I have a 750cfm Jet Performance Stg 1 Quadrajet that I've been having rich mixture troubles with. I'm wondering if I can jet down the carb, or if I'd be better off to sell it off and go looking for like a Holley 625 or Edlebrock 600. Here's what I have:

I have a Otherwise bone stock LG4 305 no longer running emissions equipnement. I have SLP 1 1/4 Primary tubes with a 3" flange to a 3" exhaust with a small reducer from 3" to 2 1/2" with no cat to a Hooker Aerochamber with 2 1/2 dual outlets. A ZZ4 intake manifold with the EGR crossover blocked off. I have a stock Curved GM HEI coil-in-cap distributor and the 750 Jet performance Q-jet. It doesn't have a pronounced bog, but the motor seems sluggish and it is definately running pig rich. Mixture screws are 3/4 turn out from full lean, any more lean than that and it won't idle. Behind it is a BW T-5, so vaccum is not an issue with the trans. Doesn't seem to have any vaccum leaks and idles pretty evenly right around 900-1000rpm. I'm running about 24-26 degrees of timing. Generally I find when the secondaries open, it begins to "diesel" or detonate. adjustment of the part throttle screw only improved it slightly, so I'm Thinking I might need to jet it down or get a new carb. I don't know the jet size that's in it, so I don't have a reference point


Does anyone know the primary and secondary size they put in these things? Could I contact Jet Performance and find out?

Thanks in advance.

 

How many inches of vacuum does it pull at idle? Is the needle steady at idle and what happens to the vacuum needle under acceleration?

I'm thinking you have "nozzle drip", causing the main system to "tip in" to supply the engine with fuel. Idle air mixture adjustment probably can't cure this.

Second, is there a mechanical advance in that distributor? If so maybe it's sticking causing too much advance intermittently?

 

You could also be idling on the transition slots!

 

stock lG4? no need for that much timing or that fast of an idle speed.

Go to 12* base, and 36* full timing, run ported vacuum advance.

Set for 600RPM idle or so. Pop off the carb and check your transition slots, shoot for ~.030" visible slot. Use a wire gauge style spark plug gapper to check that.
Adjust mixture screws for highest vacuum.

Clean your plugs (or at least a few). Lock out your secondaries. Do a WOT run from stop to 70MPH or so, shut it down and check those plugs. You can either go by plugs, or time it from 0-60MPH or so. Rejet it, (use diff rods too, try and keep the metering area the same, just go up or down a jet size to change the WOT mix). Once you've mastered the main jet, recheck your cruise mix, plug reading is best for that.

Then re-connect the secondaries and check those.

Chances are, once you re-do your timing, and check your air valve adjustment, you'll be done.

Some q-jet tuning info here:
https://webdisk.ucalgary.ca/~jmknopp...ning_Paper.doc

 

be real careful of setting the throttle blades correctly. 0.030 is a good rule of thumb. tune from there. if you want to know what the original jets and rods were for the carb the shoot me the number on the throttle linkage side and I'll look it up in the book. also you can check HERE for all the q-jet tuning information I have found on the Internet.

 

Took part of your advice and reset timing with a Base of 12 degrees and 16-18 degrees of advance (gauge isn't 100% accurate) Idles now pretty smoothly at 800rpm. Still likes to detonate if you give it more than 3/4 throttle with the secondaries open.

I called Jet Performance and they said the primary size should be is .074 and secondary is .108 for that carb. They suggested going down to .069 main and .098 secondary jets for a "mildly modified" 305.

Thanks for the help so far. I'm gonna pull the carb off next weekend and get into it.

 

secondary jets?? huh, didn't know jet did it that way, weird...

detonate at WOT, usually means it's lean doesn't it? Mine did that a little while ago, i'm curious as to what causes that....