I've been noticing a slight stumble off idle and reduced fuel economy on my 75 F250. 400 engine, T18 4 speed transmission and Dana 60 with 3.73 gears. So I started checking it out and quickly found a bad vacuum advance canister. Got a new one and got it installed this morning. I decided to try out our friend from down under
@Bewy's advice and attach the vacuum canister to manifold vacuum.
Starting the engine up and got some heat in it, then attached the canister to manifold vacuum. I also had my digital timing light on it which also has a digital tachometer. So I started pulling timing in until the engine stopped idling up in RPM. I moved the timing mark back with the light and discovered it was at 44 degrees initial. That's just too much. So I backed it down to right at 30 initial. Although it did idle down some, it was only a little. Remember, when it gets under a load, the vacuum can will fall out of the equation and the timing will fall off.
I have to go pick up a work table from a friend either later this afternoon or tomorrow across the county, so we'll see how it works. I have tried manifold vacuum on Vixen, my Valiant and it did not respond as well as it does with ported vacuum so I put it back on ported vacuum. Since the Valiant has pretty high compression (over 10:1) I believe that may be why it did not respond as well with manifold vacuum. The truck however is only around 9:1 at the absolute most and more likely around 8.5-8.7, so it may respond better, especially with the somewhat lumpy Crane Fireball cam with only about 7hg of vacuum at idle. Although I have not rechecked the vacuum with the advance canister on full manifold vacuum, it might have more now. I wanted to tag
@Bewy in this, so he could put in his 2 cents, since he is a fan of manifold vacuum.
