1973 800 CFM Mods

[Jim Rodgers]

First off, hi everyone.  

New here but have found lots of interesting and beneficial information.  Gonna see if I can post a pic of my qjet to see if I can get some feedback/ideas.

[Jim Rodgers]

And the bottom.

[Jim Rodgers]

I will start by saying I do not consider myself an expert when it comes to carbs, much less q-jets.  The work I have done to this carb has progressed slowly over time (3-4 years), and I feel like it runs as good as it does in spite of the work I have done to it, not because of the work I have done to it.

Second, this is a factory stock appearing 455 Buick race engine, 540 ci, roller cam, factory iron manifolds in a factory stock appearing 70 GS Stage 1 at 3770 race weight.  The car isnt driven on the street, it is a 1/4 mile car.

You can see I have removed the front part of the choke horn, but I left the rear portion for fear the secondaries would pull fuel from the primaries.  I removed the outer ring from the primary squirter, and ported the primary venturi and installed the larger Holley butterflies.  I also ported the rear venturi, and installed butterflies I made myself out of aluminum sheet.  (They dont make a perfect seal with the base plate so care has to be taken to set idle speed or you start pulling open the secondary air flaps)  I trimmed down the throttle shafts.  It has 88 jets in the front with no metering rods, and the smallest rods in the rear that I could find. 

I have the bigger needle and seat and I have opened up the factory carb inlet fitting to 1/2 inch.



 

[Jim Rodgers]

Fuel cell in trunk, -12 to Magnaflo 500, -10 to Magnaflo 2 port regulator, (2) -6 to a Y block, then 1/2" line to the factory fuel pump that has been gutted and the 1/2" line loops out and connects to the factory 3/8" line and goes up to the carb.  This setup will pump a gallon to the carb inlet in 15 seconds.

I have an O2 sensor on the car and it has always been lean, but the past couple times out we have been able to richen it up with the bigger jets and removing the primary metering rods. 

The car idles pretty nice and never, ever stumbles.  It always runs clean on every pass, no hesitation or miss.

The car has been a best of 10.23 at 127 and change, 1.37 60'.

I feel like it should run more MPH.  In high gear it acts like it is just lopeing along.  No converter troubles and the trans is good.  I know I have iron exhaust manifolds and that certainly doesnt help MPH matters, but I cant help buit feel it is due better MPH.

[Jim Rodgers]

Fuel system.

[Jim Rodgers]

Engine. 

My goal is a stock appearing 9.  I feel like I am leaving something on the table with this carb and I am hoping you guys can point me in the right direction.

[willie]

beautiful work,if cliff can offer any advice he probably will.He seems to be a great very smart guy.I do not have the experence to be of any help.I am still learning  myself,good luck. willie

[Cliff Ruggles]

Something isn't quite right someplace.  I say this because of the jetting required to get a clean fuel curve, and MPH is down some.

I've used the large Buick castings with the booster rings removed, but never touched the castings otherwise or went to larger throttle plates.

At least one of the cars that uses them runs high 9's at over 135mph.  It has 71 main jets and custom machined secondary metering rods in it.  

The owner has a full set of custom machined rods from .028" to .055" in .003" incriments for fine tuning at the track.

I'm thinking the mechanical pump may be too restrictive and the carb isn't staying full on hard runs.

Could need smaller airbleeds as well, to get more fuel and less air.

Could also be the specific mods to the carb choke housing and primary bores, but not sure?.....Cliff

[Jim Rodgers]

The factory pump has been gutted.  The 1/2" aluminum line just goes into the pump bowl and loops back out.  Sitting still my fuel system will pump a gallon to the carb inlet in 15 seconds.  But I realize the carb could still be going dry.

I wish I could find one of those 135 mph qjets to borrow to test back to back with mine.  Or send mine off to be tested.  

Cliff, is there a way to bench test these carbs?  

[Cliff Ruggles]

Not really.  The dynamic test always prevails anyhow (track runs).  No matter what the carb does on the dyno, it will act differently at the track.

A carb sitting on a car capable of low 10 second runs, is like holding a glass of water in your hand and getting shot out of a cannon.  Imagine what the fuel inside the carb is trying to do?  Let alone the fuel in the tank, lines, etc......Cliff

[Grych]

Nice work on that quadrajet! I always like those 70 GS models.  540 C.I. stock appearing.  Hows does Cliff say, rip your head off and throw it in the backseat!

Anyway, I myself had wondered if an adequate vacuum signal would remain at the main if the casting ring were removed in addition to the outer booster ring.  One would think a fairly high velocity would be required to create the pressure differential for enough fuel flow.  IMO, main air bleeds would need to be reduced.  Have you played with the main air bleed sizes? 

Would you post the other primary settings? Idles tubes, channel restriction, especially main and lower air bleeds.

Cat
 

[Grych]

What were your Air Fuel Readings at the 127 MPH mark?

Cat

[Jim Rodgers]

Grych,

This carb was originally a basic stocker build by Hedworth.  The carb was dead lean from the get go.  All of the additional mods were done by me including the porting, removing the casting and booster rings, modifying the air horn etc.  I have upped the primary jet size significantly and removed the primary metering rods, went to a smaller float and a needle seat with the windows.  It is still lean but not head gasket killer lean. 
I have not touched the main air bleeds, idle tubes, channel restriction or main/lower air bleeds since I got the carb from Val, hence the reason for my post.  I'm looking for advice and tips on these areas.  Also want to know how anyone has gotten larger than a .149 needle and seat in these things.  I'm a fairly creative person, but havent figured that one out yet.  I want in the neighborhood of a .175.  I'm not looking for anyone to divulge any top trade secrets on a public forum so please dont take me the wrong way.  Just looking for tips from anyone who has been there and done that. 

[Cliff Ruggles]

Considering 88 primary jets and the smallest rear metering rods you could fine, I'm still thinking that something is fundamentally wrong someplace.

I've removed the booster rings from the early large cfm Buick castings many times, and set them up to run straight off the jets.

Jet size usually ends up around .070-.073" with no metering rods, with the secondary rods in the .040-.055" range. 

It's possible the fuel bowl isn't staying full on hard runs, or there is simply too much emulsion air, or both.

Most racers run smaller seats and more fuel pressure these days, but we have successfully managed a .160" seat in those carbs at 7 psi fuel pressure..... Cliff

[Jim Rodgers]

Cliff,

Sent you a PM when you have the time.  Thanks,