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suddenly runs really rough

6K views 16 replies 5 participants last post by  23 tbucket 
#1 ·
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
 
#2 ·
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.
 
#4 ·
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.
 
#8 ·
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.
 
#10 ·
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.
 
#11 ·
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
 
#12 ·
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
 
#13 ·
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?
 
#16 ·
As long as you know that it's a hydraulic flat tappet cam (or you know it's a roller, etc.) you can get a good idea by measuring the lift of an intake valve. The cam makers all have to keep the lift vs. duration within a safe range so the lift can be used as a guide.

For example if you have a flat tappet cam w/0.480" lift, you'd have a pretty good idea the cam is not going to be much less than 224 degrees @ 0.050" lift- that would be considered to be a "fast" lobe, like seen on the XE Comp cam series. Comp recommends their valve spring p/n 986 for a wide range of their cams (including a cam like I made up above), including the XE street hydraulic flat tappet cams.

Comp also sells a good (but more expensive) beehive spring for the Vortec heads. It takes a different retainer than stock, though. There are two springs part numbers for different lifts.

If you do a search you can look for info on using OEM GM beehive springs on these heads as well.

Here is a list of valve train points to check.
 
#15 ·
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.
 
#17 ·
the dry side was on the passenger side, and the spring was busted on the passenger side not sure if they were related, but I replaced the spring with one off a 96 pickup truck with vortech heads the same as mine and it seem so be running good again!! Thanks for all the help
 
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