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This is a cross post previously listed on the 120 board. Thanks for your patience.
My engine (with OEM electronic dist/ignition system) was sourced from a '75 240, and still is in original spec except for the the one-piece intake/exhaust manifold (secondary butterflies removed), HS-6 SUs, and compression increase (somewhere close to 9.2-1) achieved by decking the block. The vacuum retard on the distributor has been left unconnected. I rebuilt this engine in '04 and the car is still a few months away from a road test, but yesterday I started and ran it for the first time - fired right off! As I was going through the adjustments I noticed that the ignition was statically set WAY advanced, and it smoothed out nicely when I pulled it back. It was running at a high idle (1800 RPM or so), indicating aprox. 30 deg advance. I noticed that it only advanced 5 deg or so when the engine was revved up, but it did indicate that the centrifugal advance was functioning. I set the timing (at the high idle speed) at aprox. 25 deg, yielding a max of about 30 deg, then started backing off the idle screws to bring it down to a normal idle speed. The RPM dropped, but continued to drop after I stopped adjusting the screws. The exhaust note became more & more labored as the speed fell, and eventually the engine died. I started it back up while holding the throttle open slightly and checked the timing marks with my strobe. I noticed that at the lower idle speeds the ignition would retard down to TDC or perhaps a bit after. Seems that when I lowered the idle speed at the carbs, the ignition started to retard, which in turn slowed the engine more, and retarded the ignition more, and so on until the engine stalled. A steady idle was obtained by bumping up the timing a bit more, but that resulted in a max advance above 30 deg.
I have no technical information on this distributor other than the tribal knowledge from this forum. It seems to me that it has quite a bit of centrifugal advance - perhaps as much as 30 deg. Secondly, it seems that the advance comes on pretty early - perhaps as much as 25 deg or more by 2000 rpm. And thirdly, 35 deg max advance may be a bit too much once on the road given the change in compression ratio and combustion chamber/squish band shape.
I did disassemble, clean, and lube the distributor back when I was building the engine. I can twist the rotor by hand - it advances a normal amount & snaps back as expected. Although I recall nothing out of the ordinary, such as missing or broken advance springs, that is not to say that something is not amiss.
What I am describing may be normal operation, but this is my only source to verify that it is or isn't. Any input is welcomed, and any leads towards technical information (description of operation, specifications, parts breakdown, etc) would be appreciated.
Thanks for your time,
Joe in St Louis
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