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Old 07-16-2012, 02:41 PM
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suddenly runs really rough

Hey guys hopefully someone can help me, I have a 23 t bucket sbc 350 mild cam, vortech heads, performer intake, summit brand 4 barrel 600cfm carb, hei ignition, all parts new and seem to be in good working condition, I jumped in it yesterday to bring it to a car show, it was idling just fine in my driveway, then all of sudden rpms jumped to 1300 from 1000 and is running REALLY rough, shaking the whole car!!
Thanks Phil

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Old 07-16-2012, 03:46 PM
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Quote:
Originally Posted by 23 tbucket View Post
Hey guys hopefully someone can help me, I have a 23 t bucket sbc 350 mild cam, vortech heads, performer intake, summit brand 4 barrel 600cfm carb, hei ignition, all parts new and seem to be in good working condition, I jumped in it yesterday to bring it to a car show, it was idling just fine in my driveway, then all of sudden rpms jumped to 1300 from 1000 and is running REALLY rough, shaking the whole car!!
Thanks Phil
Does it run rough at the hit of the key now (after cooling it off), or does it still start off good and then go bad?

An increase in idle speed like that could be from a vacuum leak. Check the PCV hose, PB booster if you use one, any/all vacuum accessories and hoses.

If the distributor worked loose, it might have advanced although you'd expect it to retard more than advance- but anything's possible.

Lastly, the fuel filter might be clogged causing a lean condition or the gas tank may be too low, not enough fuel pressure.
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Old 07-16-2012, 07:48 PM
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Good points Mark, too me it sounds like the choke if so equipped is being enabled, is this the electric choke version? how is it energized?
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Old 07-17-2012, 01:07 PM
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Its rough right from the start, and it seems to have gotten worse now it will idle up and down on its own from 900 to 1300, I tried the simplest things first a new fuel filter, drained the old gas and added half a tank of preium, checked the timing none of those seem to make a difference.
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Old 07-17-2012, 01:09 PM
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yes it is an electric choke, with 12volt key power
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Old 07-17-2012, 01:35 PM
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Sounds like a vacuum leak. Check for a cracked hose or one that may have blown off during a backfire.
The choke is easy enough to check, should be wide open after about 2 minutes.
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Old 07-17-2012, 01:46 PM
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the choke is wide open while running, I beleave it is a vacum leak gonna try working on that a little later
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Old 07-17-2012, 01:46 PM
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Quote:
Originally Posted by 23 tbucket View Post
Its rough right from the start, and it seems to have gotten worse now it will idle up and down on its own from 900 to 1300, I tried the simplest things first a new fuel filter, drained the old gas and added half a tank of preium, checked the timing none of those seem to make a difference.
With the engine idling, use a rag and partially choke the carb to see if the idle improves or not. If it smooths out and runs better choked some, that's a sure sign it's getting air from somewhere other than through the airhorn. If the idle doesn't change, or gets worse, it may be too rich or has some other issue.
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Old 07-17-2012, 02:40 PM
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I just took the carb off looking for obvious cracked gasket, and discovered looking down the dual plane intake the drivers side is soaked, and passenger dry as a bone any opinions on why that is?
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Old 07-17-2012, 03:57 PM
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Quote:
Originally Posted by 23 tbucket View Post
I just took the carb off looking for obvious cracked gasket, and discovered looking down the dual plane intake the drivers side is soaked, and passenger dry as a bone any opinions on why that is?
Obviously something ain't right.

Are both sides getting a shot from the accelerator pump when the throttle is opened? Was the accelerator pump circuit check valves (top is usually a small steel ball or pin, bottom is probably a reddish 'umbrella'-type check valve) reinstalled? If it's missing fuel could be siphoned out of the pump circuit.

Check to be sure the power valve isn't loose or blown out (leaking). Also see if the air bleeds are all open and I'd still want to double check the float level and to be sure the needle and seats aren't hanging open or fouled w/teflon tape remnants (do not use teflon tape!).

Have you looked at any plugs to see if they're fouled w/fuel? Starting to sound like the bad side is overly rich. If that's the case, half the plugs will look basically normal, the other half will be either wet or sooty black or both.
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Old 07-17-2012, 04:22 PM
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thanks for all the advice, I've actually been having alot of trouble with the rear float on this carb, it will be fine for a while then all of sudden some days it rises and stayes up and thats happend twice, so I called summit today and there gonna exchange it for me, should have the new one tomarrow evening I'll install it and let you know if that solve the problem
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Old 07-19-2012, 07:32 PM
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I put the new carb on tonight, and go figure no change, so just for my peice of mind I swapped it over to my dads GTO and his car actually runs better then it did so I've eliminated the possibility of it the carb being the problem, I'm gonna pull the intake on my next day off maybe I'll find a vacum leak there, or could it be a bent push rod or vavle, or even cam lobe gone? maybe? any thoughts would be appreciated
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Old 07-19-2012, 09:19 PM
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You'd expect to hear valve train noise if a push rod were bent or a rocker arm stud pulling.

Have you done a compression test? What's the vacuum look like at idle- needle steady and low, jerky, or ?

Does the engine smooth out if you increase the speed? Have you pulled a valve cover to see if it has any broken valve springs or any other obvious probs?
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Old 07-20-2012, 02:54 PM
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after further inspection I found a broken valve spring on the intake vavle cylinder 6, now because I dont know actual cam specs how can I find out what valve springs I need?
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Old 07-20-2012, 05:11 PM
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Good to see you found the trouble..I can't answer your question but what I am curious to know if somehow there is a connection and what it is between the broken spring and half of the intake not taking fuel. Was the broken spring on the same side and the of the dry intake plenum? Someone will be able to answer your spring pressure question.
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