F.A.S.T Car Falls Flat After Leaving The Line....

[Seamister]

I've been chasing what I think is a carb. issue over the past several months but I wanted to get some input from others.  Here is what I have:

Car:
1972 Buick GS 455 (Stock Appearing)
3860# with driver
11.23 @ 119MPH on Slicks with a 1.55 60ft. time

Engine:
464 CI
12.0:1 CR
255/265 @.050 on 108LSA
Cast Iron Intake
Cast Iron Cylinder Heads
Cast Iron Exhaust Manifolds
2 1/2" Exhaust

Carburetor:
1972 Buick (7042240) 800 CFM Q-Jet
79 primary jets w/ .041B rods
K-Hanger w/.044 DA rods
Factory Choke Pull-off (1.2 seconds) to release
Air Valve Opening: 1.312"
Air Valve Spring Tension: 1/2 to 3/4 turn
Float level: 5/16"

Problem:
When I leave the starting line at 2000 RPM, the car hooks and goes.  However, when watching a video of the car leaving the line, it appears to "porpoise".  The front end rises, falls back down, and then comes back up again.  In watching the playback tachometer, you can see the car flashes to 4400 RPM (which is correct for the converter), then falls off 200-300 rpm for a split second, and then climbs back-up to 5800 RPM where I shift.  My 02 sensor reads 13.95 to 1 at idle and about 12.4 -12.8 to 1 going thru the traps.  I'm pretty confident it is not a fuel delivery issue because I have 7psi (read at the inlet of the carb) at all times.  I can fill a gallon of fuel in 16 seconds at the carb. inlet and have the fuel regulator mounted up front on the core support.  I have a -8AN line feeding the carb. from the regulator.  I've played with the spring tension of the air valve and the car still "porpoises", and I've tried diffferent choke-pull offs (0.5-2.0 second release time) with no help either.  When the air is good (less then 2000ft. of DA) the "porpoise" isn't that bad.  However when the air is bad (greater then 2000ft. of DA) the "porpoise" is real bad.  So here are my questions:

1). Would having the carb. jetted "too rich" cause this "porpoise" ?
2). Would "fuel slushing over into the secondaries" cause this scenario ?
3). Is it even a carb. issue ?  Could it be transmission related ?
4). Any suggestions ?

Thanks for your help.

Steve

[Cliff Ruggles]

Steve, have you addressed venting in the airhorn in addition to stopping fuel from chasing up the metering rods?

I would also consider removing the primary metering rods and running straight off the jets.  We do not use primary metering rods in any of those applications, there is no need to add any variables to the situation, and pulling straight off the jets will produce a clean fuel curve at WOT. 

The jets are also larger than I've ever seen used in that particular carburetor number. 

We've set up quite a few of the 71-74 Buick carbs for pure stock, FAST, etc, and have never had to use jets anywhere near that large. 

We just got a report from a FAST customer the other day that made nearly 700hp with one of those carbs we set up for him, and it used much smaller jets....FWIW........Cliff

[Seamister]

Cliff,

Thanks for the reply and help.  To answer your questions, I've epoxied fine brass mesh screen in all the vent locations in the air horn and also on the air horn gasket over the main vent.  I seen this done on a "super stocker carb" and figured it was worth a shot.  Is there something better that I should try to address the venting issue?  Also, I'm not sure "how to stop fuel from chasing up the metering rods"....could you explain what I should be looking for or what to do to prevent this from occurring ?

I'm in the midst of getting a spare carb. so I will have something to experiment on over the winter.  I purchased your book and I'm about half-way through it.  Of course I went straight to Chapter 6 (High Performance Modifications) and read it so I would like to try the "fixed jet" option along with your "recipe #3" on this spare carb.  However, before I touch anything, I need to finish reading your book from front-to-back !!!  In the mean-time, I will jet down several sizes on my current carb. and see what happens.  Do you have any suggestions on where I should start for jetting ?  Again, thanks a ton for your help.

Steve

[Cliff Ruggles]

Steve, the brass screen deal is 30 year old technology.  Plug all of the vents or vent areas rear of the front vent solid.  Make sure to drill or slot the POE wells back to the main bowl.  A piece of electrical tape across the gasket for the metering rods is fine.  Apply a small amount of wheel bearing grease to the metering rods when you push them thru the tape.  Keep them well lubricated so that they return to the lowest position when the flaps close.

I'm not sure why or how the carb likes jets that big right to start with.  We usually set them up with 73 or 74 main jets if using metering rods in the front.  If running straight off the jets, start out around .065-.068" and go from there.....Cliff

[Seamister]

Cliff,

Thanks again for the reply and advice.  When you say "plug all the vent or vent areas rear of the front vent solid", I would assume you mean with some sort of epoxy ?  If so, what brand and part number would you recommend ?  Also, if I were to put headers and a single plane aluminum intake on this car in the near future, what would you recommend for a starting-point for jetting ? 

Thanks,

Steve

[Cliff Ruggles]

Steve, I tap the holes and install threaded plugs, no glue to come loose.  I use a tapered tap, and only thread the holes deep enough so that the threads jam in and hold the set-screws.  I also put red loc-tite on them.  You must vent the POE wells back to the main bowl if they were externally vented in the first place.

Jetting with or without metering rods?  You are already using jets way beyond what we have used here in the same carburetor number, so you may have to experiement at the track to find out exactly what it wants?

If the car sees mostly strip use, I would not use primary metering rods at all, and run it straight off the jets.  It will actually drive fine on the street once you determine the correct jet size.  At the track all metering changes are done with the secondary metering rods.  This is one of the nicest features of the q-jet, I can dial it in for any track or any weather in seconds without taking the carburetor apart.....Cliff