Part idle bucking and surging - holley 750 - Team Camaro Tech
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post #1 of 11 (permalink) Old Jun 25th, 10, 07:01 AM Thread Starter
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Part idle bucking and surging - holley 750

My 67 recently started to have an issue with poor idle and a bucking and surging condition under part throttle. If I lean into it a little bit it accelerates fine and smooths out - likewise at highway cruising speeds - 3000 to 3500 rpm. This seems to get worse when the engine heats up, although it is not overheating. Engine is a 350 with stock 041 casting heads, Comp Cams 280* cam, Performer-RPM and 750 vac sec Holley. Ignition is stock with the exception of a Pertonix igniter to eliminate the points. I am thinking that I may have an issue with the idle circuit. Adjusting the idle mixture screws did not seem to cure the problem. Could the surging be an issue with the vacuum advance? Looking for insight from the minds of TC!

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post #2 of 11 (permalink) Old Jun 25th, 10, 07:18 AM
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Re: Part idle bucking and surging - holley 750

Trash in the carb?
Fuel filter?

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post #3 of 11 (permalink) Old Jun 25th, 10, 07:35 AM Thread Starter
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Re: Part idle bucking and surging - holley 750

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Originally Posted by SPARKY69 View Post
Trash in the carb?
Fuel filter?
It's possible that there is some crud that is getting into the idle circuit. I would think a clogged filter would manifest itself more in high speed and full throttle leaning out, since not enough fuel would be reaching the carb. I was also suspicious that the vacuum advance could be causing the surging by not operating smoothly, but it is probably a carb issue. I might pull the vacuum line and see if that makes a difference.

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post #4 of 11 (permalink) Old Jun 25th, 10, 08:47 AM
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Re: Part idle bucking and surging - holley 750

Might try lowering the fuel level one turn.

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post #5 of 11 (permalink) Old Jun 25th, 10, 09:00 AM Thread Starter
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Re: Part idle bucking and surging - holley 750

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Originally Posted by Everett#2390 View Post
Might try lowering the fuel level one turn.
Everett, are you referring to the float level?

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post #6 of 11 (permalink) Old Jun 25th, 10, 10:16 AM
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Re: Part idle bucking and surging - holley 750

Yes,

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post #7 of 11 (permalink) Old Jun 25th, 10, 08:23 PM Thread Starter
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Re: Part idle bucking and surging - holley 750

I drove the car to a cruise tonight, and it is definitely a lean idle condition. It will stall when in the idle circuit even if the idle speed is cranked up to 1800 RPM or so. When I give it gas, the accelerator pump gives a shot of gas and it instantly smooths out. Idle mixture screws have no effect. It runs fine at higher RPM's - at 3000 RPM on the interstate, it ran great all the way home. As soon as I got off the exit, it wanted to die. What do you guys think? Blown power valve? I always thought you got backfiring through the carb when that happens. No symptoms like that at all. It just won't idle or run smoothly at lower RPM's. I guess I'll pull it apart this weekend and put a kit in it. Is there a procedure to test the power valve?

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post #8 of 11 (permalink) Old Jul 9th, 10, 12:26 PM Thread Starter
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Re: Part idle bucking and surging - holley 750

Quote:
Originally Posted by SPARKY69 View Post
Trash in the carb?
Fuel filter?
Ha ha, WHAT fuel filter? The knucklehead that put the car together neglected to install one - it is just piped straight through to the carb! I installed one, and cleaned the carb and put a kit in it. Now it idles great, more responsive, but it is falling on its face above 4500 RPM. I'm going to go back and double-check the float levels tonight, then if that doesn't help, start looking at ignition issues.

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post #9 of 11 (permalink) Old Aug 2nd, 10, 09:35 PM
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Re: Part idle bucking and surging - holley 750

Sounds like your primary butterflies are exposing too much of the idle transfer slot. If you flip the carb over you'll notice two slots under the primary butterflies on the forward portion of the venturi's. These are the idle tranfer slots and the primary butterflies should expose no more than .040" of these to the intake manifold. If you can close the idle mixture screws all the way and the engine does NOT die, then the idle transfer slots are probably exposed too much. Some engines require more air than this normal setting, so some guys drill a small hole in the primary butterfly to allow more air to bypass. I would suggest opening the secondaries a 1/4 to 1/2 turn more that where you are at now. This will alllow your engine sufficient air to idle while still making the idle adjustment screws effective.
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post #10 of 11 (permalink) Old Aug 3rd, 10, 05:01 AM Thread Starter
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Re: Part idle bucking and surging - holley 750

Thanks for the suggestions, Kelly. I adjusted the secondary float setting, and now it seems to be running much better at WOT. It starts right up and idles well too. There is still some tuning to be done, but right now I need to focus my energy on getting my boat back on the water.

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post #11 of 11 (permalink) Old Aug 6th, 10, 06:00 PM
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Re: Part idle bucking and surging - holley 750

280 Cam - What's the Vacuum at Idle? The Power Valve and Vacuum Advance Pod should be 1.5 Hgs lower then Vacuum at Idle; other words' you will probably never get the throttle plates below the transfer slots and the advance will be haunting.

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