Hi guys, I've been trying to tune in my 408 that is FINALLY running, and I've noticed that while driving, there is a pretty small, but very regular, and kind of disruptive dead spot aaalmost as soon as you give it any gas from idle, right around 17-1900 or so, maybe even lower. The engine purrrrs, very smooth idle once it's warm, and very smooth after that, as it's a very mild cam, but juuust as I'm sinking into the gas, I lose power a little bit, and if I don't pass that spot quickly, it tends to make the engine a little choppy.

What causes dead spots generally, and what could be causing mine? The engine is smoooth smooth right up through the upper RPM's, and the dead spot doesn't happen when I'm not in gear.

Thanks, AleX

 

My vote would be an accelerator pump that needs a little adjusting.

 

I was off on the RPMs, it actually happens right around 1000, juuust as you start to throttle. I didn't know how low this new engine was idling, so I guessed high.

 

like fellow above said , i would bet on the accelerator pump. my truck was doing the same thing when i first got it. i tore the carb apart and found a small pin size hole in the diaphram of the accelerator pump.

 

Hmm, OK, my accelerator pump is very accesable, I guess I'll go out and look at that, I guess I might just need to put a different little plastic cam on it?

 

Is your vacuum advance hooked up? to ported vacuum? could be the cause of an off idle stumble i.e. flat spot if it's not, it makes for a smooth transition from intial advance to mechanical advance.

 

Ahhhh, it's not! You know I forgot about that after having such success with everything in the past few days, but nope, it's not, I'll try that tomorrow.

What is ported vacuum though? Is that from a "port", one of the intake runners? Or what?

 

Ported vacuum is above the throttle blades, manifold vacuum is below the throttle blades.

 

Ahh OK, well I have some ports that are just capped off now up there, so I'lll do that.

Thanks! Hopefully this fixes the little hiccup, I'll be back here if it doesn't.

AleX

 

Hmm, well, smoother, but not smooth. I'd say it helped by about 1/3, but it still sputs riiight at about 1000 RPM, any ideas?

 

For reference, what carb are we working with? A Holley? Eddy? Autolite? Motorcraft?? Demon?

 

A Holley 600, vac secondaries, electric choke, about a 10 year old model.

 

I am having the same problem, same carb but brand new. Just did intake and cam swap and now i have the same problem you are having.

 

Have you messed with your accelerator pump cam at all? I haven't, wondering if they might just be a little weak right around this range....

 

yes, there are two positions the cam be put in. the other position made it worst. Holley.com has a good video clip to watch on adjusting the carb.