Truck runs bad after high RPM. *New Problem*

[y5mgisi]

It's done this exact same thing before. Scenario, hold a gear out to say 5500 RPM or so and when it comes back down to idle it runs bad. Very rough and very rich smelling. But only at idle. Once it gets off idle younwould never know anything was wrong. Like I said it's done this before but last time it cleared up after a couple days of driving. It's been almost two weeks this time.

My initial thoughts are that one of the idle circuts was pluged but then why the rich smell? Any ideas? Truck is a 78 gmc 350 mostly stock but with headers, edelbrock performer intake and 1406 carb.

 

[4xcrazy]

damn, and i feel mine is screaming at 3000 rpm going down the freeway at 55mph...

it's probably the motor telling you,,,"HEY, THAT HURT, DON'T DO IT AGAIN"

 

[76zimmer]

rephrase:

check choke plates, vacuum pulloffs (if equipped), metering rods sticking?
How much fuel pressure do you have?

 

[Jason4x4]

i'm thinking stuck open throttle plates on the secondaries.

 

[trukker]

Don't feel bad i'm having the same probs. with my 1405. It has done it since it was new on 2 diffrent motors

 

[y5mgisi]

damn, and i feel mine is screaming at 3000 rpm going down the freeway at 55mph...

it's probably the motor telling you,,,"HEY, THAT HURT, DON'T DO IT AGAIN"

Ya i cant verify that rpm. I dont have a tach. But i think that RPM shouldn't be out of a stock 350's league.

check choke plates, vacuum pulloffs (if equipped), metering rods sticking?
How much fuel pressure do you have?

Not sure on the fuel pressure. And if it were choke, i would think it would run worse at mid throttle then at idle? No?

i'm thinking stuck open throttle plates on the secondaries.

Interesting one. Haven't thought of that. I'll check for that when i check everything else.

Don't feel bad i'm having the same probs. with my 1405. It has done it since it was new on 2 diffrent motors

Same problem im having? Where after a high RPM blast it begins to run rough at idle? Does it clean itself up after a bit of driving then come back after another bit of high RPM?

 

[GsxrMike]

Just a guess but could the weights inside your dizzy be hanging up?

 

[y5mgisi]

That's another idea I had. Just thought that would start to create some dieseling issues, but I will check that.

 

[trukker]

Yep same thing.

 

[76zimmer]

check your fuel pressure, I heard those carbs are finicky about no more than 5#

 

[y5mgisi]

Well it seems to have been a stuck float. Put a set of spring loaded needles and seats in it and its all better now!

 

[y5mgisi]

So i put the spring loaded needles and seats in and that fixed my problem. But now i have a new problem!

Between about 1/4 and 1/2 throttle the truck runs really rough and feels like its missing. Go heavyer or lighter on the throttle and it runs great. Once it gets warmed up its not as bad but still noticeable. Im thinking maybe a plug was fouled from when it was running bad before? or maybe my floats arent set right? or something with the metering rods? Going to pull the plugs and see if they tell any storys.

 

[y5mgisi]

TTT for the week day crew?