be careful.
this method is not complete.
advancing the distributor will always increase vacuum because advancing timing automatically increases idle rpms.
here is what gets left out.
you are suppose to lower engine rpms just above stall at original initial timing.
so lets say original initial timing is 6 . you lower the engine rpm till just above stall, plug the vacuum advance, so you have no mechanical advance and no vacuum advance, take note of engine rpms you want smooth idle just above stall,, lets say this is 600 rpms.
so your starting at 6 with engine at 600 rpms.
hook up the vac gauge. take note of start HG reading.
now advance the distributor,, vac goes up, so does rpms. Readjust rpms back down to 600 take note of HG reading. repeat until highest reading. take note of timing, now you might be at 10-18 degrees now you have to think does that make sense for a street car, because you have tuned for max engine vacuum and power but will that work in the entire power curve.
reset Idle rpms to normal and go for a drive leave the vacuum advance off,, most times she pings, now you have to go back and reduce initial until she stops pinging. but you might need to back off another 1-2 degrees on top of that to tune back in the vacuum advance.
before you know it your back at 6-8 degrees initial after messing around in the 12-18 range. then you have the issue of a hot start, or dieseling. if you doing all this on a 60 degree garage day, what will happen in the middle of july on a 90 degree day after a highway run, you shutdown and she keeps running for a couple of seconds, or you get some fast food and park and she will not start to much timing.
all these involved type of tuning, methods usually work well in a small tolerance range but for a daily drive cause problems and before you know it your back at stock timing and 1.5 turns out on the carb idle bleeds because you had all kinds of little issues, shes bouncing at stop lights, engine stalls under heavy braking.
you get this weird balance were you want to tune the motor to run as efficient and clean as possible by the book numbers but it just doesn't work on the road in all situations.
now another interesting thing and many times people don't know an engine is pinging like crazy, they go by the seat of there pants. i had a situation a buddy with a chevy he has had it for years. finally got a chance to ride in it.. first thing i said was omg its pinging like crazy. he had no clue. i had him back off the timing and listen to what the engine sounded like night and day, also solved his buzzing noise, hot start problem.
now the outline i gave for a vacuum tune is missing even more information and steps. you then get involved with the idle bleeds and having to tune them for specific idle rpm. you can seriously spend days doing a vacuum tune.
again just a warning, for the simplest method of hooking a vacuum gauge then just turning the distributor getting the highest reading. you are increasing idle rpms and usually advancing way too far. it will make the car seem more powerful. your increasing idle rpms for a launch and advancing timing faster then usual if you have a stock distributor spring set.
down side, your dangerously closer to detonation at higher rpms, and one day you gun it on the highway miss a shift on a manual and the engine grenades. for an automatic you are causing long term ping/detonation that you may not recognize until a piston top melts or cracks and falls into the crankcase.
And i started out at one of those people that had an engine that pinged everywhere and did not know it because it seriously pinged all the time at every rpm. what saved me, was i had so many other problems i never drove the car and i was using a very cold set of spark plugs, because of a cooling issue. it took me years to figure it out.
there used to be a very comprehensive vaccum tune video on you tube it was about 90 minutes long but it is no longer posted.
be safe have fun...
You've got it dialed in. If you want to take it a step further with your new vac gauge, try setting the timing by the vacuum. Leave the carb alone and advance the distributor to highest vac and take a reading with your light.
