Carb question

[dracor1117]

Just curious about a couple of things dealing with carbs. My understanding in choosing a carb is that the more horses you unleash, the more thirsty the engine is, thus... the bigger the carb. How big is too big and what is the effect if you have too big.
example: My 79 L-82 was rebuilt to produce somewhere in the 400 horse range. It's not been dynoed yet, so I can't be sure what it's actual output is.

 

[Jack]

Safe bet that your Qjet will be plenty ... it's rated at about 750cfm ... just freshen/tune it.

 

[dracor1117]

Sorry, let me try and clarify a little more. The engine has the original block, Dart Iron Eagle heads, Crane cam, EB performer intake and a Holley Street Avenger 870 on it.
I had a local performance shop doing all of the work since I'm still rather new at all this and went with their guidance on parts. The carb was added after the rebuild when it was clear that the engine was starving with the old EB 600 that was on it. Since the 870 was added, the engine sputters in lower rpms, but screams in the mid and high range. I don't have to give it more than half of the pedal range even when hitting it hard, and because the primaries dump so much fuel, I'm not sure that the secondaries have done much more than make sandwiches and drink beer while laughing at the primaries for doing all the work.

 

[Sensei]

I'm personally not that familiar with your particular new carb (I'm a Q-jet guy), but the Edelbrock 600 could have been tuned for the set up you have and worked fine. I have a friend that built a very beefy 350, and found it easier to tune an Edelbrock 600 up than a 750 down to get top performance. Sounds like you may have the classic "too much carb" problem (often accompanied by the "too much cam" problem)! Good luck and God bless, Cris

 

[JohnZ]

Since the 870 was added, the engine sputters in lower rpms, but screams in the mid and high range.

You need to decide what your objective is for the car - if it's normal street driving, you're way over-carbed for 95% of your operation. If it's purely WOT high-rpm operation (unlikely), you'll have to accept poor driveability for 95% of your driving. There's no free lunch - either way requires compromises. There's no way any streetable 350 requires more than 700CFM, and most are a LOT happier in normal driving with a 600 or 650.

 

[Evolution1980]

The original Q-Jet 750 cfm woulda been perfect for your setup. Most people/shops will steer you towards anything other than a Q-jet because they don't want the task of servicing/adjusting it. However, a properly setup Q-jet 750 is a fantastic carb for a car setup for 'modified street', such as what yours sounds like. The Q-jet 750 has the smaller primaries which are perfect for burbling around town and the massive secondaries that will open up to feed the need for speed. If 870 is representative of the CFM's of the carb, yeah, that's way overkill for your engine. Even at WOT you'll still have more gas available than the engine can/will use...

 

[dracor1117]

Thank you all for the advise and input. Sounds like I need to rebuild the old EB 600 and pull the 870 and sell it or both. I'll decide when I get to that point I guess.
I do have a goal for my car. I would like to be able to go to the strip or autocross and be able to perform well at either. I don't need to be top dog, just be able to have fun without embarrasing the car for the poor performance that it was delivered with.

 

[Evolution1980]

Sounds like I need to rebuild the old EB 600 and pull the 870 and sell it or both. I'll decide when I get to that point I guess.
I do have a goal for my car...I would like to be able to go to the strip or autocross and be able to perform well at either

Sell 'em both and buy yourself a Q-Jet 750. With your setup, I could probably even tell you the correct metering rods and all that to save you some of the guess work. Or at least get you close...

 

[dracor1117]

That sounds good Evo. I'll keep you posted as I get there. Probably be a couple months before I get there though.

 

[rgtrough]

What Carb?

Hey Dracor, I just recently purchased a Q-Jet completely restored to original specs for my '74 shark. I have tried many different setups serching for the one that is just right. Hopefully this will do the trick. I have been listening to the members here praising the Quad that is set up right. I ran into a carb guy who has set up shop just up the street from me. He tells me he does alot of mail order business. If you want his phone number let me know. I haven't installed the carb yet, I had shoulder surgery last month but will post my findings.

Evo, I hope that your very generous offer of help is extended to me. This carb is off of a 1979 L82. He restored it to spec. I am sure with my mods I will need some adjustments.
Thanks Bud

 

[Evolution1980]

Hey Dracor, I just recently purchased a Q-Jet completely restored to original specs for my '74 shark. I have tried many different setups serching for the one that is just right.
Evo, I hope that your very generous offer of help is extended to me.
I am sure with my mods I will need some adjustments.

Yeah, absolutely! It's not any secret or anything. I have a ZZ4 which is spec'd at 355HP and 405 Tq. 10.5:1 compression. I have a pretty good baseline right now with the needles and whatnot that were changed out in the late fall right before I put the car away. I noticed a nice difference right from the get-go. Before, I was was dumping too much gas when I punched the throttle and would it would hesitate. Now, it's more immediate, the way it should be.
I'm not at home right now, but when I get the chance, I'll go out and see what I have written down in the ol' log book for carb settings. When I do, I'll post the info for ya guys. I'm gonna be hitting the dyno as well in the next month or so, so I can really get it dialed in and therein find the best definite carb setup.

 

[dracor1117]

Thanks rgtrough, I would like the number for that guy. My current plan is to rebuild the old EB 600 and sell it, then buy the new carb before I pull the Holley 870 and sell it. I ordered the rebuild kit for the 600, so I guess I know what I'll be doing next weekend. I think the cheapest I've seen for a Qjet is around $340-$350 for a rebuilt. What is this guy asking for his?
Evo, I appreciate all your help. What you described with the hesitation is exactly part of the problem I'm having now. When I had the new carb put on, it was supposed to be in the 700-750 cfm range. When I picked it up from the shop, and noticed the 870 on it, I didn't have time to mess with it because at the time, the vette was my daily driver. Now I have another car and don't have to worry about how long she's down for repairs or mods. There's not really been a winter here in Texas this year, so I haven't really had to put her away. There's at least one day every week when I can put her on the road, which is great.

 

[bobs78]

Safe bet that your Qjet will be plenty ... it's rated at about 750cfm ... just freshen/tune it.

View attachment 9746 If your thinking of doing it yourself this site should help Quadrajet Manual http://www.tocmp.com/manuals/Carbs/Rochester/QJet/HTML/MCarbRoch1973_4M_0001.htm

 

[dracor1117]

thanks Bobs78. I've got it bookmarked and will check it out.

 

[gmjunkie]

You need to decide what your objective is for the car - if it's normal street driving, you're way over-carbed for 95% of your operation. If it's purely WOT high-rpm operation (unlikely), you'll have to accept poor driveability for 95% of your driving. There's no free lunch - either way requires compromises. There's no way any streetable 350 requires more than 700CFM, and most are a LOT happier in normal driving with a 600 or 650.

I have to agree with JohnZ and Evo!! Bigger is not Better!!! My 78 L82 runs Like a Raped Ape with Stock Q-jet!! Just my 1/2 cents worth!!
I'm a Hillbilly ya know!!
bang bang bang ps. If you stick with Holley,Pay up your Fire Ins.

 

[bossvette]

I always was a Holley man before I switched to EFI but you should be able to get a Qjet for very little money from a junkyard GM used them on everything for many years. Holley even makes/made a Qjet replacement with the small primaries and big whopping secondaries. I will look in my barn and see if I still have on laying around but I doubt it.

 

[Sensei]

Decent used Q-jets can be bought on ebay for around $15 and kits can be purchased at auto parts stores for around $10. Another $10 for a gallon of laquer thinner and you're in bussiness. I have built a few of these for friends just for fun. It only takes a few hours, including some easy performance mods.

 

[Drjohn]

Sorry, let me try and clarify a little more. The engine has the original block, Dart Iron Eagle heads, Crane cam, EB performer intake and a Holley Street Avenger 870 on it.
I had a local performance shop doing all of the work since I'm still rather new at all this and went with their guidance on parts. The carb was added after the rebuild when it was clear that the engine was starving with the old EB 600 that was on it. Since the 870 was added, the engine sputters in lower rpms, but screams in the mid and high range. I don't have to give it more than half of the pedal range even when hitting it hard, and because the primaries dump so much fuel, I'm not sure that the secondaries have done much more than make sandwiches and drink beer while laughing at the primaries for doing all the work.

Bigger is not always better. I'd rather see you buy a smaller Holley, but if you really want one of those Q-jets, then you could even get one of these (depending on core availability): http://www.holley.com/BrowseCatalogs.asp?Catalog=Reman&Page=282

Given your comments regarding the performance shop above, it would save you rebuilding one yourself, which isn't really that hard to do.


You could always go with a smaller Street Avenger:

http://www.holley.com/types.asp?type=7

 

[bossvette]

Does Holley still make a Qjet replacement?

 

[Drjohn]

Does Holley still make a Qjet replacement?

I think this is what you're referring to: http://www.holley.com/0-80555C.asp

We sell quite a few actually.

Buy as many as you want. I have a son in college.....