Falls Flat at 4,000 RPM
 

Second time I’ve noticed this:
Run up to 4.000 in low gear (full throttle) and get a hard stumble(s) that almost feels similar to what a rev limiter kicking in feels like. It does not have that. Not sure though if it’s fuel or ignition related. New HEI (Cardone) distributor and reman quadrajet (17057262). All this on my 1968 Firebird 350 -stock. Both times this has happened the engine is around 180 degrees and out side temp is low 80s. Ideas??

Sounds purely fuel or air related to me.


Check the vacuum choke pull off can on the passenger side of the carb.

Pull the vacuum hose off it, push the lever that the rod is in all they way in and hold it there., then put your finger on the hose nipple to seal it off.

If that can is good then when you stop holding the lever it will stay in.

Another way to do this choke flap test is to cable tie the choke wide open and then take a drive and see if the issue takes place.

Thanks Steve. Will do. This is a late 70s quadrajet with two choke pull offs. When I replaced the cam earlier this year, I went back with a new Carter fuel pump. First assumption is it should be what’s needed. The fuel cap is vented. I’m running 93 octane. It starts easily however I’m only driving it maybe once a week so usually the carb is dry and has to be cranked over a little longer than it probably should. I incorporated an electric choke kit so it doesn’t stay engaged very long. It idles great and cruises perfectly. It just doesn’t like pushing past 4,000 RPMs.

Check your fuel sock on your sending unit inside the tank.

They can deteriorate and clog up the pickup tube.

Upgrading the pump to the Carter M6907 mechanical might be helpful for better fuel delivery.

I see that the M6907 is 120 gph and the M4690 I used is only 40 gph. Maybe that’s it. Man I’d hate to have to change fuel pumps. Used helicoils in the timing cover fuel pump holes and skeered they will back out. Can’t imagine this stock 350 needing more than that stock pump can handle. It’s a new replacement fuel tank that was done before I got the car but I can’t assume the fuel sender-pickup is also a replacement. Sounds like another project…after all it is my “retirement project vehicle “.

Do you have about 14 volts at + on HEI, engine running?

I’ll check the voltage at the HEI. If less than 14 is that a possibility of the stumble?

Appears the 6907 has gone the way of the Dodo.

https://www.jegs.com/i/Carter/180/M6...8CKUbYcmKtoY15

Summit, Amazon, Butler…all show out of stock. ??

Saw that.

Also saw this:

https://www.a1racing.com/carter-m690...et-f-p-v8.aspx

Sometimes these smaller retailers have stuff still sitting on the shelves since they do less volume than the big names.

I have one of these pumps (no fuel pressure regulator) feeding my Tri-Ppwer 421 through the stock 5/16” chassis fuel line and it pulls to 5000 rpm (which is as high as it wants/needs to go) no problem. My car has factory A/C so it is equipped with a 1/4” vapor return line.

Stock points ignition system including the coil, always starts with a flick of the key. Distributor was rebuilt and has a custom ignition curve.

A 1968 Firebird should have a 3/8” chassis fuel line if I’m not mistaken.

To find out if it’s fuel supply related see if you can drive it at a time and somewhere such that when it acts up up can shut off and then see if it will immediately start right back up and idle normal.

If you have the smallest air leak on the suction side of the pump during high levels of fuel demand then the pump will just simply suck air since it’s way lighter then fuel.

Many of us have this take place not just by poor clamping, but by dry rotted fuel lines ( be they rubber or steel. ) and not seen a drop of leaking fuel.

Those stock pumps are not always up to the task. Had a stock smog 400 with 800cfm Q jet and the factory pump could not keep up and as it would run out of gas trying to make a pass. Put a Holley mechanical pump with a regulator and it cured the fuel starvation.

Yes. I have a 3/8 fuel line from the tank.

I have battery voltage to the HEI with ignition switch to the run position. Is that the same as engine running?

Valve Springs?

Yes that’s the same, and if your charging system is working right then at idle and with the headlights on you should never have less then 13.4 volts and at 2000 rpm and up not less then 13.7.

If valve springs where weak the motor would be back firing BTDT with a 283 SBC.

On of the strong points of a Pontiac even in there bread & butter motors is the valve springs .

Lots of talk about fuel pumps, pressure regulators, and such.

HOW OLD IS THE FUEL FILTER? WHERE is the fuel filter--in the carb inlet, or in-line somewhere?
WHAT IS THE FUEL PRESSURE AT 4K RPM?

Will the ignition system reliably fire a spark-tester calibrated for HEI ignitions with the engine fully-warmed up? (I mean a REAL spark tester, not some piece of crap that has a glowing light-bulb when there's spark energy present. I want one where the spark jumps an actual calibrated gap.)

I would want to know the voltage when the ignition is actually WORKING, not just powered-up and static.

What is the voltage at 4K rpm?

Fuel pressure gauge will answer your questions about the fuel system.

Put a temporary gauge on the car, tape it to the windshield, go out and duplicate the problem. The gauge will tell you if it's fuel related and then you'll know if you're looking in the right area or not.

The fuel pressure gauge will not tell him about a plugging fuel filter if the filter is in the carb fuel-inlet fitting. The restriction would happen downstream from the pressure-tap for the gauge.

If the pressure reading is taken downstream from the filter, it should be a reliable indicator assuming the carb has appropriate float-drop.

New valve springs done with cam replacement. back in late December. The fuel filter is in the carb. Also new at time of cam replacement. One thing that I also has me wondering….this new Carter fuel pump has a port for a return line. My Firebird doesn’t have a return so I capped off the port. Not sure if that has any effect on how much pressure the pump can make. At any rate, it’s time for a fuel pressure gauge to ensure proper fuel pressure. If good I’m thinking I should check the float level. The secondaries on this carb never feel like they are fully opening but with a 2.56 rear the only way to check it is winding up first gear. It will run 90 mph in second… Not sure if the secondary air flap could cause this type of issue though. If all of the fuel system checks out then on to the ignition. Where would be the best location to tap 12 volts? Horn relay? I know that the original ignition system had resistor wire but it appears that there’s battery voltage (13 volts) on the HEI battery wire when the ignition switch is in the on position. Any suggestions for a fuel gauge that doesn’t require cutting my hard lines. The pump to carb fuel line is a factory type hard line.