Back in '91 I converted from the dual HIF SUs on my 73 MGB (long since sold)
to the Weber setup for many of the reasons stated by others. I regretted that
I did so. In the end I could have had the SUs rebuilt for cheaper, and the
simplicity is somewhat illusionary, IMHO, due to the large variations in
available main jets, idle jet, emusions tubes, on and on. "Infinately tunable"
means just that - a lot of fiddling. Without the benefit of the fantastic
advice on this list (geez, I still had an Apple IIE back then...) I found that
by advancing the timing, changing to lighter centrifugal advance springs
(rumaging through a friendly sports car shops collection - oh, heres one to
try....), changing idle circuit emusion tubes (special order from Weber in
California) all helped. Sorry, its been a long time and I didnt keep any the
data on what was done. But the biggest factor in curing the stumble was to
modify the throttle linkage so that the secondary butterfly would open sooner.
On the stock carb when the secondary opens there is not enough fuel in the
main circuit to compensate for the sudden rush of air volume, and it
momentarily leans out, hence the stumble. By increasing the overlap by opening
the secondary butterfly sooner air is moving past the jets in the second
barrel and the fuel is there when you "step on it". The result was to
eliminate the stumble. The car felt faster, probably due to the advanced
timing, but gas mileage was decreased. I was running stock exhaust, head, and
cam, so little was done to improve "volumetric efficiency".The whole process
was very trial and error, frustrating, and in the end I wished I had rebuilt
the SUs. Just one mans experience and YMMV (literally). Cheers, Andy Blackley
|