q-jet tuning Question

[Justin Fleming]

I have a 86 k5 with no emission stuff hooked up.....The truck runs well and is responsive after the carb re-build. I did the mods so you can access the air fuel screws. In my tuning days I was always told to tune for the highest manifold vaccum.... of course this is when the vaccum advance is disconnect, operating temps and so on. How ever I am like 5 turns out which should make the idle circuit pretty lean. The truck exhaust still stinks like it is rich...... and I had though anything over 3 turns out is really not doing anything...however the further out I turn them the R's continue to increase....

whats your thoughts....

And always a great topic and a matter of oppinion..... Should the vaccum advance be hooked to manifold full vaccum or the ported vaccum at the front lower left port of the q-jet

thanks

 

[dyeager535]

Most engines will have a better idle if you can give them high advance at idle, which means manifold vacuum. Some engines might be different, but EFI small blocks run 20*+ at idle, so that should say something.

Turning the idle screws in leans out the mix doesn't it? It's been awhile, but isn't that the idle fuel discharge that your are controlling with them? The farther in they are, the more they close off the orifice.

 

[Justin Fleming]

Most engines will have a better idle if you can give them high advance at idle, which means manifold vacuum. Some engines might be different, but EFI small blocks run 20*+ at idle, so that should say something.

Turning the idle screws in leans out the mix doesn't it? It's been awhile, but isn't that the idle fuel discharge that your are controlling with them? The farther in they are, the more they close off the orifice.

I thought it was aire fuel...maybe I am wrong.... I also thought turning them in would close off the orifice and not allow air which would make it run richre and turning it out allowing air would lean it out.....Maybe I am all wrong....

Can anyone set me in the correct direction......

thanks

 

[dyeager535]

Short of Doug Roe's book, here's a pretty thorough article:

http://www.highperformancepontiac.c...hester_quadrajet_carburetor_tuning/index.html

Either way, if it's air/fuel or solely fuel, turning the mix screws in will limit, since bottoming out means they seal off the discharge port into the primary venturi's.

 

[ryoken]

As Dorian mentioned, you have it backwards...

The idle air/fuel screws meter fuel to the primary idle circuit... no air involved... in for lean lean, out for rich... Roe's book recommends out 2 turns...

 

[pauly383]

I have always done mine for smoothest idle . I do have the egr and such hooked up though .

On these the mixture screws are idle only . The APT has the most affect of driving performance , since its the part throttle mixture ( between idle and WOT ) .