Fine-tuning carb/timing - Pontiac GTO Forum
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post #1 of 5 (permalink) Old 12-09-2012, 03:05 PM Thread Starter
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Fine-tuning carb/timing

I did a Holley carb swap last summer, and me and a friend finally tuned it for my setup with the carb screws today. She now purrs at idle with no roughness like she used to, and I noticed a tad more power driving around.

Then, I half romped on it to make sure everything was fine even with a decent amount of throttle. There was a bit of a delay, my friend suggested that maybe the timing was retarded slightly for the new setup. No vacuum leaks either as far as I can tell, fixed those last month.

I haven't adjusted the timing since I did a full tune 2 or so years ago. Think this could be a cause?

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post #2 of 5 (permalink) Old 12-18-2012, 04:57 PM Thread Starter
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Checked timing, it was advanced 6 deg from when last set! I dialed it back to factory 6 BTDC.

The carb ran fine "out of the box" but I had a feeling it had more in it as far as tuning goes. Checked vacuum after timing set, only reads 14 psi (15 actual, +1100 DA).

Think I should advance timing just a bit or play with the idle mixture screws? Don't want to backtrack. Car needs more throttle than it should, especially above 40mph cruising. Highway speeds are practically limping along or push the go pedal down more..

For anyone thinking about swapping nonfactory carbs, not worth the PITA! I should have heeded geeteeoh and everyones advice. Live and learn..
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post #3 of 5 (permalink) Old 12-18-2012, 05:24 PM
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If you have a stock cam, you should be at about 18-20" vacuum at idle speed. Something is wrong. You could have a slipped or mis-marked balancer, and your initial timing may indeed be too retarded. You could take out number one plug, and turn the engine until #1 piston is at TDC. Verify that by pulling the dist cap and looking at the rotor...should be pointing at #1 plug terminal. Now, look at the balancer. Is it at 0? If it is, your timing is probably ok, but you may have a vacuum leak. If all else fails, try more initial advance, until you're at about 16-18" vacuum on the gauge. Then test drive. If it pings or cranks hard on start up, back it off some.
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post #4 of 5 (permalink) Old 12-19-2012, 09:45 AM
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14 inches is way too low for a factory cam. 14 inches is what my car has, and I'm running a moderately rowdy solid roller cam in a 461.

Prime suspect would be a vacuum leak somewhere, perhaps a PCV valve that's stuck open? You can sometime "mask" a leak at idle by finding an idle screw adjustment that seems to run smooth --- what's happening is that you've been able to "add" enough fuel at idle via the adjustment screws to compensate for the leak, but off idle the car's going to be lean everywhere else. That could show up as lost power, detonation, tendency to overheat, etc.

Best way to dial in your balancer marks exactly (other than doing it with the heads off and using a dial indicator) is to use a piston stop. You have to be careful though -- you MUST back off both rocker arms on that cylinder to "disable" the valves, otherwise you're likely to have a valve hit the piston stop and bend a valve. ---voice of experience on that one I'm embarrassed to say---


Springtown, TEXAS
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post #5 of 5 (permalink) Old 12-19-2012, 09:35 PM Thread Starter
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Found a vacuum leak! The line was torn pretty bad in 1 spot. Only gained about 0.4psi though when I hooked the gauge back up. Had a tad bit of trouble starting/idling nicely the first few seconds so I know the mixture was "masking" the leak a bit. Forgot to mention my timing mark moves just a little when I shine the timing light, blipping maybe 1 degree away from center. Signs of a slightly slipping balancer?

Will try what geeteeoh said tomorrow morning, may advance the timing if all else fails and go for a spin and see what happens. Thanks so much for the suggestions thus far. You're helping a guy who literally didn't know how to do more than change a tire 4 years ago actually work on my car, a REAL car, might I add.

Now just to get her running right already.
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