Idle and timing - Pontiac GTO Forum
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post #1 of 7 (permalink) Old 10-19-2015, 07:08 PM Thread Starter
 
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Idle and timing

Well here we go again.... Got the motor in and the new Vintage Air installed. Even got a new dash bezel and rally gauges...happy, happy.... I can't get the engine to run right. Timing, I think is better set a little advanced...Have a tri power.... with a mild cam.... I connect the vac advance and the idle goes up significantly... I believe it's not suppose to do that.. I've been playing around with the idle mixture and rpm...and the timing... propably have it all screwed up now...seems to run OK? but for the increased idle plugging in the vac advance.... also it runs hotter now that I've messed with the whole thing again....frustration....I've read some other posts and come to the conclusion that I may have a bad Vac advance....not suppose to advance at idle...RIGHT? concerned also about total advance...
thinking of replacing the vac advance and starting over...with a little more initial advance setting...with the Crane adjustable advance set fit my application.

PONTIAC GTO Crane Vacuum Advance Kits 99600-1 - Free Shipping on Orders Over $99 at Summit Racing
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post #2 of 7 (permalink) Old 10-20-2015, 06:45 PM
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It IS supposed to advance at idle. Set the base timing at 6 degrees with the vacuum line disconnected (if a 389) or maybe a little more....say up to about 10 degrees if it doesn't ping when you're all done. After setting the timing, re-connect the vacuum hose. Then, adjust your idle speed to 500-800 rpm. THEN, adjust the center carb idle mixture screws. This is done by turning them clockwise until the engine starts to run rough, and then backing them out just enough to smooth out the engine. Re-check the idle speed and you are done. ALL Pontiacs of this era had manifold vacuum to the advance, which means there is advance at idle.
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post #3 of 7 (permalink) Old 10-28-2015, 07:35 PM Thread Starter
 
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thanks... I'll give it another try.... The engine just won't relax, no matter what I do.... I have the initial timing at 8 degrees now and the total advance at 2500 rpm is around 22 degrees. Shouldn't it be closer to 36 degrees? Also, after about 20 minutes messing with it the temp is at about 210 or so... whereas before I starting changing things around to get to run better...it was staying at 180..should of left things alone.. LOL.. I'm wondering if I might have a small vacuum leak preventing me from getting it to smooth out. Don't know where it would be though.. Any ideas on how to get to the carb adjustment screws? Having trouble adjusting the thing with a long screwdriver... can't seem to get to the driver's side very well.. the vac nut/line for the power steering is right there... I have two, ported vac above the plate on the center carb. one goes to the automatic chock and the other to the distributor and has a tee in it to branch off to the transmission. The large vac line out of the manifold has got the tall tee hose thing and goes directly to the PVC valve... does all this sound right? I believe I have a good tri power...It was running pretty good before I took it off to have the motor rebuilt... Tri power by Mike Wasson... It was sitting on the shelf for almost two years though...maybe issues with that? I know there's a lot here to talk about...sorry for the long message but getting pretty frustrated with the it...
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post #4 of 7 (permalink) Old 10-28-2015, 09:30 PM
 
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Earl, gto guy has it right, let's build on his discussion.

There are as you know 3 parts of timing.

Base, you set by hand, 2 Centrifigal which is the springs and weights and solely dependent on RPM's

And vacumn advance.....which depends on throttle demand i.e., vacumn.

So figure what you have. Set the base, disconnect the vac can and plug to engine. Put a hand vacumn pump on the dizzy, watch the timing light and see how far the vacumn will advance the timing..if it stalls you may have to increase RPM's just to see the vac total....record the highest timing...

Now engine off remove dist cap take one spring off one weight, don't lose it. Leave vac can vacumn disconnected and plugged. Now with base at 8 or ten and timing light hooked up....run up the RPM's until timing will advance no more.
Record that number

Now you have all three critical numbers, base, vac and Centrifigal.

Base and Centrifigal should be 36 BTDC maybe 34 per 38. This is critical.

Vacumn must then be adjusted to give the difference to 52 ideally.

Use the adjustable vac can to adjust (and remmeber to adjust to crank RPM and not Dizzy RPM's....)

This test will show you what you have and what is working.....start there and adjust...10 base, 16 vac, 26 Centrifigal ......36 total (base & Cent) 16 more crank will give you 52 at light throttle cruise....is great but it can vary a little but the 36 to 38 total is most critical...

Good luck!
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post #5 of 7 (permalink) Old 10-28-2015, 09:33 PM
 
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PSdon't forget to put the dizzy spring back on after the test....

With one spring Off the weights will go all the way out fast and you won't have to rev to high RPM to see the total advance....just until advance on timing light stops advancing...
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post #6 of 7 (permalink) Old 10-30-2015, 02:58 PM Thread Starter
 
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Thanks so much for the instructions....I'll do my best to follow them..... can you explain..(and remember to adjust to crank RPM and not Dizzy RPM's....) what is the difference and how do I get it...
Also I have a dial back timing light....I'm guessing that will help in the process...
I have Pentronics ignition...the weights are under the plate...right?
Understand that the tri power...it is critical to make sure I'm not pulling vacuum from the outside carbs during this process...Thanks again for helping me...
.
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post #7 of 7 (permalink) Old 10-31-2015, 12:34 PM
 
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earl, If you see a reference to Distributor RPM's.....

You have to double that for RPM's at the Crank for timing. Because the distributor gear from the cam is smaller.....Just make sure you know which one you are talking about....If a vacumn canister says "8 degrees distributor"..

That is 16 degrees at crank for timing......

Yes dial back light is perfect and you can do it easily with that....

Petronix modules or Crane modules should not effect the weights....the weights are visible under the rotor in these distributors ...

unless you have some electronic distributor from Petronix...don't know if they have those...

Vac source is from two places Full manifold Vacumn.....or Ported Vacumn

full manifold pulls at idle, should be strong ......

ported will not pull vac at idle as the opening is on top of the throttle plate..

you can try it with both...I like full manifold...I think it runs much smoother and cooler

Take the vac from full manifold should not effect the carb unless you take it directly off from where it should be on the carb...find another spot if you can...
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

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