Kim could very well be right on his prediction of the dizzy's mechanical advance
mechanism causing the problem. I've torn down about 175 roadster points dizzies
and at least 75% of them have mal-adjusted mechanisms. If it's a non-smog
dizzy,
the 15 degrees of advance at the crankshaft should be reached around 3800 if the
mechanism adjusted per the Nissan factory curve. I usually adjust the mechanism
to reach total advance around 2800 to 3000 rpm for better acceleration in the
lower
rpm range . But a very important part of the adjustment is that the weights
must
fully retract to the inside end of the shorter slot in the rotor shaft plate
(under
the breaker plate assembly). I've found most do not for a variety of reasons.
If your rpm test proves a problem not getting the amount of advance for the
dizzy
(35 degrees for smog, 15 for non-smog), the mechanism may not be retracting
completely
or maybe not consistently. If this happens, when you set the timing at idle, the
mechanism may already be in an advanced position, and you'll not get the full
advance
you need at higher rpms. Remove the vacuum controller and breaker plate
assembly
(no need to remove the points from the breaker plate. When you pull the weights
outward by hand, the weights should snap back to the fully retracted position.
If the mechanism is sticky due to rust or a dry condition, take it all apart by
removing the screw in the top of the rotor shaft, clean and regrease all the
mechanism
parts, including the bore of the rotor shaft and the upper portion of the lower
shaft . Make sure you re-install the rotor shaft in the same orientation. If
it
still doesn't retract completely, for the weight that engages the shorter slot,
bend the prong holding that spring outward to provide more tension on the spring
to make the weight retract completely.
This should be part of distributor maintenance whether you have the idle speed
problem
or not. And if you find the dreaded smog dizzy guts, stamped 17.5 and diagonal
slots, do your roadster a favor and install the non-smog rotor shaft (7.5) and
non-smog
springs. Even if converting to non-smog innards, you may still need to make
adjustments
to achieve a properly adjusted advance mechanism.
Gary
aka "distributor geek"
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