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MG MGB Technical - Engine stumbles badly uphill

I recently fitted new carbs and a new distributor to my 71 BGT to replace the worn originals. Now I have a problem with the car stumbling badly when I go up a steepish hill. It runs fine on the flat, but when I am in 4th going up a hill, it bogs down. When I back off the throttle it stops bucking, but slows down until I downshift to 3rd. In 3rd it surges a bit, but will continue to climb the hill. I believe the problem may be in the carb needle profile, but also might be an incompatibility with the dizzy. It is not a fuel supply problem, I have checked that already. The fuel tank and fuel pump are less than 2 years old, flow rate and pressure are good. It has a brand new filter as I initially thought that might be the problem.

The distributor is a 45D Eurospec (41427 I think), running ported vacuum, and the carbs are new HS4's, the replacements for 70-71 (Moss part #366-488). Anyone know what the AUDXXX number is for these carbs and what needles they come with? The Burlen web site shows the correct carbs for US 70-71 as AUD405, but I don't see the small tab that came on the older carbs. These carbs differ from the originals in having a vacuum port on the front carb (originals used manifold vacuum) and no poppet valves on the throttle butterfly. I am running K & N filters on these carbs.
Paul Konkle

Paul - If you address the question regarding the part number of the carbs to Burlen Fuel, they can probably give you the answer and recommend the proper needle, or range of needles. I have found them to be quite helpfull over the years. Good luck - Dave
David DuBois

Try blocking the vacuum port on the carb and going back to the manifold vacuum. I'm not sure about the Eurospec dizzy, but most US cars using vacuum from the carb port used 25D's. The only other thing I could think of would be incorrect float height, allowing one carb to either flood or starve when pointed uphill.
Ken Lessig

Needles for '70 and '71, were AAE and AAL.
Ken R
Ken Rich

Dave and Kens, thanks very much for the input. I will try switching the advance back, as that is a 10 second evolution, and contact Burlen to see about needles. What I really need is a rolling road dyno, but I don't know of one locally with someone well-versed in SU tuning.
Paul Konkle

Paul,

The distributor specifications I've seen for 45D4 #41427 indicate that it is intended to use PORTED vacuum advance (it is a common replacement for the early 25Ds). The specs on Doug Jackson's site will confirm this (and seem to warn against the use of manifold vacuum for this distributor).

FWIW,

Barry
Barry Kindig

This thread was discussed between 19/03/2002 and 20/03/2002

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