Barry, my BT7 is equipped with HD8 carburettors as well and I have the same
problem with the idle adjusting screw. Though the screw is fully turned in and
the throttle is fully closed, the engine idles at around 800rpm (but fairly
smooth).
To control the setting of the mixture it is important to lift the piston around
1/32 to 1/16" maximum. I did a basic adjustment (2 turns down after jet was
flush with bridge) and after that fine adjusted with the piston lifting method
until I had the engine speed increasing momentarily and then dropping back.
When I had done some miles I recognized the bumper getting black in the
surrounding of the exhaust pipe showing me the engine ran slightly rich. So I
turned the adjusting screws out a quarter turn. The engine now runs like never
before, very smooth, even when it's cold.
My assumption for your engine running rough when lifting the piston is you
lifted it too high. This way you get a mixture which is too rich. I used two
hollow ali rods inserted into the hollow piston rods with two wires bent at
right angles with the rod pointing at each other. That way there's no problem
adjusting the carbs and lifting the pistons not too far. These rods and wires
are for sale here in Germany and I guess they are for sale in the US as well.
They sell for about 25$, but it's worthwhile.
Regards
Eric BT7
Heinsberg/Germany
When the mixture is adjusted and the piston is lifted, how long should
the idle increase before dropping off. Mine currently idles the same or
just slightly increases for a second at most and then runs rough until I
release the "lift button". Should I increase the mixture until it gets
too rich and then back it off? I also don't get much effect from the
idle screw adjust (the larger screw) over a fairly large adjustment
range. It definitely will run rough when turned all the way down but not
much effect from 2 to 3 turns out. Barry Pate BJ8
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