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Old 06-17-2004, 04:23 PM
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350 sudden rough low idle

I have a Suburban with a new (5k miles) 350 pre-87 crate motor, a edelbrock intake and carb (1405 I think) and performer (?) distributor. EGR and PVC are hooked up.

Last week it suddenly went into a very low rough idle, maybe 400rpm and stalling , and occasionally the power brakes stop working (extra effort to stop, not fun when you don't expect it).

This had occured 2-3 times previously but only for a few minutes at most now its continuosly rough.

What should I start checking?

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Old 06-17-2004, 04:28 PM
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Perhaps the float is sticking , maybe some dirt or something in the needle and seat area.
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Old 06-17-2004, 04:50 PM
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I have nothing but grief from this carb. It was completely stripped down and cleaned when I put the new motor in.

I forgot to mention that this all started after I did a "blast" down the road maybe 4000 rpm. The reason I mention this is I have really baby-ed the motor. We were paying 2.05 a gallon back when everyone else was shelling out 1.50!

I have raised the idle to 800 just to be able to drive. The truck seems OK over 1000 rpm.

Is it possible to have a low / rough idle due to a vacuum leak, torn diaphragm ? Will dirt or a stuck float cause the power brakes to loss vacuum, maybe a sudden dip in rpm?

I'm really afraid to drive this beast, don't want to ruin someone elses day or rear bumper :-)
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Old 06-17-2004, 05:41 PM
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Have a good look around the engine compartment for a hose blown off or a crack in one. Perhaps just a vacuum advance hose got pulled off during your little blast.
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Old 06-17-2004, 05:53 PM
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Also sounds like you may have a plugged idle circuit in the carb, on one side or the other. Been there, done it before.

You can try screwing in each mix screw, counting the turns and make a note of how many. Then remove each screw and shoot some carb cleaner in each hole in the base of the carb. Then re-install each screw, matching the number of turns back out, from seated.

Then start it up, wind it up in park to around 1500 rpm and then slam the choke shut hard! That will usually suck out any little piece of crap that may be blocking an idle passage.

This has worked many times for me too, providing its NOT a Holley carb with an un-protected power valve.
Shut off the engine and switch any 2 plug wires. Fire it up and punch it a few times. Hopefully you can get a good backfire out the carb. Blown out many a crap filled idle circuit that way.
Put the wires back and see how it runs.
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Old 06-17-2004, 06:37 PM
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Sounds exciting :-)

I'll check for leaks to start then try cleaning out the card. OR any recommendations on a better carb ?

Just curious would a bad lifter cause those symptoms or vacuum leak from the power breaks, egr or pvc valve?
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Old 06-17-2004, 06:55 PM
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A bad brake booster can cause that problem. Usually the pedal is rock hard and often you will hear a big vacuum sucking sound from under the dash when you step on the brake. Sometimes the booster is mounted on a short bracket and the sound is not noticable inside.

If the engine idles fine until you step on the brakes, i'd suspect the booster. A pcv valve usually won't cause that problem, unless the guts fall out and it starts sucking huge amounts of air.

A stuck open EGR valve will for sure cause that problem too. You can usually ck it by hooking up a vac hose and sucking on it. If the engine dies or idles real bad with vacuum applied, the valve is ok. If you apply vacuum and nothing changes ( and it still is idling bad), its probably suck open. You can usually put one finger under the EGR valve and feel the diaphram move when you apply vacuum.
Mark
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Old 06-17-2004, 08:05 PM
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My guess is the EGR valve sticking open also.


tom
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Old 06-17-2004, 08:37 PM
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I had something similar happen to my caprice. It was a large vacuum leak, in my case it was at line behind the carb running to the intake.
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Old 06-19-2004, 06:52 PM
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Just wanted to thank everyone and let you all know it was the EGR. After removing / plugging the vacuum line and resetting the idle everything smoothed out.

Plug it back in and everything goes to heck :-)
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