Hey guys kinda new here and just poking around but, I cant find a thread on intake manifolds and carb spacers. Help would be appreciated!


Ive got the Edelbrock performer 400 atop my 400 I replaced my Holley 4160 with a Rochester Qjet 750 cfm with mechanical secondary's. Qjet is a spread bore and I had to bolt a spread bore spacer to a square bore spacer in order to mount the carb SO I have 2" of rise from the combo. The thing has way better gas mileage and awesome torque compared to the Holley and good throttle response still. When you open it into the secondary's the thing seems to have good power BUT it seems to have a big flat spot if I open the throttle too quick (rich symptom?) so I have to roll slowly on the throttle and cant seem to time it correctly every time to get the same response. My biggest wonder is maybe having that 2" spacer above the dual plane is creating too little velocity and making it bog when snapping straight to WOT. Is there anything to back up my idea or could it just be that I have too rich of secondary's OR do I just need to be patient with my dumping of fuel with the mechanical secondary's? Also does a spacer actually help HP/TQ or hurt it?


Thanks for your input dudes!

 

I don't think the spacer is your problem. Since the spacer has 4 separate holes it shouldn't cause what your seeing. On a Holly the fix would be the accerator pump cam or discharge nozzles but I don't know anything about your carb.

 

Hmm dang. To my understanding the Rochester Quadrajet is like the Edelbrock carbs with metering rods etc and are a spread bore design and the spacers are open plenum type not with four holes in the plenum area.

 

I think you should look for a carb spacer that is split in in the middle so that the two sides of the dual plane are separated. That may "fix" your problem. To be clear you want one primary and one secondary venturi on the carb to feed one side of the intake and the other half of the carb to feed the other. It may be that the "open" spacer causes a momentary lean or rich condition in your engine when you open the throttle quickly. Adding a divider into this space should help.

 

Thanks for your input. I think what you're saying is backing up my ideology of too little velocity in the plenum off the bottom. At 2500 RPM you can stab it wide open and it screams, anything under that if you hit the secondary's it stumbles and jolts (It looks like amateur hour at a burnout competition with the suspension bouncing ahaha)
I may end up buying another spacer and weld in a divider to meet with the manifold and see what happens, if it makes it better I can stick with it and maybe tweak it a little like the RPM air gap from Edelbrock to see what happens too. If it works or doesn't I'll post it up here

 

Have you tried resetting the carb, then backing out the adjustments for preliminary start-up(as per the manufacture)? Mark your rich/lean positions and leave the adjustments half way between rich/lean? Taking the time to mark the Rich-Middle-Lean positions well, helps down the road for a quick adjustment at the gas station or wherever. I'd most defly get into the timing also, make sure it's where it's supposed to be. With your truck bucking like a bronco, it could be both timing & fuel issues.


My gut feeling & $.02 here...I don't really think that spacer is as much of your problems but, I've been very wrong before! LOL I built a 38x TBI Stroker for a 90 Z-71 I had, it didn't initially run well. After new O2 sensors, Holley 670 bolt on T.B.I. and whatever brand 1" spacer, it made a HUGE improvement anywhere in the sweep of the tach.

 

the " big flat spot " is a lean condition in the 2nd system.

 

If you have a real Rochester Quadrajet, you have a vacuum secondary carb not mechanical. There is an air valve on top of the carb that is actuated by vacuum, so even though the bottom butterflies open with the linkage air and fuel do not pass through the secondary side till needed.( If this air valve has been removed to make this carb an actual "mechanical secondary" carb, its junk and you will never get it tuned)

First thing to do is adjust fuel pump shot and see where that gets you. To do that you need to remove the roll pin pivot, move the linkage and replace the roll pin. If that doesn't fix it you will need to tune the primary side jets and rods. Sometimes you can cheat and while the accelerator pump is disconnected (engine warm) try to rev the engine if the bog is gone youre too rich on the jets, if its worse hook up the linkage in the new spot and rev it. See what happens. The thing about the quadrajet is the secondary side doesn't need touched until you get the primary side dialed in. And if you are running an open center spacer... That will be difficult. I'll go find a video and post it up later.

 

This will get you started