The Stovebolt Site Forums The Shop Area Engine Shop Carburetor suggestions for stock 305

Hey guys quick question...

I have put a 305 engine and the 700r4 transmission into my 49 Chevy. My question is in regards to a carburetor setup.

The 305 engine came with a Rochester Quadra jet carburetor which needs a rebuilt at the very least. My question to you is what would your recommendation be for an intake and carburetor set up on that 305 engine?

I have thought about rebuilding the Rochester, or buying a brand-new Edelbrock or Holly. I'm not looking for raw power, I'm looking for a nice fuel-efficient dependable engine to use as a driver.

As always, thank you for your help.

Adam

Very few four-barrel carburetors have ever been made which could surpass the original Q-jet in fuel-efficiency on a basically stock street engine (and none made by either Holley or the e-clone).

Rebuild the Q-Jet.

You need no adapters, the air cleaner fits, etc., etc.

Jon

Make sure that the Q-Jet you are using is not a later model one that has the electronics. You may want to find an earlier model.

The Holley Street Avengers are great carbs, if you want to go that way. The 570 would be way more than enough for that engine, but with the vacuum secondaries it will work just fine.

An Edelbrock performer and the street avenger are a good combo.

Rebuild the Qjet. As already suggested, make sure it doesn't have the electronics.

Use an early 1980's Q Jet for a 305 engine. The electronic-control carbs are easy to recognize- - - -they've got a 2-prong electrical connector beside the air horn, and a 3-prong connector near the throttle linkage on the driver's side. There's no way to make a computer-controlled Q Jet work without all the electronics.

Be aware there are at least 200 separate part numbers for Quadrajet carbs, so be sure to get one calibrated for a 305 engine. DO NOT attempt to re-jet one made for a different-size engine!
Jerry

Thank you for the responses so far guys!

I definitely want to go with one that does not have electronics as I will not have much for electronics.

So do you guys have any suggestions as to where I look for one? Or maybe what years they started going with electronics? Or any other suggestions.

Thank you again!

Adam

Here's a link to 4 barrel Rochester carbs with part numbers for different size and year engines.
http://www.chevymania.com/partsid/carb.cfm?make=Rochester&model=4-brl

Hello Adam: I like Qjets also. The Edelbrock is a fine carb also. I have never had to "match" a Qjet carb to an engine. As long as it is non-E, it will work. They "sense" the engines need for fuel and are therefore an almost automatically variable carb. The 1974 mod quads and up are probably a bit better to get. QJets are mostly a 750CFM carb. Rebuild it and you are good to go. The Aluminum Edelbrock intake manifold that fits your app is what you want. The spread bore carb is the best mileage/performance for my money. You don't need to "dial" it in unless you are a racer. They are a bit more cumbersome to rebuild than the Edelbrock, which is a Carter AFB. Report the part number of your carb here, there may be some tricks and examination you need to do depending. Just my opinion and experience like everyone else here. By non-E they don't mean you can't use electric heat choke if you have that. But a real hot rodder don't need no wires to the carb. The manifold heated coil is good though. Looks like about Chevy 1980 and up can be electronic, I.E. E4M_.

Bart - minor disagreement.

Rochester made literally hundreds of DIFFERENT quadrajets. I have 609 different listed in my database from 1965~1975 (I don't list carbs above 1975, but there are many more).

Rochester did NOT make all of these different units just for the practice, or to spend money on R & D that could have been used for investor dividends. There are SIGNIFICANT differences.

So if your current Q-Jet is an electronic version, and you wish to acquire a non-electronic version; pick one that was originally calibrated for a Chevrolet, preferably a 305. You should also be able to recalibrate one for a 350 to work on a 305.

Since you will be using this with an automatic transmission, pick a Q-jet that was originally designed for use with A/T. Carbs designed for use with a standard transmission have different circuitry than those designed for use with an A/T.

Take a good look at the second and third lines in my signature.

And while it is true that probably most any Q-Jet will run on your 305, driveability and fuel economy will suffer if you try to use the wrong one.

Do your homework BEFORE you purchase, and your friends will be amazed at the results.

As to an intake manifold, if your engine currently has a factory Q-Jet intake USE IT!!! There will be NO gain and probably loss from using an aftermarket intake!

Jon.

You also might want to change the cam,alot of 305s had bad cams from the factory.My buddy just had the cam in his 305 go bad,so we put in a comp cam that was only a little better than stock with hyd. lifters.It's in a 50 AD now it's running better than it has in years.I wish I knew the cam # but he's got the cam card in his file.


Pete

Carbking correct me if I'm wrong. I'm thinking, but it been a bunch of years since I've run a Q/jet. That one for a stick shift, didn't have a place on the throttle arm to hook kick down cable for an automatic trans.

This is sometimes true. However, the big difference is that carbs designed for auto transmissions are calibrated RICHER than those for standard transmissions. There are fluid coupling losses in the torque converters that have to be compensated for in the calibration of the carburetor.

Just one of the reasons that an automatic transmission cannot give the fuel economy of a manual transmission.

The last time I changed out an automatic for a four-speed manual, my fuel economy (no other changes) rose from 17 MPG to 22 MPG plus octane requirements changed from 93 to 87.

Jon.

Understood, good advice. I am not an expert, All I can say is I have used many Qjets on many motors and never had a problem. Not matched or "tuned". Many of my friends have also. These motors probably did not perform to many peoples standards, but they ran good. That's the main reason I like them. I Don't mess with jets or metering rods. Some people I have known get over-hyped on all that from HotRod Magazine articles. I have added TV/kick down cables and all that. My real world experience matches what many say on the web. Many of the differences are simply improvements and design changes like types of chokes, Mod Quad, anti warp design, etc. I must agree that using the carb number supposedly for your engine has got to be good. Especially when it will not be hard to find a suitable one, once you get what numbers you want. You may already have the right carb.

This is from carb company in Canada.

"Because the secondary side of the Qjet opens according to the airflow requirements of the engine, the same basic carburetor size can be used on a large range of engine sizes. The basic 750 cfm QJet casting is used from 231 CID to over 400 CID - the big secondary air valves open only as the engine breathes. For example, on a mild 350 Chevy the air valve will never open all the way - because the motor can't use more than 600 cfm or so."

My comments are to give my point of view and is what I have done/experienced, Jon is the person that can advise as to what is best for you.

I would certainly agree on some were different for improvements:

(A) elimination of the internal dashpot
(B) replacement of the umbrella fuel valve with conventional
(C) replacement of the pressed-in well plugs with spun-in versions
(D) location change on the position of the float hinge pin
(E) others

But there were also many internal differences, especially in the placement and size of air bleeds, the size of idle tubes, bypasses, etc.

There were three different sizes of Q-Jets: 750, 800, 850.

1965~1969 all 750
1970~1975 most 750 (see exception list below)
1976~up mostly 800

Exceptions to 750 size 70-75:
(1) Most Buick 455 were 800
(2) Pontiac SD 1973 and 1974 were 800
(3) 1971 Pontiac H.O. and R.A. were 850
(4) 1971 Pontiac SOME S/T late production and S/R 7041263 were 850

The CFM difference was all on the primary side. The secondary side were basically 600 CFM variable depending on engine demand.

The 750 CFM have the small primary venturii
The 800 CFM have the large primary venturii
The 850 CFM have the small primary venturii with the idle booster venturii removed.

The discussion on how much CFM is necessary has disturbed MANY pixels on the internet. I have an 850 CFM on my own Pontiac 350 (and it uses it all, plan to try a 1000 CFM if I have the time) however, my 350 is no longer mild.

The change to the 800 (slightly larger primary) improved city and cruise driveability over the 750 for engines of 350 CID and above.

The book on Q-Jets by Cliff Ruggles is one of the top three (opinion of mine) carburetor books that has ever been written (at least in English). I would highly suggest this book for understanding the Q-Jet to any one, even experienced carburetor rebuilders.

And Bart, I wasn't trying to pick on you at all, merely to offer additional information.

One last comment: maximum CFM only comes into play at sustained W.O.T. Idle and cruise driveability and fuel economy are the result of holding at constant as possible A/F ratio at these RPM's. Those that understand the metering rod design and use it in tuning may see a large increase in fuel economy.

Jon.

Jon, it was I who was afraid of insulting your expertise. You are well liked, honest, very knowledgeable. I hope you will not think I am trying to challenge you or anyone else with my .01 cents. I try not to just pull things out of my tail pipe. I tell my experience and study other sources. Books, internet, your web site, etc.

I did some checking and here is the one a carb guy from this company said would work. My only question is that you guys kinda told me to stick with something from the 70's??? Will this work IF it is for a 305?

http://www.ebay.com/itm/321379397410?ru=http%3A%2F%2Fwww.ebay.com%2Fsch%2Fi.html%3F_sacat%3D0%26_from%3DR40%26_nkw%3D321379397410%26_rdc%3D1

Thanks!

Adam

Thanks Pete for the cam hint. We are going to change the cam out to fix the stock issue your friend might of come across. The guy who is rebuilding my engine knew exactly what cam was needed and I remember him saying it was mild but effective.

Just post the number of Ebay one and yours here. We need the numbers.(70xxxxx, 170xxxxx) Your link no good.

The carb number is 180-6892

They list it as a Vapex part number. Also if you copy and paste the ebay link into a new browser the carb part comes right up.

So many thanks for all the help!

That carburetor should work just fine. I see that it has an electric choke so you will need to run 12V wire to it that is controlled by the ignition switch.