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rough running 454

red

red

Being a lake bum in Texas
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Lake Brownwood, Texas
now that the new flexplate and starter are in i can drive the truck around without replacing a starter each time i need to fire up the truck :D.

its running rough. idle is pretty smooth, but anything above idle and it randomly spurts out a miss or backfire. has a new cap/plugs/rotor, not sure what could be causing it. its an 84 truck with a quadrajet 4 barrel carb and an HEI distributor.
 
My 454 has always shook the entire truck while idleing and run a bit rough that is there nature it seems. But maybe you need a carb rebuild?
 
it shakes the truck a little lol but the rough running is the concern. i'm gonna try running some b12 or seafoam through it, if that doesnt fix it then i'll check out rebuilding the carb.
 
Timing off? Can be a cause of backfire or miss.

IMHO if it's running that bad, a can of seafoam isn't going to fix the problem. I'd be checking out the jets on the carb for obstructions
 
I had a 79 chevy with a 350 and it did the same as you are stating. It was the distributor was worn out. Replaced it and never had any more problems.
 
big block 88 said:
How is it runnin bad? At what RPM's? when does it act up the most?
Click to expand...

any rpm above idle, and the speed at which it misses stays fairly consistant. when i 1st step on the throttle (if i step hard) it will miss more but after a few seconds at that rpm it calms down.

carb or distributor would both be possible, no idea how long since either has been replaced or rebuilt. idle is a nice smooth running motor, and other than that miss it runs good. a guy on another forum mentioned that it could be the vacuum advance is no longer working which would affect the timing. havent tried adjusting the distributor cap to see if that affects it.
 
I was just startin to think maybe a vcuum issue? But I fuond your problem you just said it idles smooth. 454's should never idle smooth, mine idles like monkey ass, for lack of better term. When I pulled my heads a few years back I replaced all the ignition and top end sensors. Seemed to be much more happy. Keep us posted
 
alright i'll play with the vacuum advance tomorrow 1st since its the cheapest. havent had a chance to play with it this weekend, we had an airshow here on base and they needed us (EOD) to do the special effects. 2 bombing runs and a strafing run on saterday and again today.
 
i checked the vacuum thats going to the distributor's vacuum advance and there is vacuum there. i had the truck running for about an hour, mostly at or right above idle. its smoothing out slowly which has me leaning towards a fuel issue instead. i also swapped the distributor for the one that came with the motor, ran the same once i adjusted the timing.

there was also a small fuel leak on the hardline, got that fixed and i'll find out tomorrow if it was affecting how the motor ran. whats happening is at idle its smooth (for a 454 lol) and i can slowly give it throttle to about 1/2 way without much issue. if i go above that or i blip the throttle then it gets mad and sounds like its going to explode. i poured some b12 chemtool into the fuel tank to help clean it out now, letting that settle some before i fire it up (why i'm waiting until tomorrow).

the 2nd carb (spare) is now sitting and partially torn down, if the b12 doesnt help i'll rebuild this carb and swap them out. what size jets are stock for a 454?
 
well i rerouted that vacuum line which oddly enough was still drawing vacuum at idle (tried every port on the carb) but it did allow me to get to higher rpm's. low rpm's and high range the engine was running good, at mid range it would backfire still. i took it to the high rpm's a few times, then when i brought it back down the last time it wouldnt idle and stalled itself out. got it to fire up again and it stalled again.
 
yes it has vacuum advance.
quadrajet carb 4bbl
HEI distributor
mechanical fuel pump

on another forum one of the guys mentioned that it sounds like a lean backfire, expecially since the backfire is coming up through the carb. if thats the case maybe a clogged fuel filter? only filter it currently has is the one at the front of the carb, which happens to be stuck at the moment (nut holding it in place).
 
Hmmm. usually when i am kickin fuel back through the carb my ignition is 180 off and the truck can't even idle. A lean backfire i guess could be possible. I would try that filter then rebuild the carb
 
Nope, most engines dont. If the dist isnt the problem then stick with the fuel delivery issue. It may possibly be the lean backfire, it wont hurt that engine to rich it up some. 454's can handle alot of fuel so try changing your jets or go with a bigger carb, just watch you plugs to see how the engine is handling the fuel.
 
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