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RE: Carb choice for 1275 Tue, 14 Jan 2003 17:51:07 -0800

To: jleclainche@cox.net (Jacques Le Clainche)
Subject: RE: Carb choice for 1275 Tue, 14 Jan 2003 17:51:07 -0800
Date: Wed, 15 Jan 2003 08:20:45 -0500 (EST)
Cc: spridgets@autox.team.net

Funny you should ask....

Getting my old 45DCOE all spruced up
for installation in the Spring. Judson idea
is dead unless the US Judson guru comes
up with an alcohol/water delivery system
that'll "bring back octanes" along with a 
way to reliably get the originally intended
6psi boost curve to the 1275cc engines,
superimposed on the stock ignition advance curve capped off at a
24-26 degrees considering the effective
compression ratio involved with maximum
cylinder filling by virtue of the blower.

I'd say with a Kent 276 you are better off
with the 40 in consideration of choke sizes
normally used for good street driveability (34-35).

For something like a Kent 286 or SVP3
you'd be getting into 36-38mm chokes and
the larger 45's territory. Vizard's chapter
on the subject really says it all, Jacques.

By the same token, the 45 is used with
excellent results, despite the foregoing,
coming equipped with 34mm chokes for
the stock 1275. Were I using this in your
motor, I'd say with 36mm chokes for
highway use, 35 for rapid low end and
street torque. Either will work...it's all a
matter of which rpm range to want to
"emphasize". Get at least a 5" intake
manifold for it. If a 6" will fit in there
somehow, all the better for good low
and mid-range.

You can make up for some more inlet
tract length with air horns. As Vizard
points out, not all are alike for good flow.
TWM has the "right" bellmouth curves
and also a "cool" cold air box set-up, if
you'll excuse the pun! :)

One caveat here...if you go with the larger
45, get it with the smaller 3.5 aux venturi's
which will bring the carb's "main" fuel
circuit "in" sooner than the larger 4.5
aux. venturis the carb. MAY be supplied
with out of the box. I don't think you'll
have any problem with the carb's stock acceleration pump setting but,
should there be any tendency to "bog"
when you put your foot in it (and the ign.
timing is properly set) the main circuit
coming in sooner will help to eliminate
this or prevent it altogether. 276 isn't
a wild cam and intake valve size I assume
isn't greater than 1.4, anyway...right? And
you are running a decent med. bore header for good exhaust flow velocity
at low and mid-range and thus limiting
any tendency to exhaust gas "reversion",
of course? Of course! Stock accelerator
pump settings will be just fine!! 

You might give the Dellorto a look though
from what I can see, it's simply a DCOE
with a diaphragm accelerator pump set-up. Vizard says they are easier to
for road use but I think this is with wilder
cams and the really big Longman GT17
1.48 intake valves. Never had a problem
in this respect with my 45DCOE with 
VP3 timing, 1.414 intake valves, large bore LCB and 1.5 rockers, anyway.
Only reason I went slightly "up"
from the stock accelerator pump jet supplied with the carb. was for the
speed bleed fuel supplied by the accelerator pump's fuel system. Engine
was a really hot steet motor and dyno
indicated "main system" mixture to be
"right" for everything up to about 55K,
as which time the mix began to "dry out"
a bit. A larger pump jet was just the ticket
as it didn't go "soggy" at the bottom end
when you put your foot in it, so this was
the right way to go rather than increasing
the size of the "main" jet or decreasing the
size of it's "air corrector", which made for
more economical highway cruise with the
car's stock 4 speed and 3.9 diff (later, a
3.7). This engine spent a lot of time in
the 4,000rpm range (my "younger" days!). 

The SU, being of variable venturi design is not a "response" kinda carb.
It's main
fortes are flexibility with respect to engine
health, smoothness and economy. Of
course, it can be modified but the "fixed
venturi (or choke) is designed for "response" to begin with, most
the Weber's. 

Frank Clarici tried a single 1.75 SU and
went back to the DCOE....he missed the
"response" factor!! And Frank IS the
original "drive it like you stole" guy, so
I'll take his word for it.

Long mail, eh? Aren't you sorry you asked? Bet you need a beer by now,

Cap'n. Bob (Ret'd)
      '60 Frog

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