64 GTO "Bucks" Under Hard Acceleration
 

This is an old problem I have never solved. My brother now has the car and I'm not sure what to recommend to him, new carb or new dizzy?

64 GTO
76 400 w/ Edelbrock RPM Performer 72cc Heads
800 CFM Quadrajet
Super T10 4-speed
Pertronix Billet HEI
93 Octane Gas
Taylor Wires
NGK 7373 plugs

The car bucks/kicks/stumbles at about 3100 RPM under hard acceleration. Runs fine under normal acceleration. It might be a miss but I have never heard a backfire.

This problem first surfaced about 2014. About that time I changed out an Edelbrock 800 CFM for the Quadrajet rebuilt by a local guy here in San Antonio area. The Pertronix dizzy was on the car when the Edelbrock carb was on, I could really romp on it

Timing: 12 BTDC static/about 30/32 Total Timing all in around 2500-3000 RPM. However, the timing jumps about 2 degrees under RPM

Wires were replaced during the Edelbrock era and they worked fine

Thoughts?

Thanks
Rick

Here's a quote from the guy who built my Quadrajet if anyone is curious about jetting/etc.:
"Some of the modifications that I do are proprietary such as the resizing of the internal orifices, channel restriction modifications, etc. There are about 40 different machining tricks that have to be done. All Quadrajets after 1973 were made to give lean performance to meet federal laws on emissions. That's why you can't just bolt on a stock Quadrajet and expect it to work on any engine that is not completely stock even if you change the jets and rods to bigger sizes. It still wouldn't idle right and there would be a bad hesitation when you accelerated. Jet and primary rod sizing is selected to match your engine combo. Yours would be #72 jets and APT type rods that are .041 to .044 with a .026 power tip. I include an APT (adjustable part throttle) adjustment tool to tune the carb for the best off-idle and cruise performance. The secondary hanger and rods are selected depending on the cubic inches, any head porting done, type of cam, differential gears, transmission, etc"

Take some mechanic’s wire or cable ties and lock out the secondary’s on your Q-jet just like what the levers and rods do when the choke is on and the motor is cold.

Next take the car out for a drive bringing it up to 5000 rpm several times thru the gears ,

If no issues take place then you can rule out timing and spark issues and then focus on things like the carbs secondary air valve diaphragm being leaky, or what’s more likely a fuel delivery problem keeping the carb from getting the volume it needs.

Checking that diaphragm is easy. Just unhook the hose from it. Push the lever in and then use you finger to block the hole in its feed tube.
The lever should stay in all the way for as long as your finger is on it if the diaphragm is good.

You may now have a fuel volume issue because the now bigger 800 cfm carb is making more power and needs more fuel volume to do that then the Eddy carb did.
This issue could be as simple as dry rotted rubber hose jumpers that are cracked and allowing air to be sucked in instead of fuel, and yes this can many times take place without seeing a drop of fuel leaking.
It’s far easier for the fuel pump at high flow rates to suck in much lighter air then the heavy fuel in the tank!

The next fuel issue could be that the fuel sock / filter at the end of the pickup tube in the tank is clogged, or it could just plain be that the steel fuel line has a pinched off spot in , it could be that flaps of internal rust in these fuel lines are making for a restriction , or it could plain and simple be that the motor with the bigger carb is just making more hp then the sock size fuel lines can support!

I'd start by installing a high flow N/S assembly in the carb. Unlikely it got anything more than showed up in the rebuild kit that was installed in it.

If it already has one in it then take a look at fuel delivery to the carburetor.

None of the long spiel about 40 different machining "tricks", emissions, jetting, metering rods and needed internal modifications have anything to do with keeping the bowl full on hard runs.

I use a plain old 1977 Q-jet on my 455 making a lot more power and it's completely "stock" aside from slight modifications to the idle system, jets, metering rods, grinding a little off the stop so the secondary air flaps open further and a high flow N/S assembly. NOTHING will outrun it, dyno or at the track........Cliff

40 trick machining operations? My guess is, the builder performs closer to zero mods when he works on it!

My checklist from Cliffs book is close to 10-15 items to measure/check.
I’m wondering what exact carb is on your engine?

Rick, does your brother have Cliffs book?

Yes he does

There are only two "machining" operations typically needed that I know of.

Installing bronze bushings in the baseplate for the primary throttle shaft, and on some units I might flatten the airhorn just a tad if it's warped more than the main body so it can re-conform to it when it's tightened down.

I've read all sorts of stuff over the years about how the Q-jet needs all sorts of help to work well. They don't need much beyond calibrating them for the application and high quality/high performance parts installed.

Don't buy a cheap Ebay, Pep Boys or Rock Auto rebuild kit with a POS accl pump and tiny N/S assembly in it then expect to keep up with the powerful engine you just built. I get butt loads of calls an emails every week from folks who did exactly that. Most of them are wondering why they have a bloody nose when they bounced it off the steering wheel at the top of first gear when the car nosed-over on their first full throttle run!.........duh?.....

Also what air cleaner is he running ?
Q- jets and Holley’s are picky about having the cleaner lid at least 3/4” above the carbs fuel bowl vent tube at high throttle openings.

Problems at 3100 RPM sound like fuel delivery system problems. Even the small fuel valve should get you above 3100.

I have read the over 40 machining steps before, and never knew what they might be.

The earliest Q-Jets DID need a few (the 1965 probably deserved the term quadrajunk), but Rochester fixed the early issues themselves.

Jon

My vote is fuel pickup. I used to run only an electric pump. When it was on its last legs, it would get hot and interrupt fuel flow and the car bucked like a bronco.

Steve, that's a darn good point, the only air filter I could find that fit without hitting the hood was the Mr. Gasket 1 inch. Would love to hear a recommendation for a filter that works with Q-jet & RPM Performer intake.

Throw the edelbrock carb back on it…. See if your problem goes away

Wish I still had it, that would make troubleshooting easy but it's long gone.

I think I'm going to recommend he take some of the steps Steve mentioned and if we can isolate to fuel then first check flow, if flow is good probably order a new Quadrajet from the guys in Boerne TX.

Take it for a quick spin without the air cleaner assembly and you will know right away.

"Steve, that's a darn good point, the only air filter I could find that fit without hitting the hood was the Mr. Gasket 1 inch. Would love to hear a recommendation for a filter that works with Q-jet & RPM Performer intake. "

I've covered that deal a few times on here.

Folks are quick to install the Edelbrock RPM intake. It woln't outrun a factory intake anyplace to at least 500hp. Not to mention the cobbling that goes with using it including having to move the air cleaner lid closer to the carb. That move alone can kill a LOT of power, WAY more than you ever hoped to gain going to a taller intake.......FWIW....

A one inch tall filter section is a massive restriction in and of itself!
I mean at least the standard factory 4 bbl air cleaner held a 2” tall filter!

Since your brother has Cliffs book, he obviously has some good knowledge of what to look for. Most Q-Jets will have a problem with a 1 inch air filter.
I’m believe’n the solution is close at hand!!

Cliff, I hear ya on the intake, would not have been my choice but the motor was built when I got the car.

Agree, the one inch is an issue: Anyone have a recommendation for an air filter that will work without a clearance problem?

I have Cliffs kits in 3 of my cars. The drag car is 73 SD455 stock PMD parts,qjet yes its special carb. 12 teens 110 ,12.30s mostly ,3850 lbs no problems. Street SD same ,stock exhaust when hot it will surge in top of 2nd. Need good fuel delivery. Other is 67 ,qjet, 400 auto,Firebird. It doesnt buck or surge. I've read all qjet books I can /could find over the years,Doug Roe. Now I follow Cliffs book. I had secondary air valve flap that wasnt adjusted right from factory,would bog then leave but not all the time drove me crazy.i just loosened screws, readjusted ,fine now.

My moneys on a fuel delivery issue, The stock mechanical fuel pump probably can't keep up with the demand for fuel, especially with the smaller quadrajet fuel bowl. You may have to install an electric pusher pump back by the tank.

I`m in the weak ignition camp. Check all primary wiring. Check cap and rotor for cracks, tracking etc. Check plug gaps and read plugs.