help with tuning 350sbc - no vacuum

[MannyDantyla]

I'm trying to dial in the engine. Here's what i know:

• edebrock 600cfm carb. I don't think the cam is too crazy or anything
• the idle mixture screws do absolutely nothing to either rpm or manifold vacuum. But they used to! Just a week or two I adjusted them and I was able to get the rpm to increase
• getting zero vacuum on the manifold vacuum port on the carb, but if I blip the throttle then I get a little vacuum for a second
• if I plug the vacuum gauge on a port on the back of the manifold, then it shows about 17psi vacuum
• the vacuum advance is hooked up to the manifold vacuum port on the carb, which is zero, so is the timing completely off then? I have it statically timed to 12*BTDC iirc
• its a 350 block but it has 305 HO heads! this means compression should be way up
• I'm running 89 octane gas and I don't think it is pinging
• fuel pressure is a good 6psi, idle rpm is about 600 (manual tranny), and the choke is working fine

So you're probably thinking, whats the problem? Well its just not very well tuned. Especially when its cold, it really wants to stumble when starting from a stop. And I just sense that it cold be better.

And why is manifold vacuum zero?? h

 

[PrairieDrifter]

To adjust timing you're supposed to disconnect the vacuum advance, so your timing should be ok. You could maybe advance it a tiny bit more but 12 isn't bad.

I couldn't tell you why your manifold vacuum is zero(im not a carb expert). In my opinion edelbrock is a terrible carb

 

[Irishman999]

It should read full vacuum at idle with the butterflies on the carb closed, if you open the throttle the vacuum is supposed to drop. Is it possible you have some kind of huge vacuum leak somewhere? Maybe your gauge is ****.

 

[Quadrajet Power]

Yes, you should have full vacuum from base of carb. Check your base gasket for leaks. You may have some internal carb issues. I'm not a big fan of the edelbrock design either, but they are common.

Your base timing will be fine, but you are missing the advantage of the vacuum advance for cooler running and smoother idle and transition.

 

[Georgeb]

I would make sure thats the correct port for manifold vacuum.

 

[MannyDantyla]

Yeah I'm wondering if it's a vacuum leak...

I should also mention that I have a 2" carb spacer - the open stytle, no the 4 hole kind. My intake manifold is an edelbrock performer, dual plane. Just like this: http://www.summitracing.com/parts/e...N6RlalaCBglhsFlUhYtmPoxfAfQ3eagoiQaAnkE8P8HAQ

I'm thinking I should have got the 4-hole style carb spacer. I think i'll order a new one ($20 from summit, cheap and easy to install).

For now though the engine is running fine except idle is a little weird and coming off a start can be problematic, especially when cold. But going down the road there's no problem, it's not running hot and MPG is good I think. Maybe the vacuum leak is only effecting the idle circuit?

I'm all out of brake cleaner and my buddy accidentally took my bottle of starter fluid, so I can't check for vacuum leak at the moment.

 

[MannyDantyla]

I FOUND OUT WHAT THE ******* "PROBLEM" IS

Edelbrock carb #1400 (not 1406, but 1400 which is what I have) does not have a full manifold vacuum port!! Both ports on the front are ported vacuum!

Of course I want manifold vacuum for my vacuum advance on the diistributor (see: http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm ) - so I'll have to figure out how to do that...

Oh, and I went under the hood just a couple of minutes ago and started tinkering with the idle mixture screws again. And this I was able to see a change in RPM when fiddling with them. I adjusted them to the highest RPM and then reset the RPM to 600. Hmm, maybe the other day the chock or the throttle plate was doing something funny or something, or maybe I was just not noticing the subtle change in RPM.