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1973 Formula 400 - Carb Questions


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#1 CBJoe

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  • Engine: 400
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Posted 4 weeks ago

Hello,  I'll post an introduction to the forum soon, but I just picked up a  1973 Numbers Matching Formula 400 w/ ram air.  Car runs and drives very smooth...but I have some red flags when it comes to the Quadrajet Model that is installed.  Also the carb linkages and secondary plate operation is pretty gummed up.  I wonder if the secondaries are even working properly under load.

 

Carb on there is a Rochester Quadrajet 7040247.  This number indicates it was for a 1970 Buick with a Manual Trans.  Does anyone have any feedback about this model of Carb as it relates to a 1973 Ram Air 400?  I've worked on vintage motorcycles for years, but this is my first adventure into cars.  

 

Here's a quick video of the carb itself and the condition.  The EGR is not connected and the Valve cover breather is just floating around not connected to anything.  I'm going to be going through to understand all the vacuum lines to see what isn't set up properly.  But I figured I'll start with the carb.

 

 

 

Thanks for the feedback and any other info I can provide I will.

 

Cheers, Joe

 

 

 

 

 

 



#2 CBJoe

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Posted 4 weeks ago

Whelp...I should also add that it appears a previous owner epoxy'd the fuel inlet and that has failed so there's some gas leaking at the connection.  I'm going to see if I can repair it for now, but I may start looking for a correct carb model for this engine.  Still interested in feedback on it though in case I can repair the inlet.



#3 JoePeek

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Posted 4 weeks ago

The secondaries should have a little bit of tension so they don't just flop open like in the video. There is an Allen head set screw you can loosen, and then using a small screw driver, you can wind the spring adjustment screw just a little tighter, then tighten the Allen head set screw back in to keep the setting. Also, the choke pull off diaphragm not only pulls the choke off, but after the choke is off, it acts as bleed off diaphragm to control the rate the secondaries open. If you pull off the small piece of vacuum line, you can see a restriction hole for that diaphragm - the smaller the restriction, the longer it takes the diaphragm to bleed off and the slower the secondaries open. When you nail the throttle, manifold vacuum drops to almost zero and the diaphragm can no longer hold the linkage back that is connected to the secondary flap, and lets the secondaries open at a controlled rate as the vacuum bleeds off. So the secondaries open as a function of two things....the air flap tension spring, and how fast the choke pull off bleeds vacuum. Remember, the secondaries have no accelerator pump, so fuel must come out the two small holes at the top of the air valve opening. You have to give the beginning fuel time to start up. A well set up Q-Jet opens the secondaries quick...but not too quick.



#4 CBJoe

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Posted 3 weeks ago

Thanks for that description Joe.  Because of the stripped out and Fuel Inlet on the body of the carb I went ahead and ordered a correct Rebuilt Quadrajet for my year (7043266).  I think I'll keep this one and repair it and keep as a spare.  Hopefully it will be mostly plug and play.  Hopefully it gets here in a week or so.

 

I'm very used to motorcycle carbs, but this is my first adventure into these.  This car has a stock 400 to my understanding.  But it has an X-Pipe Exhaust and the Oversized D-Port Ram Air Headers https://www.ramairre...-oversized.html

 

If this were a motorcycle I'd be considering changing the jetting.  But Not sure about this setup and if thats even necessary without cams and engine work.

 

I'll check the vacuum at idle and adjust mixture screws, but not sure if other work should be done to compensate for exhaust.

 

joe



#5 JoePeek

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Posted 3 weeks ago

I would just bolt on the new carb, adjust the idle screws and see how it drives. After you see what a difference it makes,you can still adjust the secondary opening rate to an optimal setting. Since your engine is pretty much stock, the "as is" settings of the rebuilt Q-Jet should be close, but it can't hurt to see if the engine likes the secondaries opened just a tick quicker. All it will do is bog if you go too far, and you can always back up to where it was before. I would not mess with the jetting just yet. Sometimes a few jets sizes richer can help after full length long tube headers are added, but I would see how it goes right out of the box. It may very well be you drive it after putting on the new carb and it makes such an improvement you just leave it as is



#6 CBJoe

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Posted 3 weeks ago

Thanks Joe...that will be the plan.  I see you are not to far away in Westfield...maybe we'll cross paths at a function.

 

While we're on the carb and airflow topic.  Can you tell me if this is correct.  The "Formula Ram Air" that is called out on a lot of these formulas for 1973 and later isn't actually ram air right?  As far as I can tell from reading, the last TRUE factory ram air option was in the 1971 Firebird.  After that there was the SD and HO...but even in 1973 they were not actually the Pontiac Ram Ai IV design right?

 

I guess I just want to understand is that the Formula Firebird Intakes are really just cold air intakes that feed into the airbox.  

 

Cheers, Joe



#7 JoePeek

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Posted 3 weeks ago

So there are two things going on with the Ram Air thing. The first can be described as an "airflow path" description of ram air. If the Formula hood is functional, and ducts air into the dual opening air cleaner that seals against the hood then it has a "ram air", or should we all say cold air, induction set up. Weather or not the Formula hood openings are in a location that actually rams air into the engine efficiently to make an increase in air flow or not is up for debate, but at least it is cooler air. This terminology of Ram Air has nothing to do with what the engine is, but only describes the path the air takes through the hood into the engine. I'm not sure if by 1973 Pontiac had closed off the Formula hood openings or not, the Trans Am shaker was blocked off in 1973. The other way Pontiac used the term Ram Air was to describe the 400 engine itself. There were basically two versions, they are the Ram Air III, or sometimes called a 400HO, and the much more rare '69-70 only Ram Air IV. The Ram Air III was the most popular performance 400, it was a D-Port head 400 but with a little hotter 068 cam, dual exhaust, and different carb jetting and timing....an improvement over a run of the mill Pontiac 400. Most Firebirds and GTO's had a Ram Air III 400, but many had the decal letter callouts on the hood that said RAM AIR (not Ram Air IV). Then there is the Ram Air IV, which is very different than a Ram Air III. While both are Pontiac 400's the RAIV had different round port heads, like the 455HO, and SD455 used. Aluminum intake, even hotter 041 cam, special exhaust manifolds, etc. The RAIV was only available in 1969 and 1970 in GTO and higher end Firebirds, and even then they were rare. I'm thinking when compression ratios dropped to 8.5 in 1971, so did the Ram Air terminology of the Pontiac 400 engine. I guess if you had a 1970 Formula with a Ram Air III, or even a RAIV, and a functional hood, you could say you have a ram air Ram Air.


Edited by JoePeek, 3 weeks ago.


#8 ponyakr

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Posted 3 weeks ago

To further add to the RA confusion, there was a 400 Ram Air engine in '68, with #96 round port heads, & the 041 cam.

 

http://www.classrace...13&MAKE=Pontiac

 

There was also a Ram Air engine in '67, with #670 D-port heads.

 

http://www.classrace...95&MAKE=Pontiac

 

There were some '72 Ram Air cars, which had a 455, with #7F6 round port heads. But the engine was actually referred to as a 455HO. That engine could come with the RA option, or not. 

 

http://www.classrace...70&MAKE=Pontiac

 

http://www.classrace...69&MAKE=Pontiac

 

Confused yet ?  :smile22:

 

http://www.wallacera...ginesearch4.htm

 

A guy named Brad Burton races a '72 Formy, with RA. He's won lots of races, including some national championships, in NHRA Stock Eliminator, in the D/SA class. 

 

Up until last year, when he sold it, Brad's dad, Scott Burton, raced a '70 Formy, with the RAIV engine, in B/SA. It is said to be the quickest Pontiac powered Stocker ever. 

 

The SD455 engine, which had #16 round port heads, also came either with or without RA, in '73 & '74. So, maybe '74 was the last year of the RA option.

 

http://www.classrace...81&MAKE=Pontiac

 

http://www.classrace...94&MAKE=Pontiac

Attached Files


Edited by ponyakr, 3 weeks ago.


#9 CBJoe

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Posted 3 weeks ago

Thanks All...that all does help me understand the background better....now for the next question.  Can you clarify what (if anything) should be connected to the items identified in the following pics?

 

- the one on the passenger side block looks like a temperature sensor...but its not connected.  Theres one on the driver side block that is connected.

- Not exactly sure what #1 and #2 are in the next picture.  

 

please educate me :)

 

Cheers, Joe

 

 

 

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#10 JOE68

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Posted 3 weeks ago

pic one looks like a temp sensor   pic 2 is a vac control valve and I can't see the second thing circled



#11 CBJoe

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Posted 3 weeks ago

pic one looks like a temp sensor   pic 2 is a vac control valve and I can't see the second thing circled

Any clue if the temp sensor should be connected?  The Drive side one is connected, but nada on this one. This has a TH400 auto.

 

So the #2 item you call a vac control valve...this obviously isn't going anywhere.  I assume its a normally closed valve maybe that went to the EGR which is also disconnected?

 

Heres a better pic of the other thing...I assume this is part of the emissions system which has all been disconnected apparently.  I'm Just wanting to be sure that I understand what all has been bypassed and what needs to be capped off.  

 

Cheers, Joe

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#12 JOE68

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Posted 3 weeks ago

from what I see in wiring diagram, one temp sensor is for gauge and one is for emission.    both of those valves controlled emission functions.  70's cars got crazy with vac lines as the yrs went.  they were just getting going in 73.  late 70's cars were loaded with hoses running everywhere






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