Dear Jack,
I modified some H-6 carbs years ago and recently ran them on an engine
dyno back to back with unmodified H-6 carbs on a modified inlet manifold.
The modified carbs had the throttle shafts narrowed(the shaft section on one
side of the butterfly was completely removed and the other side was narrowed
and streamlined), the butterflys knife edged and streamlined, the vertical
step on the bridge was completely milled away, the air valve was radiused on
its lower leading edge and modified so that it could go all the way up out
of the air flow , and all internal corners in the airstream were radiused or
at least had the sharp edges knocked off. The transition areas between the
round parts of the interior of the carb and the air valve area were not
"squared off" as per Vizard's instructions. The difference in power output
on a 2187cc full race engine was 160 hp with the stock carbs and 163 with
the modified carbs. I had expected a greater difference.
If you look closely at an H-6 or an HS-6 carb, you will find that the
entry of these carbs is 1/8 inch smaller than the 1 3/4 inch that the carb
measures at the butterfly side mounting flange. I would venture a guess
that if you measured the area occupied by the butterfly and shaft and
subtracted it from the area of a 1 3/4 inch hole, the result would be the
area of the 1 5/8 inch entrance for the air at the front of the carburetor.
To achieve greater airflow, I think that the front of the carb needs to be
opened up to 1 3/4 inch, then all of the other mods need to be made as well
as using good radiused velocity stacks,(which we did while doing the dyno
testing). The problem may be that there is very little material in the
carburator body of a H-6 between the air valve and the front face of the
carb. The body of the casting in this area is very thin. These carbs have
been referred to as "constant vacuum" carbs and also "constant area" carbs.
The "constant area " name would be consistent with the 1 5/8 inlet, 1 3/4
outlet with a throttle shaft in the middle. In order to get the carb to
work properly, it may be necessary to maintain the original "constant area"
ratio as the air passes through the body of the carburator.
Removing the vertical step at the top of the bridge under the air valve
seemed to make it necessary to use the next richer needle to achieve that
same fuel flow as we had had before this modification was done.
Regards, Greg Solow
----- Original Message -----
From: Jack W Drews <vinttr4@geneseo.net>
To: <vintage-race@autox.team.net>
Sent: Saturday, December 04, 1999 4:39 AM
Subject: S. U. Carbs
> Out of curiosity, has anyone on the list modified their S. U.
> Carburetors per the David Vizzard book on BMC "A" engines?
>
> --
> uncle jack
> I ama curious -- blue
>
>
>
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