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View Full Version : Vacuum Advance port on Q-jet


Novi2kRT
08-19-2003, 07:24 PM
Hello.. I am ashamed I can't remember for the life of me what type of vacuum reference signal the vacuum advance on a distributor wants to see. Manifold, or ported.. (something like that)

I have a MSD 8361 that has been made to fit, or work with the Caddy 500cid, and can't remember which port on the carb the vacuum advance wants to hook to.

I have been have been having some tuning headache's with it while workin' towards getting it on the road. In Neutral, or Park, throttle response is great, crisp.. no problems. In gear with brake on hard I can blip it, and romp it a bit with no audible issue's.

But on the road at moderate acceleration it will stumble and buck, then go.. and sometimes sounds like it's pinging. Seems to be better when I disconnect the vacuum advance hose on it though. I thought I had carb issue's, so I freshened up the OE Q-jet that was on it.. still did it.. so I swapped to a reman Q-jet for that specific application and same things happened. Though it does start and idle a bit nicer now though.

I swapped from the light silver spring/light blue spring to a heavy silver/light blue spring today to see if that helps but haven't gotten to test it on the road. At first thought it seems even smoother on accelleration in park at any throttle opening rate. Though it did half "stall" once, when I disconnected the vacuum advance hose and checked again, it didn't.

The other thing is that when I held it at 2,000 rpm after it warmed up a bit.. it would stumble, and pop a bit, but randomly.. not in a rythm.. also a fair bit of oil smoke was present while at this rpm and seemed to plume a bit more when the popping occured. So this maybe oil consumption related. More junk to check and work on.. GREAT! LOL..

But advice on timing specs for this engine, all stock with headers and Edelbrock manifold, as well as the vacuum port location would be greatly appreciated.

msdtech1
08-19-2003, 09:41 PM
You will want to connect the vacuum advance canister on the distributor to manifold vacuum. This is generally the lowest vacuum port on the base of the carburetor. You can also check to make sure you have manifold vacuum by putting you finger over the vacuum port while the throttle blades are closed to see if you have vacuum present. If it is there you have a manifold vacuum source if is not then you have a ported vacuum source. The lighter the springs, the faster the total timing will occur. In a heavy car with a lot of initial timing you may get some pinging with the real light springs. Normally you want your total timing to be completely in slightly before your cruise rpm is reached. The blue springs should probally work best on your application and will bring all the timing in by 3000 rpm. Here is a list showing what rpm the total timing will occur at with the different spring combinations with a blue bushing installed.

2 Silver (heavy) springs - Total timing in by 4000 rpm
1 Silver (heavy) 1 Blue (light) springs - Total timing in by 3750 rpm
1 Silver (heavy) 1 Silver (light) springs - Total timing in by 3500 rpm
2 Blue (light) Springs - Total timing in by 3000 rpm
1 Silver (light) 1 Blue (light) springs - Total timing in by 2500 rpm
2 Silver (light) springs - Total timing in by 1800 rpm

MSD Tech1