Now connect the Vacuum Advance and slightly adjust the Throttle Screw and Idle Air Screws to compensate for the Vacuum Advance. If you note the Timing with Vacuum Advance haunting on the Damper it probably means you have Too High of a Vacuum Pod for the Engines Vacuum at Idle or you have a Vacuum leak - This is why you plug all Vacuum Sources to ensure there are no Vacuum leaks when establishing Initial Timing for a particular Engine. The Power Valve and Distributer Vacuum Pod should have approx 1.5 to 2 Hg's below the engine's Vacuum at Idle. Once you have establish the initial advance write it down for future reference and take a Vacuum Reading.
Disconnect all Vacuum sources such as Vacuum Advance, Tranny Modulator Sx, Power Brakes and PCV - The last thing you want is a Vacuum leak when establishing Initial Advance. When setting Initial Advance (which is where an engine will start and idle by turning the distributer) ensure the Carb is setup with throttle plates in the idle circuit, the Secondary Plates set per Mfgr's Specs and Idle Air Screws approx 1 1/8th turn off seat. Generally a SBC will take 12 to 14 degrees BTDC Initial with some modifications - Every engine is different. Sometimes using a 1 quart Vacuum Reservoir "T'd" into the Vacuum Advance hose can help eliminate this condition. I'm guessing that the solution is probably a simple one that's right under my nose, but I'm kinda at my wits end.Īgree you need more Initial Advance also if you have pin holes before the mufflers the unburnt fuel in the pipes, with the throttle closed, will super heat from the inrush of air and ignite - a good way to blow the mufflers too o) This is a recent occurrence, and not associated with any engine work. However, if I downshift and decelerate under load (engine braking), I get all kinds of popping and backfiring through the exhaust. I have strong, smooth acceleration under load, and smooth no-load deceleration (clutch in, load off engine - decelerates quietly and smoothly). Timing is set at 6* BTDC and have a strong spark. I have a '68 Camaro with a pre-81 GM Goodwrench crate engine (350ci - made in Mexico type) with Hooker headers, Flowmaster exhaust (2.5"), an Edelbrock 1806 Carb, Mallory HEI (nothing fancy), mild cam, and Accel wires. Or at least, others that I've read have not described the same symptoms.
I've done a lot of searching on this forum and others on this topic, and I know there have been a lot of answers, but I'm not sure I have the same issue.
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