Hanging RPMs - Team Camaro Tech
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post #1 of 6 (permalink) Old Feb 19th, 07, 09:47 PM Thread Starter
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Hanging RPMs

Hey guys,

This is sort of a revisit to an issue I had earlier.

69 Z, everything stock with 140 cam. Before, I had an issue with erradic timing/idle that was chalked up to a tweaked advance spring. Fixing that got it consistent.

Now is another issue that I think is similar in the cause, but not the same symptoms.

When cold, its fine. Once it warms up, if you bring the RPMs up, it slowly comes back down to idle. Really slowly.

Ive ruled out carb/linkage. No binding, and when you put some load onto the clutch, it brings the idle down and stays there once the clutch is released.

Next thought was that the vac advance was sticking. Disconnected/plugged the vac advance at the carb, no difference.

I pulled the distributor the other day. Everything is moving freely without any dragging or binding. The end play is not too tight nor too excessive, and I dont think that would cause this problem. The first time I went through the distributor, I set the endplay to Jerry M's specs. After some running time, the endplay had zero slack, so I used a thinner shim.

I'm kind of thinking advance springs again, because it seems like the advance is "hanging". But the springs in it now are out of my 70Z, and it never had this issue.

Anyway, just thought I would throw this out to see if anyone had any thoughts/suggestions, because Ive never run into this issue before.

Thanks

Driving a #s-matching Z in the rain..... and lovin' it....
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post #2 of 6 (permalink) Old Feb 19th, 07, 10:41 PM
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Re: Hanging RPMs

I think you're on the right track with the centrifugal advance. You might try checking with a timing light while the engine is revving down as you described. If the timing doesn't retard all the way to the initial setting until the rpm's come all the way down, you've found the problem. This can be a subtle difference from normal. The fact that the idle comes down and stays down after applying a load with the clutch is a clue.

Another thing that comes to mind is that the throttle can work fine when the engine is not running, but can be sticky due to the air (vacuum) load on the throttle plates while running. If this was the case though, it probably wouldn't respond to a little clutch load.
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post #3 of 6 (permalink) Old Feb 19th, 07, 10:50 PM Thread Starter
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Re: Hanging RPMs

Quote:
Originally Posted by red67camaro View Post
Another thing that comes to mind is that the throttle can work fine when the engine is not running, but can be sticky due to the air (vacuum) load on the throttle plates while running. If this was the case though, it probably wouldn't respond to a little clutch load.
Appreciate the input.

To discuss your point, the carb ran perfect on an identically setup Z (all stock, same cam). It pulls basically no vacuum at idle with the cam, and it only has this issue once its up to temp. Before it reaches temp (ie cold), it will rev and come back to idle normally.

Driving a #s-matching Z in the rain..... and lovin' it....
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post #4 of 6 (permalink) Old Feb 19th, 07, 11:27 PM
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Re: Hanging RPMs

Your carb is probably just fine, particularly if the throttle feel doesn't change with the engine off or running. It came to mind because it's something that can be very temperature sensitive.

I think the timing thing is a better place to look. What may be happening is the timing is affecting how the engine runs to a greater degree when warm if that makes sense. Another thing to try may be to retard the base timing a few degrees and see what effect that has on the idling down problem.
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post #5 of 6 (permalink) Old Feb 20th, 07, 12:02 AM Thread Starter
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Re: Hanging RPMs

Once again, appreciate the input very much.

The total timing is at 35-36*, with stock 480 weights, plate, vac adv, etc., so I havent really wanted to mess with that too much.

I'm sure its something simple that I'm overlooking, its just that I havent encountered this issue before.

Aside from the "significant lack of streetability" that goes along with this engine combo (along with this issue), the thing runs great. Sucks gas like no tomorrow and can barely get out of its own way from the stop sign, but really comes alive at 5K or so...

Driving a #s-matching Z in the rain..... and lovin' it....
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post #6 of 6 (permalink) Old Feb 20th, 07, 04:50 AM
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Re: Hanging RPMs

Vacuum leak, I wager. Correct PCV?

Have you thought of taking a carb lift plate, loosen the rocker arms, and plumb shop air, 10-15 PSI, into the intake, then Snoop the intake/gaskets?
Also feel the oil filler cap for air pressure.

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