I'm having trouble with the vacuum advance on my 1978 US spitfire with
the stock Lucas 45D4 distributor with an Allison Electronic Ignition. I
hadn't checked the timing for about 2 years due to a broken timing
light. I recently bought a new one and found that my initial timing was
spot on, but that I was only getting about 10 degrees additional advance
at 4000 rpm. I dismantled the distributor and discovered that the
mechanical advance was sticking and cleaned that up and now I get full
mechanical advance. I took the car for a test drive and it accelerated
fine but "hunted" at cruising speed, there were very small but constant
changes in speed at constant throttle regardless of whether I was going
25mph or 60mph. But when I disconnect the vacuum advance, the problem
goes away.
It seems to me that the timing is "jumping" when the vacuum advance unit
comes into play and I can't figure out why. Initially the vacuum unit
was mounted loose on the distributor but I have tightened it down and
that made no improvement. I have also checked for a vacuum leak in the
hose and carb fitting by the rather low tech method of sucking on the
line, the distributor advances smoothly and stays in place.
I don't know what else to look for. I'm also confused why a problem
with the vacuum advance suddenly occurred after I fixed the mechanical
advance. Any and all suggestions welcome!
TIA!
Greg Rowe
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