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.

I think that's a given. Where ever you mount the gauge will tell you everything you need to know before the gauge. Most importantly it'll tell you if your fuel system is keeping up.....or not. Then it's time to dig further. Without a gauge everyone is just guessing.
You want to eliminate the fuel system as a possible cause, a gauge will do that.

Hopefully you guys are changing your fuel filters semi regularly.

Takes a little more work but in cases like that I remove the hard line and run a temporary rubber line, T into it before the carb and run a rubber line up through the cowl and tape the gauge to the windshield.

Remember it's just a quick test, you'll know right away if there is a fuel related problem, either pump, fuel sock, line restriction somewhere, etc...
In almost every case Ive done it's been fuel pump related

Hey Schurkey, please refrain from partially dissecting and mis-critiquing what I posted if your not going to get right!

I very clearly posted back in #15 that the voltage checks where to be at IDLE and then ABOVE 2000 rpm.

Some secondary pulloffs can be eliminated. Might be worth a try and at least take that part out of the equation.
I believe there is just a little bit of re tuning the choke.

If the engine cuts off/hard stumbles at high RPM at WOT, windmills down in RPM and then re-lights, the carb has run dry. I had this exact same thing happen to me in my old 400. Top of 2nd gear, suddenly silence, a few seconds goes by and then it lights again. I thought I had turned the key off.

I was checking wiring, ignition, everything. Changing to a 70 GTO 455 big body fuel pump fixed it until I went electric Mallory 140.

You've outrun the fuel delivery. Harder to do with a P350, but if the float level is low and the pump/lines are marginal, I can see it laying over at high load.

Here’s some more info…drove it a little while ago. Outside temperature is already in the mid-eighties. Stopped at the gas station and refueled with 93 octane. It did not want to restart. Seemed the carb may have been dry. Got it started and drove towards home. Wound it up a couple of times and no stumble. It ran fine. Got home and pulled the air breather. The fuel was percolating in the carb. Temperature gauge showed 180 degrees when I parked in the garage. Could this be causing the stumble? Maybe needs a carb to intake insulated gasket? Maybe it needs the fuel return line from the pump to the tank? I do run more initial timing than I may need at 18 BTDC. Vacuum advance is ported vacuum. No spark knock or labored cranking. Dunno but it shouldn’t be boiling the fuel. The heat riser flap was removed from the exhaust manifold but the intake crossover is not blocked. Stock cast iron intake (68-71).

I have the same problem, however it is intermittent under certain conditions. Once I put a fuel pressure gauge on it I found it is a fuel delivery problem. Pressure will go down to zero, and car will nose down, rpm drops. In low gear it will rpm up to when I shift it, even close to 6k. As long as there is pressure, even 1 psi it will continue to rev. It usually goes to zero either a bit below or a bit after the 2-3 shift. Depends on the shift rpm. This with either the Carter 6907 or RobbMc 550 pump. Qjet, stock modified intake.

When you have a heat cross over blowing 500+ degree exhaust temps under a carb with today's fuel, it for sure will cause all kinds of drivability issues in the summer heat.

I'd still have a gauge on it to see how it's acting but it already sounds like there are a few things I'd be addressing on it.

Ok. Let's dissect this conversation.

Post 6. Kenth asks a very important question.
Post 13. Original Poster sort-of answers, having tested with the ignition ON, but the engine NOT RUNNING.
And you come back and tell him in Post 15 that testing voltage with the ignition on, not running, is the same as testing with the engine running. (Which is NOT CORRECT.)
Then you go on in the same post to discuss his charging system when what's important--and Kenth's original question was "What is the voltage AT THE HEI WITH THE ENGINE RUNNING."
But if you're talking "charging system", where is he to measure the voltage? At the alternator? At the battery? You aren't specific about this voltage test being AT THE + terminal of the HEI.

But I want to know the voltage at the HEI when the actual problem occurs, in Post 17.
Like you, I could have been more clear--"What is the voltage at the + terminal of the HEI at 4K rpm."

I'm not perfect, Steve. Usually you give good advice. But not this time. Your #15 was incorrect in one place, and unclear in the other.






And...from Post 6 to this one, we STILL DON'T HAVE AN ANSWER to the voltage at the HEI, engine running (preferably at 4K.) We STILL DON'T HAVE AN ANSWER as to what his fuel pressure is (also at 4K.)

I think you might have located the problem, possibly ? Try a insulater Gasket.

GT

GT

Take the internal filter out of the Q-Jet. Those things create a restriction, there just isn't enough surface area for HP use. Get an inline filter to run between the fuel pump and the carb. RobMc sell some nice aluminum inline filters.

https://robbmcperformance.com/products/filters.html

Butler also has some nice filters

https://butlerperformance.com/c-1459...el-filters.htm

Ethanol gas? I run 93 E10 in my 67 and I was having boiling issues with a holley 4150. The issue I was having is it was going pig-rich at cruise as the fuel boiled and the vapors were pulled into the engine. The symptom was a burble stumble and random misfire. AFR gauge was showing 9-10 something AFR during the random stumble.
I blocked off the crossovers under the intake and that solved it. The ethanol gas boils a little easier than straight gas. Engine temps are 180-190, but thats water temp. The crossover area under the carb gets hotter than that due to the exhaust heat. I still think your laying-over is the carb running dry, but the only way to 100% verify is a fuel pressure gauge reading when the laying over happens.

Try some ethanol free gas.

https://www.pure-gas.org/

Boiling gas doesn't shut-down an engine at a specific RPM "almost like a rev-limiter"--4K in this case. The boiling gas is no doubt "a" problem, but it's not "the" problem.

Good point.

Did t read through all the posts but wanted to add that RF signal issues show up at 4000 rpm
If your MSD wires run parallel to each-other they can trade signal. They need to be shielded or braided so they pass at right angles over and over . I chased this once for a full season of racing on and off. Just like a rev limiter , it once cut power so hard when it came back in it backfired and blew the glass hood off the car at 1000 ft mark
Sadly I’d inquired about RF signal w MSD at the beginning g of the season and they said no couldn’t be. At the end of the season I spoke w someone else there and he says oh yeah 4k almost definitely RF signal. Wasted a year. Cam changed , springs changed, timing chain changed. Drove me crazy.

I'd have never guessed that RF interference would be RPM related.

Thanks for that.

While RF interference might be a possibility its not applicable to the OPs issue here to me.


Go back and read his reply in post #26.

No. My issue is definitely fuel-vapor related.. Wished I would have blocked the intake crossover passages. Ordered a fuel tank sender that includes the fuel line return (1/4), return line, 1/4 phenolic carb spacer, and a length of fuel line heat sheath. My fuel pump has the return port. If this effort doesn’t fix it I’ll be pulling the intake to add the block off plates. All of the cruise ins and shows are during warm weather so don’t need just a wintertime car.

If making block off plates, 321 stainless is a good option because it's alloyed with titanium. 304 stainless will eventually experience chromium carbide precipitation at high temps, which can cause it to essentially rust through at exhaust temps.

https://www.azom.com/article.aspx?ArticleID=2867

https://www.aceraceparts.com/blogs/n...gt79yrxKRlbgpr

Good info. Thanks.

Happy with the results of the phenolic carb spacer and fuel return line addition to the tank. Ran and hot started as it should. It was 87 degrees outside when I took it for a drive. Glad I dropped the gas tank to swap the sender. (Needed the sender with fuel return). The gas tank straps were just hung in the front slots without the screws to secure the straps. Not unlike other things I’ve found that were not right. Brake adjusters backwards, Pittman arm loose, dry suspension joints, rubber insulators missing from the radiator support to fenders, and a multitude of loose bolts-nuts. I purchased the car for the nice body and interior so pleased with the progress. Now the ring and pinion change…

Good news to hear!
Now enjoy the summer without getting stuck somewhere.