Welllllllllll ... since you asked for 'theories:'
Seems possible that SU used the same size passageways for the HD8s as for
earlier carbs (HX6s). Since the BJ8 has a
'hotter' cam, allowing greater airflow, the passageways may be too small to
allow enough mixture flow at idle. Or maybe
the passageways are larger, but still not large enough to allow for greater
necessary mixture flow. Also, I believe
earlier carbs had no bypass circuit at all, so idle was set in the
'traditional' fashion.
At any rate, I never liked the idea of the throttle plates being allowed to
slam shut against the throttle bore, so I
tune my BJ8's HD8s with just a bit of throttle open at idle. That way, the
throttle stops perform their named purpose,
instead of the throttle bores, and I can fine-tune the idle--e.g. for air
density changes at altitude--easily and return
them when I land.
Bob
On 4/7/2012 5:49 AM, warthodson at aol.com wrote:
> When adjusting HD type carbs, I generally prefer to back off the fast idle
> screw so that the throttle is fully closed (after disconnecting any
> interconnecting linkage)& attempt to set the idle speed with the idle screws.
> I have no trouble doing this with HD4& HD6 carbs. Including tricarb cars with
> both HD4's& HD6's. Yes, I know those are not the original tricarbs. My
> problem& question concerns the HD8's which seem to defy being adjusted this
> way. I never seem to be able to open the idle screw far enough to increase the
> idle speed to an acceptable level. In many of these cases the HD8's have been
> recently fully restored professionally& the cars are in very good condition.
> Am I correct in believing that this is a common problem? If yes, what are the
> theories that explain why HD8's behave this way? Yes, I know there are two (or
> more) schools of thought on how to adjust carbs.
> Thanks,
> Gary
>
>
>
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Bob Spidell San Jose, CA bspidell at comcast.net
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