351M basic tuning
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Junior User
Joined: Apr 2006
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351M basic tuning
I lost track of a thread discussing incremental timing changes of 2 degrees followed by a test drive looking for pinging. The part I wanted clarification on was the 8 turn sensitivity adjustment using an allen wrench on what I thought I remembered was the vacuum advance unit. I have the single diaphragm vacuum advance with the vacuum line on the end and an atmospheric vent (hole) on top next to the distributor. I can't find anything that resembles an adjustment screw. So, if anyone has a clue what I'm talking about, do I have "sensitivity adjustment" on this distributor/vacuum unit and, if so, where is it? Thanks
Laughing Gas
Joined: Apr 2016
Posts: 858
Likes: 118
From: Buffalo NY
You will need a hand-held vacuum pump (Harbor Freight # 69328). Disconnect the vacuum line at the carburetor and hook the line from the pump(also works as a gauge) to the spark port. With engine idling, reading should be 0. Open the throttle slightly and note the vacuum reading. Remove the distributor cap,and connect the vacuum pump to the advance .Apply vacuum to the advance until the breaker plate just starts to move,note vacuum reading. Using allen wrench,adjust the advance until it cuts-in at the same vacuum reading you got from the carb. Once they match,you're good to go.
Tuned
Joined: Sep 2003
Posts: 348
Likes: 2
From: Northern Michigan
vacuum advance in the clockwise direction until a spark
knock is noticed while driving the car in highway an
d city conditions. We recomm
end that the vacuum advance be
adjusted in small increments such as 2-turns at a time.
8. Then back out the adjustment screw (c
ounter-clockwise) in 1-turn incremen
ts until spark knock is eliminated.
The vehicle should be driven after each change.
Copy paste from Jegs GM HEI adjustable advance unit.
Take care of the centrifugal advance first, then start playing with the vacuum advance.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
^^^ This. Vacuum advance is the last thing adjusted in curving the distributor. After the initial crank timing and mechanical advance is where you want it. Then adjust the vacuum advance full bore and back it off incrementally.
Note the "ping" associated with vacuum advance is produced under different conditions than crank timing. Excessive vacuum advance shows itself at steady cruise speeds, (when vacuum is at its highest) and part throttle acceleration. Light, occasional ping now and then is OK. Excessive ping on a dead stop acceleration for example is too much crank timing or mechanical.
Note the "ping" associated with vacuum advance is produced under different conditions than crank timing. Excessive vacuum advance shows itself at steady cruise speeds, (when vacuum is at its highest) and part throttle acceleration. Light, occasional ping now and then is OK. Excessive ping on a dead stop acceleration for example is too much crank timing or mechanical.
Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 118
From: starship enterprise
Most of the distributors I have seen lately do not have the adjustable vacuum any more, hopefully yours does. Now, I would start by a good thermostat that is the right one, make sure that the nylon timing gears have been changed to new metal ones, maybe a good clutch fan, unless it has one. Then just go from there, free flowing exhaust, tinker with plugs and timing, maybe headers if you care for the fuss, they do tend to help but they are a pain in the ***.
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