Q-Jet install on edelbrock performer

bobs77vet

bobs77vet

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arlington va
well i am finally getting back to old buisness that was interrupted when i moved from NJ to VA..... i am getting ready to swap my Lars tuned 750 Speed Demon for a Lars tuned Q jet. i have the fuel line stuff i need, i have vacuum caps, i have the choke tubes that i need, the Q jet bolts and the bracktery for the 700r4.... what else am i missing that i haven't thought about??? i guess i need to know if the Q jet will work on the performer as a direct bolt up application or if i need to put a spread bore spacer in there so the bottom butterflies don't hang up on the performer? thks bob
 
Bob -
If it's a square-bore manifold, you have to use an adapter - the Q-Jet secondaries won't clear a square bore pattern. Many Performer series manifolds have the dual pattern to allow either carb - it is obvious when they are dual pattern due to the Q-Jet-appearing secondary holes in the manifold.
Lars
 
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ok I am ready to start the swap and its scheduled for Saturday.....looks like i have a dual pattern manifold and you can clearly see the bulge for the secondaries, i can't remember which of the vacuum ports on the Qjet are direct vacuum or ported vacuum is there an illustration any where that shows which is which?

believe it or not but i am getting psyched for the Qjets little primaries.....the speed demon is great for "getting on it" but most of the driving i do around the DC area never hits above 2500 rpm range, and with the 700r4 even at highway speeds i am under 2000 rpm. i also will not be sad to see some of the rich fuel smell go away either. and who knows maybe even my gas mileage in the pure city driving i do will increase.
 
On divorced choke Q-Jets (up through 1974), the ported vacuum nipple is the one located on the forward driver's side of the carb, in the middle of the float bowl. All other nipples are manifold vacuum. On integral choke carbs (1975+), the ported nipple is located in the carb base plate (throttle plate) in the forward passenger side corner, pointing out at a 45-degree angle. All other nipples are manifold vacuum.
Lars
 
the swap went well, no real problems, we only missed one pre planning item and had to run to the hardware store for a vacuum fitting for the brakes. we only did the initial install and some neighborhood runs. it is exactly what i hoped for the .....the low rpm acceleration is much better then the 750 cfm speed demon and i think this is because the smaller primaries just get a better signal at the unfortunate low RPMs of the driving in this area. today after it stops raining i will get out the test tach and vacuum meter to set the idle speed and idle mixture.



i was surprised at how similar the 700r4 TV cable final cable measurements were to the speed demon set up. I am supposing that there must be some basic rules of how far the accelerator cable travel is vs the relationship of the linkage hookup to the radius from the throttle shaft that makes many of these carbs linkage travel very similar.

Lars i have a question on setting my idle speed and you have an email

and also does this apply to my situation from your other email on setting the a qjet?

"Once timing has been set up correctly and verified, set your idle rpm to about 750-800 and start with your idle mixture screws at about 4-6 turns out from lightly seated. Turn them both in 1/2 turn at a time until you get a 50 rpm drop in idle speed. At this point, your A/F mixture will be about 15:1, which is a tad too lean for ethanol-added fuel, so crank the screws back out about 1/2 turn from that point. This will get you right about at 14:1 on your idle mixture, which is perfect." i normally use a vacuum gauge for setting the mixture can i use this method on qjet?

thks bob
 
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You can certainly use a vacuum gauge, but you really don't need it: If you have any "feel" for the engine at all, you can hear and sense the changes to the engine before it even shows up on a vacuum gauge. Run the mixture screws at the leanest setting that produces best quality idle. You can hear and feel this easily without the need to use a gauge.
Lars
 
life is good again.......turns out i had a vacuum canister that was broken and not pulling any advance and leaking as well as a brake booster grommet that was leaking, Lars got me squared away with his usual dead on trouble shooting techniques and i now can go the the final tuning stages. even with not touching the idle mixture screws from the rebuild the car runs great as it is.......i can't believe how much of my familiarity with the car i forgot/lost in the 1 1/2 yrs due to the move and building of the house.
 
so i was explaining to the Mrs how the vette is tuned for performance with the light centrifugal advance springs and the advance coming in as quickly as it does and she asked me, how is that different then tuning it for either economy or emissions standards and why does it make a difference....i said it has to do with making the spark plugs fires early in the compression stroke but beyond that i dont really know.....so whats the answer? thks bob
 
Retarding the timing results in elevated combustion temperatures which will lower HC and NOX emissions. For this reason, pre-catalytic cars used ported vacuum to the vacuum advance to keep timing retarded at idle and low speed. Advance curves were kept slow and retarded to keep combustion tempertures high. This, of course, is not conducive to performance.
Lars
 
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