idles too high

[petro1143]

72 stock 455 ,stock rebuild kit from cliff.i set my initial timing to 10 degrees,mixture screws to highest idle/vacuum  ,, vacuum reads 18 and a little jittery. i back idle screw all the way out and idle wont go below 850,please help,it runs great just cant get idle down

[Cliff Ruggles]

Vacuum advance on manifold vacuum at idle?

Did you install bushings in the throttle body and index the throttle plates, check to make sure they are fully seated, etc?.....Cliff

[petro1143]

yes vauum on manifold straight out of base  plate in rear ,,, no i did not index the throtle plates. i did install your bushing kit and all the other parts from you ,,,,i will take off the carb and look the throttle plates,,,, tang for idle stops about 3/16 of an in from the casting if i back adjustment completely out i tried pushing the throttle back further but no difference,,also mixture screws work perfectly,both are out 2 3/4 turns runs great...no bog ...no hesitation

[Cliff Ruggles]

If ported is available I'd use that as a source to the advance instead......Cliff

[petro1143]

i took carb apart and got the air valves to seat better.got the idle down to 590 rpm,then when i hook up manifold vaacum it goes up to about 800 rpm i will try the ported vauum thanks cliff,all your parts worked great

[Cliff Ruggles]

Keep in mind that adding timing at idle is not needed for many applications.  In addition, a good ported source does EXACTLY the same thing as manifold vacuum right off idle, so fuel economy and engine efficieny are the same.

There are hundreds of pages of reading on this subject, some of it is very miss-leading as to how the vacuum advance works, and how it should be used.

My book describes it in detail.  There are "experts" out there that continue to advocate using MVA only, and that if you don't you are NOT doing it correctly.

The truth is that there are engine combinations out there that don't need that much timing at idle, and one quickly finds out that you can't get them to idle down and have control with the mixture screws running a lot of timing on them.  The educated tuner knows that they do NOT want that much timing.  Using a good ported source we can still give them the timing they need at light load (cruising) for best efficiency......Cliff

[petro1143]

thanks cliff i have your book,,is the vacuum port on the base plate between mixture screws a ported vacuum?th only other vacuum port is up high and goes to transmission

[Cliff Ruggles]

Test it with a vacuum gauge to make sure.  It should have no reading at idle, and FULL vacuum right off idle, or it isn't designed for distributor vacuum advance.....Cliff