Need help with carb problem - Team Camaro Tech
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post #1 of 9 (permalink) Old Dec 27th, 04, 06:04 AM Thread Starter
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Gary
 
Join Date: Sep 2004
Location: Papillion, Ne.
Posts: 16
Question

My engine will run just fine at cruising speed for a few miles and when I stop, the idle jumps way down to 500 and wants to stall. On initial start up it idles just fine and if I cruise around the neighborhood at 30 to 35 mph, it will still idle fine. Here are the specifics:

396 bored .030
Comp Cams 280 Magnum muscle solid cam
Edelbrock Ram Air manifold
MSD billet distributor and MSD 6al ignition
Holley Street Avenger 770 Vacuum Secondary
70 Primary jets, 75 Secondary jets, 65 Power valve
Timing 14 deg inital 36 total
Yellow spring in the secondary
Floats checked and below the sight glass
Vacuum reads 12 - 14 at idle
Idle at about 900 RPM

I've tried 72 primary jets (too rich) and 68 jets (too lean). It stumbled off idle and at cruise speed increase until I changed up to size 70 and the yellow spring in secondary. I also bumped up the timing from 12 to 14 initial and now the stumble is gone and the throttle is more responsive. If I cruise 5 to 6 miles at highway speeds and then stop, the idle wants to fall to 500 rpm and the engine wants to die. If I keep feathering it to keep it idleing, it finally comes back up to 900 and will idle without dying. I have NOT messed with the idle mixture screws yet. Am I flooding out or starving ? Is my power valve too low?

BTW....this is after a total engine rebuild.

1968 Chevelle SS396
396/375 hp L78
4-speed Muncie M21
3.73 12 Bolt
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post #2 of 9 (permalink) Old Dec 27th, 04, 06:50 AM
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Are you using vacuum advance?

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post #3 of 9 (permalink) Old Dec 27th, 04, 06:56 AM
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Al
 
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Is your electric choke hooked up? It almost sounds like at initial start-up your fast idle is coming on allowing it to idle fine, but after the choke heats up it kicks the fast idle off. If that's the case, it sounds like your curb idle is set too low.

But then again, it could be something entirely different.


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post #4 of 9 (permalink) Old Dec 27th, 04, 07:03 AM Thread Starter
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Gary
 
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Location: Papillion, Ne.
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Yes I am using vacuum advance hooked to manifold vacuum port on front of carb. The choke is electric and it will come off fast idle when it warms up. I'll try setting it down to kick off sooner because I have it set to full rich and maybe it is creeping back on?

1968 Chevelle SS396
396/375 hp L78
4-speed Muncie M21
3.73 12 Bolt
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post #5 of 9 (permalink) Old Dec 27th, 04, 07:11 AM
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What are the specifications of the vacuum advance?

"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
Semper Fi! L/Cpl Edwin L. "Tim" Craft, B Co 3rd AT's, Khe Sanh Combat Base, February, 1968
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post #6 of 9 (permalink) Old Dec 27th, 04, 02:54 PM Thread Starter
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Gary
 
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Location: Papillion, Ne.
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I believe the can on the MSD billit distributor adds 20 deg. I set total timing to 36 deg at 3000 rpm with the vacuum DISCONNECTED. Would messing with the idle mixture screws help? If I were to set them it said to do it with the air cleaner installed but that makes it almost impossible.

1968 Chevelle SS396
396/375 hp L78
4-speed Muncie M21
3.73 12 Bolt
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post #7 of 9 (permalink) Old Dec 28th, 04, 04:40 PM
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John
 
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Adjusting the idle mixture screws is essential; it probably idles fine at cold start because the choke is on, richening idle mixture. Connect a vacuum gauge to manifold vacuum and adjust the mixture screws evenly for highest steady vacuum (engine warm, choke off, normal idle rpm), reset curb idle as required. [img]graemlins/thumbsup.gif[/img]

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post #8 of 9 (permalink) Old Dec 29th, 04, 07:13 AM
 
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Ditto John^^^The vacuum gauge is your best friend. I usually go highest vacuum the back them both off 1/8 turn. Also set timing with advance hooked up to regular port before switching to full manifold vacuum port.
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post #9 of 9 (permalink) Old Dec 29th, 04, 03:18 PM
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Timing should be set with the vacuum advance hose disconnected from the can and plugged, to make sure there's no vacuum at all on the advance can. [img]graemlins/thumbsup.gif[/img]

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