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Discussion Starter · #1 ·
Hello,
Hey Swheaton If your there need your and the other gurus on this. Posted a topic and made the changes to the carb and timing. Timing is @ 16* and in by 2800 @ 36*. I replaced my p/v from 6.5 to 4.5 because of vacuum @7-8. Now I have something going on I can't figure out. When I start the car and at idle all of a sudden the idle will rise fast and will not return back to normal, and If I adj. the idle screw to lower it will take awhile then go to low. This started after the p/v replacement. What would cause this. Thanks for your help.

Scott :confused::mad:
 

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4 things come to mind,one is worng vac adv if your running one for the 7-8 " idle vac causing an unstable idle.

Is that 8" idle vac with autotrans in gear/ If your running auto trans and it want in gear for te idle vac reading it will be ower then 7-8" in gear just so your aware.

And the second is that the mech adv springs could be a tad too light allowing the mech adv to partially activate at idle or just above idle by like only 50-100 rpm or so also making the idle unstabel and wandering at times.

But my money is on the vac adv if your running one so with that said what type dist are you running?

For example if the current vac adv your running doesnt start to activate untill maybe 6"-7" of vac and isnt fully activated untill 10-11" vacuum it will be searching up/down/iup back and forth becasue there is bearly enough vacuum at idle to partially activate it but enough vacuum to keep it fully activated to keep the timing stable thus the idle stable.. So in that case the vac adv is searching up/down becasue its never fuly activetd which in turn add's & takes timing from the motor back and forth making the idle search up/down resulting in an unstable idle.

You need a VC1810 fvac adv or stock type gm points dist or a VC1862 vac adv for gm hei both being Napa/echlin pt#'s.

If you dont have a napa store in your area you can either order them online from napa like i do or you can have your local parts store cross ref them to their brand and hope they get it right.

If you arent running a vac adv then you likely need to run a little stiffer spring/s on the mech avd but not so tight that is keeps the total from fully comming in past 3k rpm.

If installing the correct vac avd for your idle vacum doesnt fix it then the 3rd thing is that the mech adv & wieghts could be hanging up at times. That can also cause an unstable idle that will do what you describe so check that out too and clean/lighly lube the mech adv mechanism and weights but use the lube very sparingly.

If the mech adv springs are too light it will basically do the same thing the wrong vac adv will do accept intake vac doesnt affect the mech wieght springs,its engine rpm/dist shaft rpm along with the strength of the mech adv wieght springs that affect the mech adv & when it activates.

4th is very lean idle mixture and or vacuum leak so did you readjust the idle mix after advancing the base timing and relacing the p/valve which by the way doent affect idle at all. The p/valve can only affect the carb mixture when the prim throttle plates are out of the idle circut and the thorttle is opened far enough that the intake vacuum is lower then the p/valve rating,then it opens up to enrich the fuel calibration when the motor is under a higher load requiring more fuel.

Scott
 

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Discussion Starter · #3 ·
Thanks Scott,
The dist. is a mallory hei with a adj. vac. adv. The reading is just running not in gear. The weird thing is it was fine until I put in the p/v. Could something be in the air bleeds or something? Could I somehow adj. the vac. adv. that I have? Yes I adj. mixtures with the vac. gage.

Scott
 

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The only was i can see that changing the p/valve affetced the idle is if the prim throttle blades are almost or fully out of the idle circut Thats because when the thottle baldes are full in the idle circur the p/vlave should not be activated.

Maybe you now have a lrg vacuum leak after going into carb to change the p/valve.

Has the vac line for the modualter valave in your auto trans come off at the trans or the line for power brakes is loose creating a lrg vacuum leak?

Strange how this happended post p/valve change,you definately went leaner in a sence going from a 6.5 to 4.5 p/valve because it should not activate/open to enrich the fuel calibration when out of the idle circut untill your intake vacuum is below 4.5 ".

You need to remove the carb to see if the transition slots for prim blades look like a square shape which is normal,are a rectangle shape showig blades are partially out of idle circut,and or the transition slots are ful exposed shoiwn the prim blade are fully past/out of the idle circuts in which case the p/valve can affect it when the intake vacuum is low enough to activate it.

Scott
 
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