73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: VeeTwin60 on December 30, 2009, 03:03:15 pm
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350/TH350
Holley 1850C/S 600cfm Carb w/Holley Street Performer Intake Manifold
Pretty much stock everywhere else (no emissions hooked up)
Conditions: driving at normal road speeds (35mph +), when I let off the gas to coast sometimes the engine will start "idling" at a low RPM. Then when I depress the gas (normal, not hard) again it stumbles for a second and goes.
I have rebuilt the carburetor and it helped other issues, but not this one. I also am only getting about 8-9mpg... could this be related?
Ignition/Distributor? Carburetor/Intake?
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My guesses:
Ignition/Distributor? Vacuum advance wire could be worn or broke, check this under the dist. cap from your vacuum advance module toward the center of the dist.
Carburetor/Intake? Accelerator pump located somewhere on the carb (I'm not too familiar with holleys).
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Sounds like fuel to me... Go over your carb again.
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Any specifics on what to look for in the carb?
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Do have another carb laying around you could swap for testing purposes?
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Unfortunately, no.
Could this be a power valve? I've heard that those can be a real pain in the keester on Holley carbs.
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Depending on the year of the carb it may have power valve backfire protection already. Does it restart hard when it's warm?
From Holley's Website:
If you have a carburetor older than 1992 (or you have experienced
an extreme backfire) and expect a blown power valve,
use this simple test. TEST: At idle turn your idle mixture screws
(found on the side of the metering block) all the way in. If your
engine dies the power valve is not blown.
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Does your engine have an egr valve? If so unplug the vacuum line to it, plug the hose and test drive
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Does your engine have an egr valve? If so unplug the vacuum line to it, plug the hose and test drive
Yes. I'll check that out too.
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I was tinkering today, and realized that you said EGR valve... I was thinking PCV valve when you said that for some reason. No - my engine doesn't have an EGR valve.
However, that got me to looking around.
I found that my PCV valve is connected to vacuum, instead of venting to atmosphere or to the intake. Could this be part of the issue? I've disconnected that, but haven't test driven it yet.
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you can hook the pcv valve back up to vacuum, that's where it's supposed to go. (either on the intake itself or on the carb)
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does it do this at idle also or just moving, check to see if the pcv valve came loose like 4x4 said around your carb or intake and now you have a vacuum leak
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It only does it while moving.
Cold starts, warm starts, idling is all fine. Drive-ability is good most of the time - except for when this happens. And it isn't a real huge problem... the engine hesitates for a second and then all is normal again.