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MG MGB Technical - 45 Weber jetting

Any suggestions on where to start with jets on my 1950 B race engine. I have a 45 DCOE with 40 mm chokes, and a selection of jets!

Colin Parkinson

Hi Colin
I ran a 1970cc race engine in my midget hillclimber for a while and used a 45 Weber with 40mm chokes The specs that worked in that engine
Chokes 40mm
Main jet 1.85
Air corr. 1.60
Idle 60F8
Pump jet .60
I found that when the weather was cold or damp I could run 1.90 mains but not on hot days
My motor was very touchy on exhaust systems as I am sure Yours will be. I ended up with a three into one system with Nos.1+4 being 15/8inch od and the centre pipe 13/4 all being 24 inches long and joining together into a 21/2 inch collector 18 inches long
One thing to check that caught me out is the needle and seat. Although I had a healthy fuel supply to the carburettor the original needle and seat wouldn't flow enough fuel at full load with the 185-190 jets. Drilled the seat out as far as I could while still maintaining a seat for the needle to work on and away she went.
Your motor being a few ccs smaller might take the 190 jets ok Also i used open ram tubes but found that with
any air cleaner at all performance really dropped off at the top end of the rev rage.
Hope i've been of some help . Good luck Gary.

F16 emulsion tubes

Thanks Gary, that looks pretty much as I thought.
Can you remember what the needle valve was? I have a 200 in at present, also got a 250. But what pump were you running and did you us a pressure regulator.


Colin Parkinson

Sorry mate but I don't know what size needle valve it was I just drilled it out as it was the only one I had at the time.
The fuel pump setup I ran on that engine-- (here I go giving away all my secrets) It was an in tank mounted Bosch fuel injection pump with a 5/16 line to the carb and at the carby I had an adjustable blow off valve from an old Hillbourn fuel injection with the return line back to the tank. With this setup the pressure in the main line can be set at anything from 0 to any pressure you want and the excess just goes back to the tank. Most EFI cars run about 45 psi under load so the pump is more than capable of supplying enough fuel for a carby. With the Webber around 6psi is the go anything over that flooding starts to become a problem. Set at 6psi there was a supply of about 1/3 a litre per 10 seconds which is more than a 4 cyl B will ever use.I use the same setup on my mate's 4.5 litre V8 racer and it feeds that without a problem at 5psi with a holley 4bbl carb.
When I first started running this pump I tried one of those in line regulators in the supply line and no return line but by the time it was screwed down to 5-6 lbs there was hardly enough fuel coming out to run even an unmodified engine and the pump sounded as though it was working really hard - which I guess it was. They might work ok with an ordinary pump where they wouldn't need to be screwed down as far but keep an eye on fuel amount as well as pressure there have been a lot of good engines ruined through lack of a good fuel supply at high revs-load That better do for now hpoe it helps sometime Gary

Hi. Forgot about this one--- A friend down the road has an E type Jag that he uses for competition events. For the fuel pump on that we fitted two of those little Facet solid state pumps. We devided the line from the tank into two using a Y piece and fitted two of the pumps that are rated at 3-4.5 psi side by side and then joined them back together into one line to the carbs. This gave 51/2-6 psi and heaps of flow and no regulator needed. A good setup cheap and works well. Might be woth looking at. Have a nice Easter Gary

This thread was discussed between 17/03/2008 and 22/03/2008

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