I’m replacing a defective vacuum advance on my 69 427 390 HP (L36). The one that is on the distributor now is marked MS 360 12. Is this an original part number or a service replacement.
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Jeff------
GM number 1115360, stamped 360, is the original vacuum control used for the distributor originally installed for your L-36 application. It's impossible to say whether the vacuum control you described is actually the original unit installed on your distributor because the 1115360 was available in SERVICE from GM for many years and, thus, could be a replacement. Regardless, it's the correct vacuum control, originally installed or not.In Appreciation of John Hinckley -
There is no longer an exact replacement for the MS 360 12. In case you are not aware, 360 is the last three digits of the GM long part number and 12 is the maximum crankshaft advance, which is provided at 12 or more inches Hg manifold vacuum.
SMP used to provide an exact replacement, but it was discontinued at least 15 years ago. The closest is now the B26, NAPA VC-1765, which can be cross referenced to other "brands" like BWD, Standard, Delco, and most others.
This part starts to add advance at the same 6" Hg as the 360 12, but max advance is 16 @ 12" rather than 12 @ 12". This might cause some driveability issues since in my experience early closed chamber big blocks don't like as much low speed, low load advance as small blocks with similar valve overlap. Go ahead and install it and see what happens.
To install with the distributor in the engine, pump down the can as far as possible or manually force the pin to the limit. This will expose the forward mounting screw. Then remove the screws and carefully wiggle the pin out of the breaker plate. Pump down the new VAC to pull the pin to max advance and install.
Duke
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Thank you gentleman, very informative. I have the NAPA replacement on the way and it will be fun to check the advance at full idle vacuum. When the original was working I would get 12 degrees right on the nose at idle vacuum. I guess if I get 16 from the NAPA replacement and it becomes a problem I could fudge the initial back 2 degrees to split the difference. Also, I assume there is probably an adjustable advance available for this distributor. Either way, it’s going to be way better than the “no advance” I’m getting now😀. It failed slowly so the diagnosis was not apparent. The car would run and idle well, then it wouldn’t, with no rhyme or reason. I assume the diaphragm was slowly tearing, and would sometimes pull some advance and then other times not. I have the distributor out so replacement will be easy.Comment
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Duke,
For the L36 engine, the factory spec (factory service manual) for vacuum advance says 0@7”. So I assume the advance starts around 8”. The VC 1765 is in at 6”, whereas the VC 680 (B1) starts at 8”. Both provide a max advance of 16 deg, though the VC 680 requires a higher vacuum to reach max. However, the L36 should make plenty of vacuum to be 2” above that. It seems the VC 680 would be closer to the stock spec, but I assume there are reasons you prefer the 1765?Comment
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