[Fot] SU carb smooth bore or leave rough?

[Mike Harmuth]

Thanks to everyone for their insights. I think I'll just look for any
obvious rough spots but mostly leave well enough alone.

mike h

On Wed, Nov 25, 2020 at 4:52 PM Enquiries Road & Track <
enquiries at roadandtrack.net.au> wrote:

> I have had a few goes at shaving throttle shafts with very mixed
> results..typically fracturing where they meet the carby body.  if you
> planned to change them routinely, thats not such an issue
>
> Some recent flow bench work leaves me wondering completely about the
> wisdom of any of this without real-world testing
>
> We found some head/intake arrangements where the air flow was too high in
> certain places and we had to dimple the port to slow it down. Sometimes,
> just enlarging the port does not work as intended
>
> The very best SU research ive seen was done by David Vizard . There are
> plenty of on-line references to his work, but if anyone can't find what
> they want, i can scan relevant parts of his text book (tome) on tuning BMC
> A series engines. Ive used his analysis on  various shapes of ram tubes for
> many years
>
> of far more importance seems to be the distance from the valve to the
> throttle plate, with the ideal varying considerably with the RPM range
> being used. As a general comment, it seems like many folk have it far too
> long
>
> I have proven that fiddling with the damper in a SU has a dramatic effect
> on throttle response, and not always what you think. i'll offer the comment
> that its very engine set-up specific. Sadly, this aspect is all-but
> impossible to test on a conventional flow bench
>
> Terry
> Australia
>
>
>
> On Thu, Nov 26, 2020 at 2:35 AM DAVE HOGYE via Fot <fot at autox.team.net>
> wrote:
>
>> I know that Huffaker does this.  I believe they also remove the damper
>> oil and or the damper completely and shave down the throttle shaft to
>> almost nothing.  So, I don't think you'd be trashing them, but I don't know
>> what the dyno results are either. As far as I know, they also replace head
>> gaskets and engines quite regularly on their BMC builds.
>> Dave H.
>>
>>
>> On 11/24/2020 12:41 PM Mike Harmuth via Fot <fot at autox.team.net> wrote:
>>
>>
>>
>> I was going over my race carbs (SU H4)and wondered if there's anything to
>> be gained  (or lost) by smoothing out the casting (before the jet)? I'm not
>> sure if it's designed (or left) that way to promote turbulence and better
>> mixing as the air flows over the jet or if polishing it down would help.
>>
>> I'd hate to trash a pair of good carbs before I get some answers.
>>
>> thanks
>> mike h
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