i set the distributor up for the plow truck engine tonite,same way i do all of mine. mine all burn 87 octane without ping/detonation,and i run 12~15 degrees intial advance and manifold vacuum to the advance unit. so,here's the way i set mine up...
my engines are fully stock,or 4 barrel holley with edelbrock performer grind cam,all emissions equipment removed from both. this tune works equally well with both. i have my first 8600 cam'd 360 on the stand,this distributor is going in that engine.
i use a cheap plastic hand vacuum pump and T in a vacuum guage. i like this better than the pump with the built in guage,because this way i can put the guage where it's easy to see,not having to look at the pump for the reading.
first the vacuum advance. it's important to know the vacuum advance is the engine load monitor for the ignition. it is not the advance for increased rpm,the mechanical advance takes care of that.
also,leaner charges (higher vacuum)-like idle,cruise throttle positions-take longer to burn than richer charges (low vacuum)-like acceleration,or wider open throttle positions,or anytime the powervalve is open.
so,high vacuum= low engine load,leaner charge,and increased advance from the vacuum unit.
low vacuum= higher engine load,richer charge,and lower to no advance from the vacuum unit.
some people eliminate the vacuum advance,to overcome the pinging issue,but they do so at the cost of throttle response and economy. some think since racers don't run an advance,they don't need to on their street car either. thing is,racers run around the track at near to full throttle all the time,with a carb setup to run richer for power,often eliminating the powervalve,because it'd be open all the time anyways. racers don't need a device monitoring engine load,it's always high.
so,first,get a reading of the vacuum advance,to see where it's at. this one started advancing at about 2.5",and was fully advanced at about 10.5". this is a common reading on these distributors.
that's a problem. the powervalves on these trucks are usually dual stage,rated at somthing around 11"/8.5". the single stage units will be 6.5-8.5" units. when the carb goes rich,and the powervalve opens,the vacuum unit will still be nearly fully advanced. this is where the ping/detonation comes from. the ignition is firing too soon for the rich charge.
to tighten the vacuum unit,a 1/8" allen wrench is stuck in the vacuum nipple,inside is an adjuster,you will feel the wrench engage it. turn the wrench counter-clockwise to increase the vacuum needed to advance the ignition.
i set mine so the unit begins advancing at the vacuum level the power valve opens,or very close to that number. meaning,when i get into the throttle,my vacuum level drops as the butterflies open on the carb,gradually backing the timing off in my advance unit,until the vacuum dips low enough to open my powervalve,at which point my vacuum unit is backed out completely. i run a 7.5-8.5 valve in my holleys,that is the vacuum at which my vacuum advance begins moving,or,advancing the ignition.
you will find when you get the advance dialed in,that it will be fully advanced at around 15" vacuum,a bit high,but with a mild cam,and properly set up carb,you'll be running strong vacuum at cruising speed. also. also,these units give a lot of advance. most are 12 degree units.thats in distributor degrees,so it's 24 crank degrees.a lot. even if you vacuum wasn't strong enough to be fully advanced at cruising speed,you'll still be gettin a lot of advance from the unit. i believe it's more important to have the unit backing off the advance at the correct time.
you should now be able to run 12~15 degrees initial advance,and i recommend manifold vacuum to the unit. you will have a smoother idle,and much crisper throttle response. and ping should be gone now!
onto the mechanicle advance. this step is not near as critical as the vacuum unit adjustment.but still worth doing. the engine speed advance. fires the plugs sooner as rpm increases,regardless of charge strength. after about 2500-3000 rpm,air turbulence is strong enough that additional advance is not needed. from the factory there is too much available,but the springs are stiff enough that it usually isn't a problem. but here goes.
remove the vacuum advance unit from the distributor body.remove the rotor,pull the felt from the top of the shaft,under the rotor.
there is a wire clip,grab it with a small needle nose pliers,and pull it out and up to release. you may need to do this with both tangs.
next,the reluctor wheel. it has a roll pin locating it to the shaft,mark the hole on the relucter the pin is in,then gently pry it up and off with 2 screwdrivers.
pull the 2 screws holding the plate in the body.they can be stuck,heat the body underneath where the screws protrude to help.
and pull the plate. you'll need to rotate it a bit to clear the positioning tab for the distributor cap.
heres the mechanicle advance parts. note the slots,one longer than the other,that limit the amount of advance that is available,and the tab that is used in the slot to limit advance. you want the smaller slot,it'll be on the big slot from the factory.
so,release the springs,carefully. you may need to bend the ends out a bit to get them over the pegs. you don't want to distort the springs.
once released,romeve the upper shaft,clean and oil the spindle part of the lower shaft.
and put the upper back on,rotated 180 degrees,to be on the smaller slot.
light lube here...
put the plate back in the body,align the screw holes. anti-sieze the screws,install,and put the reluctor back on the shaft,tap it into position,align the roll pin hole with the groove on the shaft,set the pin in the groove at the base of the hole and tap it in.
put the clip back in. drop it into position,hold one side down in place with a small screwdriver,and shove the other side down with the pliers,snapping it into place. put the felt in,and lightly soak with oil.
if you did this with the distributor installed,the rotor will be 180 degrees off now,so you will need to pull the distributor up out of the engine and rotate the rotor 180 dgrees to get your timing back in place.