Chevrolet truck runs rough...


Old 04-02-04, 09:12 AM
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Chevrolet truck runs rough...

Okay, I want to state up front that the way this truck was put together was not my idea! The previous owner of my '86 Chevrolet 1/2-ton removed the OEM 305 with the Quadrajet, and replaced it with (from all the research I was able to do) a 1970 Chevrolet 350 with one '80 305 head and one '81 305 head, an unknown year intake manifold and another Quadrajet from a '79 Olbsmobile 350. The truck was running rough at idle and part throttle after I rebuilt the carb (ran okay at near-full throttle). Took another look in the carb and found a hunk of wire or something in the APT/power piston bore which I suspect was holding the piston up in the bore, making an always-rich part-throttle condition. The truck now runs better, but I'm still looking for a smooth idle. Any ideas?
Also, I changed oil and filter before starting it after the latest round of repairs, and oil level was fine. Now, the oil level seems to be creeping higher every time I run the engine. The fuel pump has not been changed; my suspicion is that there is a ruptured diaphragm in there, sending fuel into the pan. The oil doesn't look foamy or milky (which I hope rules out a water/anti-freeze leak), just thinner than it ought to. What do you think? At $12, it would be cheap enough insurance to just replace the pump, but I'd like to know if there's somewhere else I could look, too. Thanks for all input. - Chris
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Old 04-02-04, 04:23 PM
Join Date: Apr 2002
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You may have a major mismatch going on causing internal vacuum leaks.I would put matched components on the truck(a used or reman engine assembly)you could probably get away with the carb.I would just change the fuel pump thats the most likely culprit there provided your carb rebuild isn't dumping tons of fuel.
Old 04-02-04, 09:39 PM
mike from nj
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gas mixed with oil won't make it foamy or milky, it will make it look like black ink. an easy test is to hold a lighter up to the dipstick and see if it will light the fuel/oil mixture. if it have mere minutes left of your engine bearings....change the oil now! $12 for a fuel pump is very cheap insurance too, whether it's leaking or not.

it's also possible with all the different parts you have that you might have a huge intake manifold leak (like davo just said) inside the engine. that would dump fuel into the oil, cause a very rough idle and part throttle, yet run ok at full throttle (sound familiar?)

barring the intake leak, and provided all the vacuum hoses are hooked up correctly(or plugged), you need to figure the carb problems out. that might be your only problem, and if that is dumping fuel into the cylinders, it will quickly work it's way past the rings, and into the oil. a stuck closed choke on any carbed car can ruin the engine in short order. this rough running engine fits that description.

have you done the basics, checked ignition timing, timing chain slop, compression in the cylinders, manifold vacuum at idle and revved up? there are many things that cause a rough running engine before computers took over.
Old 04-04-04, 10:59 PM
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As for the mismatched heads...if you are concerned about the compatability of them, remove the valve covers and get the casting #s of the heads. Do a chevy head# search online or give the #s to a machine shop and find out the valve sizes and combustion chamber CCs. If they are the same, you shouldn't have any trouble. If the #s match, you don't have to worry about them. If they have the small intake valves, (1.67" if I remember right), then they weren't intended for use with a 4barrel, and you might be giving it a bit more fuel than it can inhale with that 4bbl on it.
Old 04-06-04, 01:48 PM
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Thanks for the help, guys. Vacuum check showed 14 in. steady (fuctuations of less than 1 in.); my Chilton's manual says this indicates late ignition and/or valve timing. I'm trying to round up a degree wheel, timing light and tach so I can check these. I changed the fuel pump, oil (it was inky and black, like mike said) and filter, and will keep a close eye on oil level.
Something new cropped up today, while I was checking vacuum. When I shut the engine off, I heard a faint hissing noise from the passenger side of the engine. Didn't see any fluid of any sort squirting out or puddled anywhere. The upper radiator hose was still cool, but the upper heater hose was warm ( truck had run maybe 3-4 minutes). As I'm also watching the coolant level, I removed the radiator cap and was greeted by the presence of a layer of small white bubbles on top of the coolant. the bubbles quickly dissipated and left no discernable odor. The first thing that comes to my mind is combustion leak into coolant passage. If I'm right, I'll be replacing either ( in order of expense) cylinder head gaskets, cylinder head(s), short block or long block. Naturally, I'd like to spend as little as possible; any pointers on how to narrow down the possibilities? Thanks to all in advance - Chris

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