# Inconsistent idling and Tach Flucuating



## timmyg (Jan 2, 2012)

:confusedWhat I have in my 1965 GTO is a 1970 455 HO Engine: #10 McKellar Solid Lift Camshaft – 1.65 Ratio Rocker Arms– HO Round Port Heads –Pontiac Super Duty Rods – Forged Pistons – 1966 Tri Power Set up. 3.73 Posit Traction with Turbo 400 Trans. Stock Converter. I have installed a new Points Ignition Distributor with vacuum advance hooked up. I was advised per this site to leave points in and not change over to HEI. Ignition Timing is set at 9 degrees per specification for 70 455 HO. I was thinking of setting timing at a total advance number, but not sure what total timing should be and at what RPM to set it at. 
The holes for the linkage arm between throttle shaft and pump actuator are worn on the center carburetor. The idle is very inconsistent whereas one time the car idles at 1000 rpm’s and the next time it may be 1200 or 800. I have adjusted the idle numerous times and it will not stay consistent. I have continued to turn the idle adjustment screw in to raise the RPM’s at idle but the next time I drive the car it will be different. I do not have Power Brakes or any vacuum operated devices with the exception of the Choke Pull-Off, the Distributor Vacuum Advance and the Transmission Modulator. I can have the car idling well and then drive the car somewhere and when I slow down at a Traffic Light or stop sign, I have to nurse the accelerator to keep the car running as it wants to cut off from idling so slow. Not from a loping Cam issue, just slow idle.
Also, while driving the car, you see the Tachometer fluctuating 3 to 4 hundred RPM’s constantly and it is real because you feel the car subtly jerking as it is doing this. I believe it is due to Fuel Flow to the engine through the carburetor. That is my guess. Any help or guidance is appreciated. Not sure what to do next or how to pursue problem


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## geeteeohguy (Feb 2, 2008)

I would check the following: 1. timing at a steady cruise (2000-2500 rpm) with a timing light to see if the timing is jumping around. 2. Vacuum leaks around the carb bases. Note that '66 TP intakes like to crack at the center carb inlets and this can cause problems. 3. Check your float levels. Did you adjust the carburetor mixture properly in the center carb? Does it do surge just at cruise or at WOT, too? Surging at cruise can be distributor advance, ignition, or fuel...check it out. Also, I'll throw this in: is your condensor tight? A loose condensor in the dizzy can cause surgining/dying out, too.


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## Rukee (Feb 8, 2007)

If you do total advance you want it at about 36* @ 2500 rpm.
Check the center carb's throttle shaft for excess play.


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## jmt455 (Mar 26, 2012)

It could be fuel, timing or a combination of both.

I would start by disconnecting the vacuum advance (plug the vacuum source) and drive the car as you typically use it. 

*If the idle is stable with no vacuum advance*, verify that the base timing is stable at idle speed. If both the idle and base timing are stable without vacuum advance, then you'll probably need to replace the vacuum advance canister with one that is better suited to your engine/cam. If you are using full manifold vacuum to operate the advance unit, you should select a can that is engaged at a vacuum level approximately 2" lower than your idle vacuum level, so the timing remains stable at idle.

If you are using a ported vacuum supply for the advance, there should be no timing change at idle when you connect the advance mechanism. If the idle and/or timing changes when you connect the vacuum advance to a ported source, your throttle plate on the center carb is open too far (past the idle slots) at idle and you're not actually running on the idle circuit. If that's the case, you have some carb work to do so the engine can idle with the throttle plates in the proper position.

*If the idle is still wavering without the vacuum advance*, you need to determine the cause. Could be a vacuum leak, incorrect weights/springs in the distributor, sticking advance plate in the distributor or a sloppy throttle shaft or air leak on any or all of the carbs.

Search this site and PY forums for help with determining optimum ignition timing; there's a LOT of information out there.


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## timmyg (Jan 2, 2012)

When the vacuum advance is disconnected and the engine timing is set to the initial 9 degrees as stated in the Specs and I hook up the Vacuum Advance which I have coming off the center carb from the rear port on the carb., the Timing jumps way up and the engine speed increases and engine smooths out. Is this not the typical scenario? I dont claim to be an expert mechanic by no means but every car I have ever worked on responds that way when you hook up the vacuum advance. They say you never quit learning so maybe I am getting some good info. that I need.Please elaborate on this whole timing advance scenario. I really appreciate the help.


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## Rukee (Feb 8, 2007)

Check this thread.
http://www.gtoforum.com/f50/gto-tuning-setup-tips-13052/


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## jmt455 (Mar 26, 2012)

Rukee's link is excellent!

Yes, the idle will speed up when you connect the vacuum advance to manifold vacuum source BUT the timing should be rock-solid stable when the vacuum advance is not connected. If the timing is jumping around when the vacuum advance is disconnected (and the vacuum source is plugged), you need to look at the mechanical advance system to make sure it is working properly.

Are the springs strong enough to maintain no mechanical advance at idle?
Does the advance plate move smoothly and ALWAYS return to the base timing position when the engine drops to idle speed?

Also, I don't know whether the 455HO timing spec is appropriate for your engine.

Total timing and advance curve should be established by what the engine needs for best performance. Then, you set the initial timing where it needs to be, based on the distributor advance curve you are using.


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## timmyg (Jan 2, 2012)

The timing is Solid at idle when vacuum advance is disconnected. When engine is revved up, the timing does always return back to where it was.


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