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MG MGB Technical - big bore carburation

Installed a 1950cc engine with 5-speed transmission conversion in my 1963 MGB last summer. just got everything running in november so I only have about 100 miles on the conversion before I put it up for the winter. I installed the original HS4 with stock jets and found very shortly this was a mistake.

Looking for advice on what has worked for others. Drivability is a key issue, have a DCOE but wanting someething more civilized abd easy to set up. Have thought about rejetting the HS4s, installing a set of HS6s and most recently considered the weber "outlaw" 38 DGES,

It just turned 50 degrees here today and my thouhgts are turning towards some short spring drives.
edward

Edward. The 38/38 Weber has the same diameter throttle plates as the SU carbs you currently have. So, I do not think it would work. Do you have a copy of Peter Burgess's book? He notes that the large engine "Feels very flat. Needs a cam to peak at standard RPM". This is on page 16 of his book. He then notes that a mild road cam gives much better performance with the -4 series carbs. The next description shows slightly better top end performance with the -6 series carbs, but the hp rating is slightly less at 3,000 rpm than shown with the -4 series carbs. You might want to drop a note to Peter, describing the engine modifications, and see what he would recommend. Les
Les Bengtson

Edward-
With a stock head and 1 1/2" SUs your engine can't breathe. I'd suggest that you get a Fast Road Big Valve head from Peter Burgess. It features larger inlet valves and is ideally suited to a Fast Road camshaft such as the Piper BP285. The increased breathing capacity of the head will show good returns with twin 1 3/4" SUs or a Weber 45 DCOE. With a Big Bore engine conversion the head is well suited to restore the peak horsepower rpm to its original position. BHP increase is approximately 25% at 3000rpm and 35% at 5300 rpm when used with a standard camshaft and K&N filters. Easily worth the money if you're sticking with the five-port head.
Steve S.

This thread was discussed between 08/03/2003 and 09/03/2003

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.