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Carb upgrade



I have a 78 Silver Anniversary. How hard is a carb upgrade to a 750 cc Holley from stock. Someone has already upgraded the cams, intake, and exhaust. The intake is an Edelbrock, I'm not sure of the model. I've been told the air filter will have to be changed out to aftermarket. Any suggestions???

What is the cars main use? Racing or driven? If your going to just race the car Holley would be a good chose. For the street the Q-jet is great with such small primaries you get good gas mileage when you're not into, and with 795cfm's to get plenty of flow wide open. Plus the Q-jet if setup correctly will get great throttle response. But if you must use a Holley I would recommend the 795 vacuum secondary Holley.

Hey Silver78
Welcome from another 78 Silver Anniversary! You have found a friendly and informative Corvette site. Hope to see more of you here.

Give the details of your set up to Jet Performance in CA. They will set you up with a remanufactured "real" q-jet or substitute which is calibrated correctly. Nice folks.
Seeing Double?

Hey Hedi:

I had to stop and look again. I thought I was seeing double. Isn't that a conicidence, but I guess there's room for two. I wonder how many more are out there.


Check with the experts. Everybody I know that put a 750 cfm on a small block has had to rejet.... it's too much carberator. Better to be sure than sorry.
Al Al
Double Take!

Hey Al,
I pointed it out to hubby and said "That isn't me!" :L
Always happy to see 78s, and especially Silver Anniversaries!
spreadbore or squarebore

I tend to agree w/ Greyghost...typical 750 Holley on typical 350 is often too much carb... thus needs rejetting. But, conversely, typical 750-795 Q-jet on typical 350 seldom requires rejetting.

You first should determine whether you have a squarebore or spreadbore (type of flange at carb/intake) setup.

Obvious differences: spreadbore has rather small primaries and huge secondaries... while squarebore's primaries & secondaries are nearly or equal diameter. The mounting flanges differ too!

What is the part number (raised casting numbers) of your edelbrock intake? (likely possibilities 2101 or 3701). If you have a manifold that accepts the standard Q-jet (Q-jet has a spreadbore flange) ... some intakes (such as 2101) accept either spread or square... then you might consider one of edelbrock's brand new "replacement" Q-jet clone spreadbores that are based on Q-jet design. Consider edelbrock pn's 1902 & 1903; typical price range about $375-$420. For street duty in a mild L82 w/ adult drivers, a good Q-jet or clone is tough to beat for all around responsiveness, economy, reliability and spirit.
Back to the meat of the orignal question.
Yes, a 750 Holly will fit on almost any of the aftermarket manifolds without an adapter. You will need to look at your linkage carefully to insure that the carb that you are getting has the proper connections for your equipment. Cruise control being the most common problem. If your current carb uses a hot air choke you will have to convert to a manual or electric choke, kits are available for both. The fuel line will have to be modified or replaced. Great ime to change to braided steel! An aftermarket aircleaner can be used, your stock ar cleaner will not fit. You might look at earlier Corvette stock air cleaners to see if they will match up to your fresh air intake. The cost of NOS will be the problem there. The air cleaner problem is due to the float bowls on the front and rear of the Holly. Any air cleaner that clears the hood on a Corvette will have to be dimpled in those areas.
Good luck with it.
This is strictly for recreational use. No racing, just the nice weather, drivin around. But on occasion, it's always nice to fulfil the need for speed:cool . I'm assuming the q-jet is a quadrajet?? I haven't seen a q-jet that big (cc's). Are they difficult to come by? Is the upgrade easier? Would this require the air cleaner upgrade?
Q-jet = Quadrajet

The 750 cfm & 795 cfm (pn 1902 & 1903) Q-jets are new "replacement" Q-jet "clones" that are manufactured by long-time carb-maker WEBER and are marketed by Edelbrock as Edelbrock. Each is essentially a copy of the original Q-jet but have several design improvements. They will replace an original Q-jet with NO modifications to original Q-jet fuel line and original Q-jet air cleaner and original Q-jet chokes. You can find these Edelbrock carbs at www.summitracing.com or www.jegs.com or www.herbertperformance.com or many others; likely can be ordered/found at your local auto parts store.

BTW, Smog Nazis recognize 1902 & 1903 as legal replacements.

And darn it ... I got it confused too ... was it 78SilvAnniv (Silver, Heidi, Hubby etc) ... OR ... Silver78 (?,?,?). Oh well, me goofy.:Silly
Carb Upgrade

Just finished with a carb change from a 795 Q-jet to a 750 Edelbrock. Yes, you will most likely need to rejet as they tend to run rich with a 250-270 hp slightly modified 350. I did, it's quick and simple with the Edelbrock and I may lean it some more yet. Just finished tonight and took a test drive, good, but maybe I can get it even better. Some folks have installed 600 cfm Edelbrocks on their '81's with very complimentary results. The Q-jet replacements are pricey, $500 or so, Edelbrock 750 or 600, around $210, with electric choke around $270. The installation on a stock manifold requires an adapter from spreadbore to squarebore everything fits well. If you do the change make sure you get the linkage adapter kit for your cruise control and automatic transmission. You could change the intake to a Edelbrock Performer for about another $110. Either set-up will fit under the hood.
14xx OR 19xx ?

What is pn of your new Edelbrock carb:confused ? The 19xx series have a spreadbore base to fit stock Quadrajet intakes; that's why I suggest 1902 (1902 especially) or 1903 as "drop on" replacements at under $400 and $420 respectively. On the other hand, the Edelbrock 14xx series do have a squarebore base and do require an adapter/spacer plate to mate w/ stock intakes. Also, I'm not certain, but I think the Edelbrock 14xx series is based on the Carter AFB design with a "typical" Holley (squarebore) base. Also, Holley has two 650 cfm carbs w/ spreadbore base; mechanical secondary carb pn 0-6610 and vacuum secondary carb pn 0-80555S ... both under $335 ... but you'll likely be faced with tuning a Holley.

I put on a 1407, and your absolutley correct it is an AFB design squarebore. $209 at Summit and I added the electric choke kit for another $43. Looking back I would choose the 1411 w/electric choke already leaned somewhat over the 07. But like I said several have installed the 1406, 600 cfm version and appear to like it a lot.

For what it's worth...
I installed the 1407 Edlebrock 750 on my highly modified engine (six inch rods, deck milled to 9", aluminium heads etc etc). I had to change the metering rod springs to the "yellow" from the orange. This cleaned up the rich situation. I am generating 12Hg vacum at idle (600 to 700) rpm with 16 degrees initial advance.

Hey redmist. Tell us about the no suspension travel thing.

This is a slight overstatement but not much. Besides the stiffest fiberglass spring I could find, the basic mechanicals are conventional, just stiff. Racing Bilsteins, poly bushings, shortned stiff springs etc. Taking about 150 lbs off the back and 200 off the front certainly contributes to the effect. Basically a Kart type ride.
Thanks for all the help. I ended up with the Edelbrock 795. I hope it just needs some tweeking. It still idles at 1k. kinda high. They said it was difficult to install. Does this sound right? It does get up and go on the highway but nothing on the line.
1K is high for an unmodified engine. What is your timing...initial and max at 3000?
First maximize you idle....turn each screw to achieve max rpm. Reduce rpm to approx 600-700.
Disconnect vacum adv at the distrib, and plug.
hook up timing light and turn distrib until total advance at 3500 is no more than 36 degrees Btdc
Play with springs & weights until initial advance and total is according to your objectives ( approx 10-16 and 36).
Then play with vacum.. adjust so advance at 80 mph intop gear is about 40-50 btdc.
now it gets a little hairy... if engine knocks reduce total vacum advance if possible (get adj vac adv) if not reduce initial advance to a max of resonable idle speed. If not enough (as far as total adv is concered) reduce max mech adv. If vac adv adj is possible reduce it.
Let me know if you need clarification.

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