This is from ClamsCanino. Worked great for my 73 1500.
Manual specs are different for each, but this is close enough to get them
all REAL close. This assumes nothing is broken or "wrong" with it. This also
assumes that the timing pointer is properly adjusted (if adjustable) to
accurately read TDC of the #1 piston.
This basic procedure will work for all the 2/3/4/6 cylinder inlines from
1960 to 1988. It's not for the newer 3 and 4 cylinder loopers.
Engine off
Disconnect throttle cable
Pull throttle arm to full throttle and verify carbs open fully. Adjust main
throttle stop so that the carbs are JUST hitting thier own little throttle
stops. The idea here is that you want the screw stop and NOT the carbs
acting as the "final stop" for your throttle lever.
Return the throttle to idle, carbs should be closed. Set all idle screws to
2 turns out from lightly seated.
Remove all plugs except #1, hook a timing light to #1. Then follow the
narritive.
When the throttle is in the idle position the carbs should be fully closed
and the ignition retarded. It is the amount of retardation that determines
your idle speed and is set later with the idle stop screw.
Timing Event One - throttle pickup.
As the throttle is advanced, the spark timing advances toward TDC and then
to before TDC. As the timing is just passing TDC the 1st (minor)throttle
pickup should hit and start to open the carbs. Use the timing light. Adjust
the tang or trigger screw to make this a reality. All of them are spec'd
different, but if the 1st pickup hits at 3-4 degrees BTDC advance - you win.
Use the timing light with a friend cranking it to complete this setting.
Timing Event Two - max advance.
As you continue to advance the throttle the timing will advance while the
carbs slowly open a bit more. The next adjustment is max advance, this is
the point where the distibutor (or trigger) can advance no more. Set the
max-advance stop to 21 degrees for "direct charge" inlines and 34 degrees
for the older crossflows. Use the light, - err to the side of caution.
Right after max advance, further throttle motion should cause the second
throttle pickup to hit and open the carbs to full. You already set that -now
verify it.