Stumbling and backfiring carb question (2 Viewers)

thebigredrocker

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A retarding distributor cannot be timed to the bulb without manifold vacuum or it will be too retarded when the rpms increase.

Are you sure your dissy is vac retard? To check remove cap, apply suction with your mityvac pump. Retarding points plate rotates clockwise, advancing rotates CCW.
 
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Are you sure your dissy is vac retard? To check remove cap, apply suction with your mityvac pump. Retarding points plate rotates clockwise, advancing rotates CCW.
Yeah. I checked it with the timing light with vacuum applied.
 
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You can optimize its operation on manifold vacuum by trial and error. However, it won’t do what the vacuum advance / retard is supposed to do: retard the ignition under WOT high output conditions where you have precognition / detonation / pinging.
 
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You can optimize its operation on manifold vacuum by trial and error. However, it won’t do what the vacuum advance / retard is supposed to do: retard the ignition under WOT high output conditions where you have precognition / detonation / pinging.
Now that is a new one, I would like to learn more. You are saying my timing should advance up to a point and then retard under high output? And this would be accomplished in my 1972 distributor how? Here is another thread where someone mapped timing advance against rpm. No where do you see a retard at the end of the curve.

 
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Like Jim C sez: “If the data above is collected by free-revving the engine, the vac advance data is not very useful, since it is a function of throttle position and load, neither of which are on the graph.”

Yes. The point of a vacuum advance (or retard) distributor is to retard the spark under high engine load and low RPM, like going up hill in high gear with the pedal to the metal.

This was accomplished in your ‘72 by the VSV applying vacuum to the distributor with a WOT And low speed.
 
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Yes. The point of a vacuum advance (or retard) distributor is to retard the spark under high engine load and low RPM, like going up hill in high gear with the pedal to the metal.

This was accomplished in your ‘72 by the VSV applying vacuum to the distributor with a WOT And low speed.

For a vacuum retarding distributor, how can the VSV apply vacuum at WOT? The source vacuum from the manifold has dropped. I guess it works because according to the Haynes manual, during acceleration between 13-41 mph, the VSV is energized to retard timing. I assume then that I shouldn’t have vacuum at the distributor at idle if the VSV is working.
 
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At WOT, there is about 3 inches of H2O which is enough to drive the retard mechanism compare to zero.

If the VSV is working (and the distributor), you shouldn’t be having any problems.
 
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Can you post a picture of your distributor and pics of your carb from a couple of different angles? I'm curious to see how things are connected.
 
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The truck is running well now. I found that the VSV connections were not correct. Here is how everything is connected now.
White goes to distributor.
Red goes to carb base (manifold vacuum)
Black goes to manifold connection under air cleaner
Green goes to throttle position diaphragm


1972_vsv.jpg


The main issue I had was trying to set the timing to 7 BTDC without any vacuum to the carburetor. With a vacuum retard carburetor, the advance is when the manifold vacuum drops when you hit the gas. If you don't have that advance it will stumble and backfire if set to 7 BTDC. I found that with the VSV connected, there is vacuum at the distributor during idle only after the engine has warmed up. That is when the timing needs to be set at 7 BTDC. If you disconnect the VSV, the timing will have to be set much higher or connect the distributor directly to manifold vacuum. If you connect the distributor directly to manifold vacuum, (as Pin Head said above) you might get too much advance and get pre-detonation under WOT though I never found that to be true in my testing. The VSV is supposed to fine tune the timing a bit as part of the ICS (improved combustion system).
 

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