019 8:48 pm

Hi Jeffrey
I am building a very similar spec of engine, and am looking at which cam to buy. Kent have recommended the 724 high torque cam - what is your opinion now 3 years on?
Rgds Nick

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Peter Cresswell
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Re: Weber Carb Jets

Post by Peter Cresswell » Tue Apr 16, 2019 9:41 pm

Hi Nick
I would recommend a chat with Peter Burgess at Alfreton, Derbyshire as he has a lot of experience with MGB engines. Make sure you know all the mods you have done to the engine and what you want to achieve with the car before you ring him on 01773 520021.
In my case the engine is 1868cc with quite a self modified head based on Peter Burgess outline in his book plus some other things I picked up from the Joe Huffaker Tuning Manual. I also fitted a Brown and Gammons lightweight flywheel. When it came to camshaft choice I went for a Piper 285 Magnum ground onto a new billet, and timed using steel Vernier cam wheels. This Piper cam becomes less wild as the bore size increases so it is not an undrivable cam. In fact so far it seems to pick up just after tickover and goes all the way to 7,000rpm.
Although I haven't yet had my car on a rolling road, as per the above postings it really is the only way to sort out the set up.
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

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