Understanding the meaning of "under load."

[MIGHTY 8]

Hello everyone,

I am having a hard time troubleshooting an issue with my Weber carburetors (dual 44 IDF’s.)

Basically I’m confused about why an engine will start, run perfectly when it’s in neutral, and will accelerate through the rev range without issues, all while in neutral. Then, while driving in any gear there is a stumble, tendency to be jerky, and all of these things occurring intermittently.

What is happening mechanically while driving (under load) as opposed to reving the motor while parked in neutral?

This is in an attempt to trouble shoot and try and fix the issues that I am having right now. If everything is great in neutral…then all the "big" things should be working….fuel, jetting, plugs, spark, etc…why is it different "under load?"

Thanks in advance.

[bobplyler]

If you are going through the rev range in neutral, the carbs aren't open much. If they were open as much as under load, the engine would rev until it blows. Under load, the carb is open more, thus there is more air/fuel in the cylinder, which results in more pressure. The pressure can cause spark problems.

Sorry, that's just the reasoning. I can't help with the fix.

[MIGHTY 8]

Thank you so much for,the response. I appreciate your input. I did some further research (used ChatGPT, as a matter of fact…) well, I guess I got something of an education along with this post!

FYI….is there an electronic in line fuel pressure gauge (linked to your cell,phone) which transmits real-time readings? For some reason I suspect fuel pressure issues, but again, can’t ascertain it while popping the hood in neutral and seeing the fuel pressure gauge in that state.

[18Fiatsandcounting]

I can't help with the inline wireless fuel pressure sender, but some thoughts on whether stumbles or jerkiness are fuel or ignition related. The differences are subtle.

Ignition-Related: Usually a miss under load, sporadic, like skipping a heartbeat, but recovers until the next miss. If severe, it can be more of a sputtering and loss of power under load.

Fuel-Related: Often due to insufficient fuel delivery to an engine under load. Feels like a drop in power output (rather than a miss as above) and recovers if the load is reduced.

As noted in the previous post, an engine can rev fine while in neutral but then start to experience issues when the fuel delivery increases and the cylinder pressures rise.

-Bryan

[MIGHTY 8]

Thank you again for these very helpful comments.
I’m now running without a fuel return to the fuel tank, although have operated also with a fuel line returning to the tank (car was previously FI.)

I am under the assumption that it is possibly a fuel pressure issue. I am running a Facet Gold Top pump located underneath the car just forward of the tank. I have installed a standard inline fuel pressure gauge which does show 3.5 PSI, but that is only available to view while in neutral.