Engine stumble at higher rpm

[Mikehess73184]

Hey folks, looking for some advice and opinions. New to me 76 highboy with a 390. Truck didn’t run, owner said it needed a new head gasket. Replaced gasket and got it running. The problem is that the engine falls flat on its face and stumbles when I’m cruising and try hard acceleration. I notice this at around 2500rpm. It’s like I put my foot into it and it won’t accelerate, just bogs/stumbles. I have to shift down and take my foot off the accelerator to recover. I thought it was a carb issue so I changed carbs with another truck of mine and did the same thing. It’s an edelbrock btw. I’ve changed accelerator pump positions, changed rods in carb for more fuel delivery and no change. It doesn’t backfire or have clouds of black smoke when this happens, so I don’t think it’s too rich.
Things I’ve done so far include:
New spark plugs and wires
Checked firing order multiple times
Checked timing multiple times 16* initial and 36* total
Vacuum advance works and on ported.
Tried running without vacuum advance /no change
Tried a brand new distributor/ no change
Tested the coil it’s good
Put a fuel pressure regular on it to 5.5psi
Engine has great vacuum at idle it’s 20
I went to stiffer step up springs in the carb

Any ideas?

[basketcase0302]

Engine has great vacuum at idle it’s 20

It sounds to me like a classic vacuum leak, (which could be one of 20 different places).

Get a long piece of vacuum line and run it into the cab. (where you can then monitor your vacuum reading under acceleration).
And get a can of starting fluid to use for spraying all the sources of a vacuum leak. With the engine running spray all the hose inlets/manifold gaskets/carb gaskets/vacuum advance/etc. The starting fluid has always been my "fluid" of choice as it leaves no residue like most carb cleaners and other sprays chemicals do to clean up for gasket sealer to grab afterwards.

Hope it helps, and maybe some other guys can chime in here too.

[pcmdub]

Maybe check for cracks in any fuel hoses. At high RPM it could be letting air into the line.

[fastEdsel]

I agree it could be a vacuum problem, chased much the same issues in my Edsel 361 for 3 years! But I'm running a Holley 600 so do not have a whole lot of Edlebrock experience to share in all my Ford years but what I found was actually 2 problems. First was the base plate screw that connects to the carb was loose and vacuum was lost there at the gasket. The next was the vacuum diaphram on the distributor was sucking air, easy to check though. Just pull off the hose to the distributor and if you can either blow through it or suck air that will be a big problem on acceleration so it's shot. And like yours the vacuum measured 19-20 strong inches so there was a lot of chin rubbing going on. Also the altitude here is 3400 feet, not sure if that has anything to do with vacuum pressure though, others may have a better idea. I installed a Holley carb kit p/n 37-1544 and what a 100% difference. Also changed the distributor diaphram so now back looking for reman thumper Camaro's to show that "the old guy has still got it!" What an amazing car. Still needs premium fuel though. And starting fluid really does work well to find leaks. Hope these ideas help, if not then let us know. I'm sure my challenges have been the same for others though with Holley carbs.

[jzjames]

A carb float sticking could also cause this.

[zgerbic]

From the symptoms it could be one of these:

Typical problems:
1. Not enough fuel coming from the accelerator pump.
2. Float level is too low. In this case when you put a high demand on fuel from the float bowl, there is just not enough and the jets starve. You have changed the carb so this problem would likely have followed the carb. The second cab should have corrected the problem.
3. Not enough vacuum advance.
4. Poor spark voltage of too wide a plug gap.

These don't seem to be your problem so the next place to look is at these two areas:

1. Not enough fuel getting to the carb. It sounds like a fuel delivery problem. Your pump could have plenty of pressure at low engine speeds because the carb is not using much fuel. The fuel pump can keep the line to the carb under pressure even if you have hardly any flow volume.
When you are at highway speeds I suspect the volume of fuel is just good enough and the pressure might be heading toward the low side because of some restriction before the pump. The float bowl fuel level could be really low at highway speeds as well. When you put a fuel demand on the carb the pump may not be able to deliver enough fuel causing starvation. This would match the symptoms. Look for a filter or rubber line restriction. Could also be rust in the metal lines or a clogged screen in the tank.

2. Too big a carb, or mechanical secondaries with poor jetting. Way too much air vs. fuel when you put your foot down, outstrips the accelerator pump capacity and then the amount the jets can provide cannot keep up. This is less likely to be the problem as just about any time you floor it the engine would starve for fuel and probably backfire.

Regards

[Ren]

Once had a problem very much the same. It was a weak fuel pump.

[Ren]

You said the pump reads 5.5psi. But I really think this is a fuel issue. Replace and/or blow through all the lines and replace the filters. Also, could be a kink in a line.

[markguiver]

I had a similar issue with my 75 after I replaced the fuel pump a few years ago. If I stomped on it from cruising speed it would react the same way yours is. It felt like it was starving out. After checking the timing, fuel lines, vacuum, distributor, etc..., I put a gauge on the fuel line and found the fuel pump was putting out 9 psi. It wasn't starving out, it was flooding due to excessive fuel. Put a regulator and pressure gauge on it and set the pressure to 4 psi, and that cured the problem. Another something to try.
Mark.

[basketcase0302]

="Ren"You said the pump reads 5.5psi. But I really think this is a fuel issue. Replace and/or blow through all the lines and replace the filters. Also, could be a kink in a line.

I documented something on BC #16 here years ago that you're describing, (with pics until photobomb held my pics hostage and I left them). And it was a factory issue done to possibly millions of our trucks! On my 71' F-350 DRW with factory Bendix PS the steel 3/8" fuel line from the dual switch/valve beside the seat that runs along the DS frame rail was kinked down to about 1/4" diameter. It was kinked where Ford ran the line behind the PS box, (not visible unless you pulled the box out like I did on my engine compartment rebuild).

To correct that was a major PIA as I opted to drill holes inside the square frame rail and rout the line inside to prevent any further issues.
Still have my pics showing how I did this if anyone is interested, shoot me a PM with an email and I'll forward the pics).