Carburetor for 350ci

[davepink53]

Hi all. I am not happy with the quadrajet on my '71 350ci Avanti Automatic. Anyone suggest a modern replacement carb to suit?

Thanks in advance

Dave...

[Gunslinger]

The Quadrajet is an outstanding carburetor. I'd look into having it rebuilt...by someone who knows how to and understands them. Many will just clean it, install new gaskets and whatever else in the kit and call it rebuilt. A good carb tech will check the needles and seats, will re-bush linkage entry points where needed, etc. For a street car you won't do better than a Quadrajet in good working order.

A Q-jet is a spread-bore design...to replace it might be difficult since most aftermarket carbs such as Edelbrock and Holley are square-bore and would require an adapter to fit your intake manifold. The extra height of an inch or more would likely create a clearance problem with the hood. Holley does make some spread-bore replacement carbs as an option.

If you're really determined to replace your Q-jet, I'd say go with an Edelbrock Performer or Performer EPS intake manifold and an Edelbrock 1406 600cfm carburetor. The standard Perfomer will take either square- or spread-bore carburetors. The EPS is specifically optimized for a square-bore carb and posts better horsepower and torque numbers, but not by a huge margin. Holley makes 600-650 carbs as well but I like the Edelbrocks...no power valves to blow out, they're easy to adjust and hold their adjustments. Normally I would prefer a Barry Grant Demon carb before Holley, but Barry Grant went out of business, though I've heard someone else picked up the product line...don't know if that's true and if so, how good the product will be.

Another thing...are you sure your Q-jet is the cause of whatever problem you're experiencing? It may be or may not be...isolate the problem before deciding what to do. If it's never been rebuilt before, it certainly could use it by this time.

[WayneC]

Hi all. I am not happy with the quadrajet on my '71 350ci Avanti Automatic. Anyone suggest a modern replacement carb to suit?

Thanks in advance

Dave...

Why are you unhappy with your quadrajet?

I'd echo what Gunslinger said, for driveability and fuel economy in a 4-barrel, it's probably impossible to beat a Rochester quadrajet. Some say an Edelbrock quadrajet-clone is better, that they improved a few things in the design, but I've never owned one and I'm always suspect that a generalized clone can best a carburetor that was tweaked to the exact year Chevy engine and vehicle, on dynamometers and at the GM proving grounds, by Chevy and Rochester experts, just before the car was first sold.

BTW, I think Gunslinger may have given you the wrong Edelbrock carb number, the small block takes a 1901, not a 1906.

- http://www.edelbrock.com/automotive_new/misc/tech_center/install/1000/1901.pdf

Here is a rebuilder that has troubleshooting tips on their website, under the philosophy that not all "carburetor issues" require a carb rebuild...

- http://www.recarbco.com/technical/

I believe this was the outfit that rebuilt my own '71 quadrajet a few years back, I can check for certain by searching old files, but I realize you are in Australia and will probably want someone closer. They do say they sell rebuilt carbs outright, but apparently you'll need to email them to find out if they can supply one for your application (here's how to identify your carburetor: http://www.recarbco.com/technical/rochester/qjet.html )

You haven't said whether a carb rebuild is a project you'd undertake yourself; I've owned a lot of Chevy's over the years and done a number of rebuilds, but in the distant past, and I'd be out of my depth if the carb needed new butterfly valve bushings, or any other sort of machining, in fact I probably wouldn't even recognize the need for it... I've only done parts replacement, cleaning, adjustments, and played with jet sizes.

Here's a couple of related articles of interest I bookmarked at some point...

- http://www.carcraft.com/howto/57178/index.html

- http://quadrajetcarburetors.com/

- http://quadrajetcarburetors.com/bushingkitinstructions.html

Edited November 7, 2011 by WayneC

[Gunslinger]

BTW, I think Gunslinger may have given you the wrong Edelbrock carb number, the small block takes a 1901, not a 1906.

- http://www.edelbrock.com/automotive_new/misc/tech_center/install/1000/1901.pdf

I understand the misconception, but I was talking about an Edelbrock 1406 Performer carburetor based on the AFB, not Edelbrock's version of the Quadrajet. Their Q-jets were discontinued several years ago and are no longer available. I understand the tooling was destroyed for some reason. They had an excellent reputation but I don't know why the tooling was destroyed (if it actually was)...the carbs were expensive compared to their AFB clones and may not have sold very well.

[WayneC]

I understand the misconception, but I was talking about an Edelbrock 1406 Performer carburetor based on the AFB, not Edelbrock's version of the Quadrajet.

My mistake, sorry, somehow I read 1406 and mentally transposed to 1906.

[davepink53]

Hi again. I had my carb rebuilt first up. The car drives nice on the road but when you put your boot into it, it sort of bogs down, you know, a bit sluggish. and most mornings needs a small amount of fuel down the carb to start. once its going, it will start fine all day. I think ( with my limited knowledge ) that the accelerator pump is not working properly. The guy who rebuilt the carb has gone now. He charged me $350 for the rebuild. Thank you all for your input.

Dave..

[Devildog]

Hi again. I had my carb rebuilt first up. The car drives nice on the road but when you put your boot into it, it sort of bogs down, you know, a bit sluggish. and most mornings needs a small amount of fuel down the carb to start. once its going, it will start fine all day. I think ( with my limited knowledge ) that the accelerator pump is not working properly. The guy who rebuilt the carb has gone now. He charged me $350 for the rebuild. Thank you all for your input.

Dave..

That is your is your accelerator piston not squirting gas into the carb. In morning when you pump a couple of times it should squirt a little gas. And, when you put your boot into it those two little 'snake fangs' should squire extra gas into the carb as you open the throttle.

Easy to check. pull the air cleaner and actuate the carb linkage by hand. If it does not squirt gas out those two 'fangs', little piston is NOT working. When you rebuilt, I am sure you saw the piston, they sometime dry out when sitting.

Sounds like that is your problem this time.

But, in general 90% of carb problems are really ignition issues on a small block Chevy. Here is a great primer on tuning the ignition on a SBC. On most any SBC, within 20 minutes you will find 20 hp and 30 ft.lb. of torque for no $s.

How to Set Your Timing for Peak Performance

(Non-HEI)

by Lars Grimsrud

Colorado Corvette Crazies (CCC)

The Ultimate Corvette Tuning & Beer Drinking Fraternity

Lafayette, CO Rev.B 4-18-01

This tech paper will discuss setting the timing on a ChevyV8. This procedure also applies to otherGM V8s.

The procedure outlined here differs from the ServiceManual, and is based on my years of experience doing this work in the quickest,least painful, most economical way while keeping the level of qualityhigh. It is recognized that other peoplewill have different methods of doing things, and may disagree with specificmethods and procedures that I use.

Howto Set the Timing

When you thinkabout it, setting the timing at idle speed makes no sense at all: You don’t operate your car at idle, andtiming changes as the rpm changes. Fact is, the timing spec at idle speed isprovided as a simple way for most people to set the timing, and is not a goodprocedure for optimum performance.

Small blockChevys (and most other GM performance V8 engines) perform best when the totaltiming (full centrifugal advance plus the initial timing setting with vacuumadvance disconnected) is all in by 2,500 – 2,800 rpm and is set to 36 – 38degrees. If you have an adjustabletiming light, this is very easy to check. If you don’t, you need to scribe a 36-degree mark on your harmonicbalancer. Here’s how:

Measure thecircumference of your harmonic balancer using a sewing tape measure (or otherflexible tape measure). Get it as accurate as you can. Take this measurement and divide by 10. The number you get is the distance to 36degrees. Measure this distance CLOCKWISEfrom your existing harmonic balancer timing mark and place a clear mark on thebalancer.

Remove yourdistributor cap and rotor. Remove the 2centrifugal advance springs. Install the rotor and the cap (without thesprings). Disconnect the vacuum advance.

NOTE: This procedure cannot be used on the HEIignition systems. Removal of the springs will cause an artificiallyover-advanced condition that will never be achieved with the springs inplace. You can use the basic techniquedescribed in this paper with the HEI units (setting timing up to 36 degrees),but to check total timing, you must install a set of soft springs. You cannot remove the springsaltogether. With the soft springs inplace, rev the engine until the centrifugal advance is pegged out. Adjust for 36 degrees total. Then re-installyour original springs.

Start the engine. It may kick back a little due to the advancecoming in immediately without the springs. If you’re using an adjustable timing light, set the light to 36 degreesadvanced. Now rev the engine just a littlewhile observing the timing marks with the light. It shouldn’t take much rpm to peg out theadvance without the springs installed. With an adjustable light set at 36degrees, align the stock timing marks with “0” when the timing is “peggedout.” With the non-adjustable light,align your new 36-degree mark with “0.” Rev the engine a little to make sure the timing will not advance anyfurther. Shut it down.

Pop the cap androtor and re-install the springs. Puteverything back together, but leave the vacuum disconnected. Start it up. For future reference, make a note of the timing setting at idle. This is your new curb idle timing spec. Now give the engine a few quick rev’s past3,000 rpm and verify that the full timing (36 degrees) is coming in. If it’s not, you need to change to a softerset of springs until you get full 36-degree advance before 3000 rpm. (NOTE:A stock set of springs will usually not allow full centrifugal advance to comein before redline rpm. If you have stocksprings installed, don’t rev the engine beyond its limits to try to force fulladvance in.)

Shut it down and hook up thevacuum. Now do a road test.

The 36-degree2500 rpm advance curve is optimum for performance, but may require premiumfuel. Lug the car around, and punch thethrottle at low rpm while listening for detonation (“engine knock”). If you’re getting any audible knock, you MUST retard the timing. Retard thetiming in 2-degree increments until engine knock stops. Engine knock will seriously damage enginecomponents if not corrected. If you getno knock, you may see slightly improved performance at 38 degrees totaltiming. This is particularly true ifyou’re running at high altitude.

If you have noengine knock under acceleration, but the car “chugs” or “jerks” at cruisingspeed (light throttle application), you are getting too much vacuum advance ontop of the mechanical advance. You mayneed to change out the vacuum advance diaphragm with an adjustable unitavailable from aftermarket sources. Adjust these units so that you get the most vacuum advance possiblewithout any “chugging” or “jerking” at cruise speed.

Your timing isnow set for best possible performance. Make note of the new setting, and use this for your future tune-up work.

Questions, Comments & TechnicalAssistance

If you have questions or comments regarding this article,or if you notice any errors that need to be corrected (which is quite possiblesince I’m writing this from memory…), please feel free to drop me ane-mail. Also, if you need any technicalassistance or advice regarding this process, or other maintenance issues, feelfree to contact me:

V8FastCars@msn.com

[davepink53]

Thanks for that info. Could be a bit beyond me though.

I have tracked down the guy who rebuilt the carb for me. He has closed his shop but now works mobile, so he is coming tomorrow morning to look at the problem for me.

Dave..

[davepink53]

" in general 90% of carb problems are really ignition issues on a small block Chevy"

this is good advice from Devildog.

The carb guy has been and found nothing wrong with the carb!

He pulled the coil lead from the dist cap and had me crank the engine, which produced a very weak spark.

I eventually got the engine running and left it idling for 5 mins while we talked. then shut it off and pulled the lead again, this time a strong spark! He said that the coil was probably faulty. Guess I will buy a new one and see what happens.

Regards, Dave...

Edited November 9, 2011 by davepink53

[Devildog]

" in general 90% of carb problems are really ignition issues on a small block Chevy"

this is good advice from Devildog.

The carb guy has been and found nothing wrong with the carb!

He pulled the coil lead from the dist cap and had me crank the engine, which produced a very weak spark.

I eventually got the engine running and left it idling for 5 mins while we talked. then shut it off and pulled the lead again, this time a strong spark! He said that the coil was probably faulty. Guess I will buy a new one and see what happens.

Regards, Dave...

Dave, I wish I could give you some guidance on a new coil. It use to be straight forward replacement, but now days there are so many junk coils out there that me and many of my car buddies have had to replace new coils multiple times. Seems if you can find a metal can AC Delco, they have a pretty good QC.

At least they are easy to swap out

Joe

Edited November 9, 2011 by Devildog

[davepink53]

Thanks Joe. I will see what I can buy for it.

Dave...

[Gunslinger]

He said that the coil was probably faulty. Guess I will buy a new one and see what happens.

Regards, Dave...

Only forty years old and the coil is bad? They sure don't make stuff to last, do they? It's no wonder GM is in the condition they are...this planned obsolescence is killing the industry!

[davepink53]

Only forty years old and the coil is bad? They sure don't make stuff to last, do they? It's no wonder GM is in the condition they are...this planned obsolescence is killing the industry!

Ha Ha Ha lol :rolleyes: :rolleyes:

Dave...

[davepink53]

I went to 2 parts shops and told them what I had and what I need. They both gave me a blank stare. Had no idea! I guess that I will have to do some asking around to get a suitable coil for it.

Dave...

[Gunslinger]

One thing to keep in mind...in 1970 Avanti Motors began using a transistorized capacitive discharge ignition so your '71 should still have it...unless someone changed it out over the years. That may take a different application ignition coil, but I really don't know that for a fact. If your existing coil has any readable numbers on it, try and cross-reference them to what's available now. It may use a standard coil...if so, it will be for a '71 Corvette 350 which is probably a generic GM coil for the time period.

Coils are one of the most over-hyped pieces of equipment out there. With few exceptions, a quality original equipment replacement is all that's needed. All the performance companies hype their coil's voltage output, but none tell you that it's the coil's saturation time...its ability to quickly recover and fire the next spark that's more important. If you have a 45k volt coil and it only requires 30k volts to fire the spark across the plug gap, that's all the coil will put out. Any quality coil will put out sufficient spark up to its maximum ability to do so. The saturation, or rise time, between voltage pulses is more important.

[WayneC]

Hate to admit, but I've owned my '71 so long that I can't remember what ignition it originally had, but I do not recall it having an old-style transistorized ignition... I changed out the engine for an '84 Vette engine in the early 80's. I also once owned a '66 Avanti that I converted to a later engine and an HEI ignition back in the late 70's.

I may be mistaken, but I think the '71 came with the old points-style distributor and separate cylindrical coil.

If that's the case, then maybe the following info would help you find a coil...

Delco lists the 1971 Corvette coil (shown as the old style cylindrical coil) as

"COIL ASM,IGN Part Number: E555"

Or, you could check junkyards for Chevy V8 cars of about 1971 vintage for a used coil.

Or maybe your parts suppliers can find cross-references to these coils listed by NAPA:

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?A=ECHIC1600_0166500374&An=0+599001+101971+50012+2012030&Ar=AND%28P_RecType%3aA%29

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?A=BK_7355274_0298616150&An=0+599001+101971+50012+2012030&Ar=AND%28P_RecType%3aA%29

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?A=BK_7355273_0298616149&An=0+599001+101971+50012+2012030&Ar=AND%28P_RecType%3aA%29

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?A=BK_7357250_0326350345&An=0+599001+101971+50012+2012030&Ar=AND%28P_RecType%3aA%29

Or, of course, you could consider swapping in a later HEI distributor from a mid-to-late 70's Chevy V8...

you'd also need to replace the spark plug wires with 8mm or 7mm, along with their support guides, and run a non-resistor wire to the distributor (to replace the current pink resistor wire); if you acquire the setup from a junkyard, also clip the connectors for the 12v power and the tachometer from the wreck to use to attach your wires to the HEI.

Corvette-style ignition shielding may be needed if AM radio reception is important to you.

Lastly, you could probably order from a US Corvette parts vendor (assuming they'll ship to Australia), such as this one: http://www.docrebuild.com/dr-r-web/COIL.pdf

Edited November 11, 2011 by WayneC

[Gunslinger]

I may be mistaken, but I think the '71 came with the old points-style distributor and separate cylindrical coil.

To look at the system Avanti Motors installed you wouldn't tell the difference at a glance. The distributor and coil looked the same as a standard GM window type unit. The coil was a round can type...maybe the same coil but I don't know for sure. About the only way to tell it was electronic would be to remove the distributor cap and see no points and condenser. There would also be a capacitive ignition box mounted forward on the inner panel on the passenger side of the engine bay. My '70 was equipped that way when I purchased it. It was made by Delco but I have no idea if GM installed that particular system on any of its own production. GM installed many of their K66 transistorized ignitions on many of their high performance engines and was optional on other cars.

Avanti Motors started installed the CD ignition in 1970 with the RQB series as standard and advertised it as such. They probably used it until replaced by GM's HEI, which is an excellent system. Of course, Avanti Motors could have stopped installing the CD ignition at any time for any reason.

[WayneC]

There would also be a capacitive ignition box mounted forward on the inner panel on the passenger side of the engine bay. My '70 was equipped that way when I purchased it. It was made by Delco but I have no idea if GM installed that particular system on any of its own production. GM installed many of their K66 transistorized ignitions on many of their high performance engines and was optional on other cars.

If my '71 came with that, I think I'd remember it, because I was aware of the K66 setup installed as an option on '64 to '71 Hi Perf Corvettes... I once bought some of the parts for that Corvette system at a swap meet back in the mid-70's, including the finned cast aluminum amplifier box that mounts on the inner fender; but I never installed it, rather I sold it to a mid-year Vette owner whose amplifier had failed. That amplifier tended to be a problem part (as the equivalent part seems to be on aftermarket transistorized ignitions like Accel), so it's possible the previous owner of my '71 reverted to the basic points system.

Here's a vendor on the internet that still services K66:

http://www.tispecialty.com/amplifiers.htm (sells parts, but $$$$)

http://www.tispecialty.com/services.htm (photo of the fender-mounted amplifier)

http://www.tispecialty.com/coils.htm (sells the coil)

Another vendor on the internet that sells replacement parts for K66 (coil, circuit board):

http://www.lectriclimited.com/shopping_cart/ff_catalog.asp?id=676501993

Edited November 12, 2011 by WayneC

[davepink53]

Thanks for all of the advice. My '71 has the window type unit with points and condenser. I installed the wrong coil first, a Bosch GT40R resistor type. the motor fired straight up and ran for a few mins before backfiring and cutting out. Installed a standard type coil and it is running great.

Dave...

[Gunslinger]

Great to hear...especially when it's a simple fix. Sometimes the simple fixes are the the one's that can give the biggest headaches.

[drat]

great thread, great info. I have a 69 with the 350 and a Q-jet as well with a 3 speed auto Borg-Warner.

I have a similar problem of bogging when you put your foot into it in drive. It bogs when the tranny kicks down to second and catches up slowly.

It will not bog when you manually shift. Could this be a similar issue.

The 3 areas I intended to check were,

carb

timing/ignition

transmission

[ernier]

A few extra cranks before starting if the car sits for awhile is not unusual. With my toys I spin them a few seconds before T touch the gas pedal and set the choke. It fills the carb bowl and builds up some oil pressure before the engine kights.

There are a couple of things to check on The QJet. If the choke doesn't open all the way there's a tab on the linkage that will keep the scondary air valve from opening. It's a very fine adjustment You can visually check it or while the car is running and fully warmed just push on the air valve. If you can open it you're OK. The air valve is held closed by vacuum so there shjould be some pressure applied before it will open. If it opened too easily you might have a leak and the valve could be opening too quickly causing the bog.

The weak visual spark from your coil could cause the hard start but it's not a good test of the coil while the car's running. If you have a points type distributor the start circuit is a full 12 volts in the coil but when it's running it's reduced to around 9.5 to keep the points from burning up. I've seen more condensors cause hard starting than coils. In the 70's when i had a couple of gas stations I could cure a hrd morning start by removing the condensor and wiping oil from the bracket and breaker plate. I converted my '64 to Pertronix and it started easier and idled a lot smoother than with points.

Back to the Q-Jet...Check to see if the carb is opening fully at full throttle. I had a '69 427 Corvette that went from ho-hum to holy Crap! with a throttle cable adjustment to get that big secondary to do its job. They seem to loosen up easily also so re-check the hold down bolts and the top screws themselves.

Lastly, Before you buy an alternate carb you will have to use a fairly thick adapter to mate a square bore carb to a spread bore manifold. You may not have enough hood clearance.

ErnieR

[brad]

Rochester Quadrajet Carburetors are very adjustable. The secondary air valve is spring loaded, and adjustable as to when it comes into play, based on the airflow vs spring pressure. The secondary valve, also has a plastic cam which sets the rate of the lifting of the secondary metering rods. They are much like the main primary metering rods in Carter and Edelbrock carburetors, except they are mechanically linked instead of vacuum operated.

The spring is adjusted to eliminate the bog, so you don't get too much air before the engine can use it. And the cam can be changed to one that lifts the metering rods quicker, to enrich the mixture sooner.

Also, there is an area between the main casting and the lower throttle plate,(just below the primary jets),that is prone to an internal leak. There used to be a kit with aluminum plugs with o-rings that replaced the welch plugs.

These plugs look like tiny cup type freeze plugs. I just usually JB weld them and let the body set-up over night.

The main inlet for the fuel line, sometimes has a pleated paper filter that has never been replaced! Other times it is a sintered bronze filter, or a fine brass screen. Extreme care must be taken when removing the large inlet fitting, as these can strip out just by looking at them wrong. There are replacement fittings that are slightly oversize, and self tapping as a repair. The accelerator pump is often just a replacement rubber that slips over the old pump shaft after removing the old rubber. Try to find a kit that has a complete new pump.On some carbs the base of the pump is the inlet check valve.

Brad