Gary -
These symptoms are suggestive of too much air or too much throttle coming
through the carb.
I've VERY successfully set up HD8s on my BJ8, and also on my friend's S1
E-type. Actually the E-type is even more sensitive as it gets very
sensitive to settings having three carbs and the slow idle bypass on the
Etype is probably slightly too much port for the engine, but I always
manage a nice steady idle around 550 rpms for all cars. I also have HD6s
on my Jag Mk IX... idles like a purring kitty.
The key thing here is as follows (in most important order):
1) The throttle plates MUST be adjusted to sit flush closed in the throttle
body. I suspect this is your problem right here. When the carbs were
rebuilt, I suspect the rebuilder did not flush the plate with the throttle
body. This is a very common mistake because on H and HS carbs, having
these plates flush isn't that important. But on an HD carb, especially the
fat throat HD8, it must be set as flush as humanely possible. The only way
to fix this is to take the carbs off the car, loosen the throttle plate
screws, and hold them up to the light and make sure little or no light is
showing through the edges of the throttle plate. If you do this, your idle
will drop like a rock and your slow idle will work properly.
2) Slow Idle screw O ring. The slow idle function on an HD8 carb is VERY
sensitive to having a good seal on the O-ring around the slow idle screw.
If there is any leakage around here, you can have significant idle
variations both up or down depending on how your mixture is set. You can
test for this problem by squirting some 3&1 oil or motor oil around the O
ring, sealing it up, and seeing how it responds. Note, whenever I rebuild
an HD8, I always smear motor oil on this O-ring... it really seams to help
considering no one makes the original type sealing rings for this anymore.
3) Finally, the throttle shaft bushings need to be sealed. On the HD8,
these seals are teflon, and should have been replaced during rebuild.
Again, you can check for failure here by squirting oil on both sides of
the shafts and seeing if that settles your idle down. If so, then your
rebuilder did not replace the teflon bushings. You can do it yourself,
it's a bit fiddly but you don't need special equipment to replace the
bushings.
Give that stuff a try and get back to me. I suspect #1. Note that HD
carbs are still my favorite.... I find they are great for setting a
beautifully clean idle for tens of thousands of miles without any need for
further adjustment (provided the carb is set up right!).
Cheers,
Alan
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