I just got a nice 74' coupe, 4-spd :^) and it has the following engines mods to the matching number 350: Speed Pro H617CP 10.25:1 pistons (based on my stock cylinder hear volume) , new Sealed Power rings, Pro 2000 series CS1014R mild cam, new valve springs, Eglin Industries stock replacement lifters, hi-pressure oil pump an old, and a NON-vette' Rochester carb. It is a non EGR model carb and frankly the egr system is pretty much gone from my car. Also has 2-1/4 pipes and Flowmaster mufflers - it is too loud for me, but that is another issue all together. The throttle shaft is binding somewhat and it drips fuel while idling (you can see it when looking down into the primary venturi area. It idles ok - sometimes, but I don;t think the secondaries open up when you give it full throttle on the road. I am simply going to replace it with a reman from Ecklers and sell this one on Ebay. The number on the carb is 7045213 and it does not cross reference in my 'Corvette By the Numbers' book. I will either install an aftermarket manifold or blank-off the EGR valve.

BTW: with the A/C on at 70 mph, 3200 RPM, with the A/C on all day it runs at about 210 degrees & idles at 190 or so - is that too hot??? Maybe its the new radiator??? (The engine compartment will get a total refresh this Winter)

My question is, based on the HP that my car is estimated to be producing (maybe 225???), what year Rochester should I put on it? I DO NOT want an EGR carb that I would get with a 74' carb. When I get the new one installed I will remove the EGR valve that is bolted to the manifold on the choke-side of the carb. I honestly don't know what the EGR valve does since it has only a vacum line that runs from it to the water pump elbow on top the manifold.

Thoughts???

 

up for attached photos.

 

Stop! Do NOT buy a remanufactured carb! Have a reputable carb shop rebuild the one that you have, specifically for your combination.

If you really want a different carb to rebuild, your best bet would be the latest model non-computer controlled Quadrajet that you can find.

 

What I have learned from being a single man for 7 years:
The sexier the girl, the more trouble they get you into!.....and the same with carbs:

Least sexy: Rochester AFB....ugly, ugly and ugly.....but super reliable
Middle sexy: Edelbrock's version of the AFB....ugly but looks like chrome
Sexy: any Holley.....sexy look, sexy name, and will cheat on you every time!
you will have gasket leaks, power valve problems, starving jets, throttle leaks, and fuel spilled everywhere when you change jets

So all said and done.......keep the carb you got.....rebuilding it is super easy with an Autozone kit for $20 and a can of carb cleaner.

By the way, you are correct, your EGR is only hooked up to the switch with no vacuum applied to the other nipple on the thermostat.........the EGR is not working and only along for the ride. My vote is to eliminate the EGR completely.

Lastly, ditch the points distributer and coil and put in an HEI with a super coil.....your high compression engine needs better spark and more initial timing. You shouldnt "feel" a correctly functioning secondary....it should just keep "pulling".

 

...did more research with a carb expert repair shop and the carb is from a '75 vette and likely just needs throttle bushing kit. They said that a little fuel bubbling of the venturis out during fast idle is ok, but should not bubble out during normal idle and that's how my carb is working now, so I'm good to go except for throttle shaft bush.

BTW: I called Summit racing to order my HEI distributor (250-some odd bucks!) and while talking to the sales guy, he said the EGR system should be left on!?!? I ordered an EGR blanking plate anyway.

 

Your research is not correct. Your carb is a 1975-1976 GMC Heavy Duty Truck carb - it is not a passenger car carb. The 1975 Vette carb is an intergral choke carb and cannot be used with the choke system on your '74. The truck carb can be made to run fine if correctly set up and tuned for the application. There is also nothing at all wrong with running an "EGR" carb. Just put a cap on the port. The 1974 carbs are probably some of the best running and most easily tunable Q-Jets out of all the divorced choke Q-Jet models.

You will gain nothing by going to the HEI. A properly curved points-type distributor with a good set of points will provide an outstanding level of performance in your engine - unbeatable by any HEI unless the HEI has been curved more aggressively. Skip the distributor swap and concentrate on curving and tuning what you have - you'll be much further ahead.

Lars

 

Lars - Terrific info, thx! No wonder it has such strong lo-end - it's a monster truck Vette'! I plan to blank off the EGR port. For now I still plan to keep this carb unless it comes back from the the repair shop with a problem. The car runs great with it. Starts up perfect when cold and starts perfect when hot - i really can't ask for much else. My distributor has lighter advance springs, but I don;t think it was curved on a bench. Does anyone even do that anymore???

 

You'll gain nothing by removing the EGR valve. The Summit guy was right.

 

Why is he right - what is the science or physics that support the assertion? The Summit guy was adamant that having the EGR valve was a good thing and actually improved gas milage and accordingly should be left in place. I asked why then should'nt all late 60's or eary 70's muscle cars repalce their manifolds and other ancillary emmissions gear and retrofit an EGR valve and he didn't have an answer. Maybe modifying the system to accomodate another EGR valve would be twice as good???

Can anybody explain why having an EGR valve left in place is a good thing? I'm all for not touching goodness

 

To clarify what Summit and Mike are saying:

There's no "science" behind what Summit is saying - it's the law. Tampering with, or rending inoprative, an emissions control device is a Federal crime, regardless of what State you're in or if emissions testing is required in your State. Summit cannot, by law, advocate you commit a crime or advise you to remove a Federally mandated emissions control device. The Summit tech guy would end up in Leavenworth if he told you to remove emissions equipment from a street driven vehicle. Removing the EGR is illegal, no matter how old your car is and no matter what State you're in. The law just isn't enforced much by Federal authorities, so many people alter their cars. That doesn't make it legal, and an employee of a company like Summit cannot advise you to alter your car illegally.

Mike is also correct: The EGR only operates under certain throttle & engine power settings, and none of these operational points is during "performance" driving: The EGR is closed at wide open throttle, and thus does not have any effect on peak power or maximum performance. No "science" behind it - just simple fact: The EGR is not circulating exhaust gas under power conditions.

Lars

 

I replaced my quadrajet in 1985 (that is 25 years ago, if my math is correct) on my L-82 4 speed due to hard starting issues, cold start issues, and generally poor idling at the time. I bought a new Holley 4175 spreadbore replacement at that time, essentially due to my experiences at that time with Holley's, simplicity of design, ease of rebuilding myself, and adjustability for tuning to my application. I still have that carb on the L-82 and it has never been rebuilt. It was a direct bolt on and I was able to play with the primary jets as well as the secondary springs to optimize the L-82 engine. I believe that I changed the primary jets to one size smaller and I currently use the lightest secondary spring that came in the spring package for when the secondaries open-no bogging or hesitation.

I am finally getting around to thinking of rebuilding it primarily since the last few years, the float bowls drain after sitting overnight (I have to seal those screws underneath with some epoxy) and the cold idle is a little rougher than it should be but under normal operating conditions, the carb is great.

I did experience the primary side float bowl leaking one year but primarily because I was taken it on and off playing with jetting and did not properly tighten the screws-I always check them ever season now-no different than checking many things every year on a 33 year old car (manufactured in Oct 77).

Is a properly running Quadrajet a good carb-absolutely but I always felt that it was overly complicated for what I needed it to do!

PS-Ditch the EGR valve! Yes it does not operate at WOT BUT why would you want to superheat the fresh incoming combustion charge with previously burned combustion gases if you don't have to (FEDS) rather than pure fresh cooler air? My L-82 immediately idled better and had better throttle response without the EGR valve. EGR valves can also contribute to part throttle detonation (see reasons above) in the early emissions engines. I am still amazed at the tremendous improvement that occurred on my L-82 once I eliminated all the emissions hardware-very very similiar to a late 60's/early 70's SB! Bear in mind that I would never mess with the emissions hardare on my other performance cars (and have not!)simply because they run so well together with the emissions gear that was much better designed and integrated into those computer controlled vehicles-that simply was NOT the case in the early emissions cars!

 

mar,
If there's one thing I've learned on this Forum, it's that Lars knows what he's talking about when it comes to Quadrajets. Do as he says!!!! One thing he didn't tell you to do was to ditch that fuel filter and rubber fuel line. Replace the line from your fuel pump to the carb with a one piece steel fuel line.

If you must have a filter, insert it BEFORE the fuel pump. That way, no crud should enter the pump and therefore, the carb. There is a small filter ( or should be!) in the inlet of the carb that should stop anything that might get past the pump. If you must have a filter between the pump and the carb, Lars has a paper on how to do it properly. Read and obey!

If you follow his advice, you will have a car that will do everything you expect it to do, and be safe at the same time. Follow the advice that some have given, and maybe you'll have a different outcome.

Caveat emptor!

Regards from Down Under

aussiejohn

 

LARS is the MAN for quadrajet info, agreed!!! I was just providing MY experience with a carb other than the quadrajet.

 

jb,

I did not mean to slight you, or anyone. It's just that there were several opinions, some of which contradicted one another. To use EGR or not to, Holley vs Rochester, etc. Don't forget, GM fitted EGR and Qjets to millions of SBCs in the '70s and they did that for good reason.

EGR helps minimise harmful emissions into the air that we breathe, except in WOT situations and, let's face it, those times are rare in a normal driving experience. Well, they are for me!:o

Regards from Down Under

aussiejohn

 

Understood! Thanx and cheers!!!

 

By the way - The way your EGR is hooked up (or, rather, NOT hooked up), your EGR is already non-functional in the closed position.

Lars