454 Stumble

[USSkoval]

I've been fighting a light throttle stumble that doesn't want to go away. I'm running a Holley 670 Street Avenger on a very mild 454. It idles fine, just off idle is fine, heavy throttle is fantastic, but it continues to stumble around 1500 rpm and light throttle. That doesn't work very well at low speeds because the truck has 2.56 gears, so driving around town I can either putt around at idle or smoke the rear tires. There's really no happy place in between.

I've had a hunch this whole time that I need a bigger pump nozzle (currently a #31) but nowhere around here has one. So I've been playing with what I do have to work with.

I've messed with the timing, no improvement in driveablity.
I've repeatedly adjusted the idle speed and idle mixture. No improvement.
I've double checked the float level.
I've changed pump cams, it likes a blue cam in the #2 hole.
I changed the mounting gaskets (I had reused the old ones before) and that showed some improvement too.

Also I have found that it doesn't seem to stumble when the choke is still on. As the choke opens, the stumble shows up.

I think I need to try a #35 pump nozzle. But before I pay annoying money to ship a stinking 12 dollar nozzle, am I possibly overlooking something?

[geezer#99]

If it works better with the choke on, check for vacuum leaks.

[USSkoval]

Originally Posted by geezer#99

If it works better with the choke on, check for vacuum leaks.

That's what made me suspect the carb mount gaskets, and swapping those did help slightly. But everything else seems ok. Intake gaskets were new when I installed the motor.

[geezer#99]

Gaskets!
You mean like in more than one!
Do you have a spacer?

[USSkoval]

It's an Eddy manifold meant to accept a spread or square bore carb. So I have a lower gasket, the flat Eddy adapter plate (about 1/8 inch thick), then another gasket.

Also forgot to mention it's a rather lightweight shortbed stepside C10, so the stumble is rather odd. Like what you would normally get from an over-carbed small block in a big heavy truck.

[geezer#99]

Have you tried squirting something around the base plate to see if the idle changes?

Here's a rather long thread on hotrodders forum about a similar problem. A long read but might find something there.
http://www.hotrodders.com/forum/holl...on-346689.html

[USSkoval]

Originally Posted by geezer#99

Have you tried squirting something around the base plate to see if the idle changes?

Here's a rather long thread on hotrodders forum about a similar problem. A long read but might find something there.
http://www.hotrodders.com/forum/holl...on-346689.html

What I get from that is I probably need to try a bigger squirter like I originally thought, and maybe an 8.5 power valve too. If that does OK, I may need to up the jets a bit. Or get a new carb, lol.

This is the first Holley I've had trouble with, and my first Street Avenger too. Most others have worked ok right out of the box. I actually had a 600 Holley on this motor originally and it worked ok, but heavy throttle caused a bog. In hindsight, I probably should have just tuned that carb.

[geezer#99]

I've followed a bunch of threads on the holley 670 problems. Some where cured with squirter or pv changes, some cases they just turfed the carb and used another carb.
I always liked the old standby holley 750 vac sec 3310. Good on most motors from 283's to 454's.

[USSkoval]

Yeah, the most reliable carb I've ever had is my Holley 750 vac secondary, but it's currently in use on one of my other vehicles. It's ok if this 670 takes a little work to make it right, as it was free. I only have about 60-70 bucks in parts (so far) and then some of my time.

[USSkoval]

Just an update, I think I have been chasing my tail because of a piece of trash somewhere in the carb.

Took the truck for a drive today, to see how it did on a longer drive. Like normal, ran well until the choke started coming off, then had the usual stumble. But then the stumble got really bad, the truck was nearly undriveable. Stopped and adjusted timing and idle one more time, it was just barely a little better. After getting through town I was very frustrated with the truck, so I nailed it and didn't let up for a bit... it ran much better after that. So I simply did that a couple more times and afterward, the truck ran great all the way to the next town and back again. Other than a very minor hesitation when I really mash the go-pedal (again, need to try a #35 nozzle) it was running perfect. We're supposed to get rain here the next couple days, after that I'll take it out again and see if all is still well.

[geezer#99]

Honestly officer, I was just trying to get some trash outta my fuel line!! LOL!!

Could be something simple like that. I found a small wire in the inlet of my carb once. It was long enough to stick beside the needle, but retract into the fuel line again when I shut it down.

Good you might of found it!

[USSkoval]

I had overhauled the carb in our bedroom (the wife loved that) because it was cold out in the garage. It sat there on my desk for a couple months before I installed it. Maybe a spider or something crawled into it.

[USSkoval]

Took the truck out for a short time today, everything still seems good. I feel confident my carb troubles are behind me.