Rough Idle but still pulls good

86 F250, 460, Edelbrock performer intake, Holley 670 carb, 480 lift cam(only specs on cam I can remember)new plugs , cap, rotor, wires are less than year old. Old plugs were a little on black side as in to much fuel but not just horrible. No vacuum leaks best I can tell. As I said it idles rough but still seems to pull good. Pulling
30' fifth wheel and would run right up hills like it always does. Seems the more on the throttle the smoother engine would run. Did not check mileage but did seem to burn a little more fuel than normal. I believe I have a carb problem here but since carbs are my criptonite I figured I would ask you experts who helped me build this beast for some more wisdom before I start tearing into it. Oh, and it does backfire through the exhaust every once in awhile. Any thoughts would be appreciated....  

plowboy34 wrote:86 F250, 460, Edelbrock performer intake, Holley 670 carb, 480 lift cam(only specs on cam I can remember)new plugs , cap, rotor, wires are less than year old. Old plugs were a little on black side as in to much fuel but not just horrible. No vacuum leaks best I can tell. As I said it idles rough but still seems to pull good. Pulling
30' fifth wheel and would run right up hills like it always does. Seems the more on the throttle the smoother engine would run. Did not check mileage but did seem to burn a little more fuel than normal. I believe I have a carb problem here but since carbs are my criptonite I figured I would ask you experts who helped me build this beast for some more wisdom before I start tearing into it. Oh, and it does backfire through the exhaust every once in awhile. Any thoughts would be appreciated....  

tune it. clue is plug color and mpg. correct that it will run smooter

Everything new except wires and they are less than a year old. Another thing I did is ground out the plug wires one at a time to see if maybe it was one cylinder. Three cylinders didn't make a difference and one cylinder actually made the engine smooth out, never seen that before. By the way it pulls there is no way three cylinders are dead. I am just plain puzzled on this.

Are you sure it is wired with the correct firing order, starting in the correct spot?

Sure did Lightning....that was exactly what I thought. It's all correct, I have been doing some more reading and I am fairly certain power valve has gone bad. Several people have reported exact same problems I have and they all said a power valve replacement fixed there problem. At least I know how to test one now, as I said carbs are my criptonite. I know enough to be dangerous but I am not afraid to dive in. It's a cheap replacement so I'm gonna try it.

I would try some new plugs first. You could close the idle screws and see if the engine dies, if the power valve is bad, the idle screws won't leans it out very much.

Holley carbs don't like backfires it tends to blow the power valve in them or the older Holley didn't like backfire through them hadn't bought one in a long time that I left the power valve in.

what I like to do is let the engine run and pull one plug at a time and if the idle dose not change then you know that plug is bad well it looks like that ways already said that's what you get for not reading all the post

plowboy34 wrote:Everything new except wires and they are less than a year old. Another thing I did is ground out the plug wires one at a time to see if maybe it was one cylinder. Three cylinders didn't make a difference and one cylinder actually made the engine smooth out, never seen that before. By the way it pulls there is no way three cylinders are dead. I am just plain puzzled on this.

leave the plug wire off the one that smoothed the engine, for the easy fix. lol

Hey DanH, I suggested that to my buddy but he wasn't going for it....  

Rabbit, yep I already did the plug wire thing, and it did things I never seen an engine do.

Plugs have already been replaced, that is the first thing I did.

Fairly certain it is the power valve, haven't had a chance to look at it yet as we had about 80mph winds here in SE Missouri yestersday and I lost a bunch of roofing so that has taken priority at this time. Just taking a little break so figured I would read some engine tech stuff to calm the nerves....  

Hehe... DanH I guess that would give new meaning to "plug" wire.