When taking a vaccum reading at idle to determine vacuum for power valve size, should the port vacuum be taken at the carb base, or on the metering block(venturi). I have always used the metering block and divided that reading /2 to get the appropriate size valve. And for the vacuum advance I always used the venturi port on the metering block. How do these differ and are these the correct ways. In the past I,ve checked total timing with an advance dial light and everything seemed correct. ie. the engine had the correct total timing. The curve sheet shows 20@ 2700rpm and 12 degrees from vac, for a total of 32. I recall running as much as 31 intial and the motor ran extremely well with no ping.

I was always under the impression that you should use manifold vacuum to determine power valve type. This would be taken at the baseplate. The fitting on the metering block doesn't get full vacuum until the throttle is opened past the idle circuit. Also when running a lumpy cam I use the baseplate fitting for the advance because the increased timing raises the rpm's and lets you close the throttle more and get back into the idle circuit. But that's me.

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'69 RS Camaro
355 5-speed 4.11

1-2 is right - you always measure intake manifold vacuum to determine power valve values, as that's what the power valve actually sees - manifold vacuum, not "ported" vacuum. Your total timing is set with the distributor vacuum advance line disconnected and plugged; total timing is initial plus centrifugal, period - vacuum advance comes and goes depending on engine load, and is totally inoperative at wide-open throttle anyway, so you don't include it for total timing.

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JohnZ
CRG
'69 Z28 Fathom Green

yet another question, with the prementioned curve how much initial timing seems logical? its at around 25 initial now, Giving me a total of 45. Like I said I have turned it up to 31 initial without ping, on the previous motor with same cam and general setup. 51 degrees seems like alot, how much seems applicable? how much is too much?

railing68,
You should check out the other post on this subject. There is probably a lot of free power waiting for you there.

-Mark.

I'm having idle chase problems, I do have a vacu-tester with an integral guage. should I pull vacuum on the vac-advance unit to see what it requires? How Will I be able to determine this? I have previously been using venturi vac for the advance I only have one vac fitting for power brakes, will using a tee for both accessories work correctly?

I would check the vacuum readings at idle and cruise and then verify the can you have will be fully deployed at idle. You can pull a vacuum on the can and visibly see when it stops advancing. It should have the number of degrees of advance stamped on top. 16 degrees is about right. If the can you have now is not right, pick a better match from the list of OEM service replacement cans or purchase the adjustable can kit.


As for hooking up the advance can to manifold vacuum, I would look for a manifold fitting that has an extra vacuum port for this purpose. Many resto houses sell these fittings. It replaces the power brake fitting you have now only it has one or more small extra taps that you can use for the vacuum canister. Obviously the bonyard will have a good selection as well.

-Mark.

thanks for the info on the timing situation string69 you have been a lot of help. I know that my idle vac is low due to a large cam. So at 2700 I should have 36 degree locked in. I'll take a reading a get the appropriate can. thanks again