Running Rough When in Gear and foot On The Brake

[CopperRanger]

is there a drop in idle speed when placed in drive? Does the carb have an idle dashpot?

Yes the idle gets slower when placed in gear. Not sure what an idle dashpot is.

 

[DirtDonk]

It's a disc-shaped canister of one design or another to act as an idle control device. Usually (if not always?) controlled by vacuum.
I've seen a ton of them that were just there, with no vacuum or electric connection that I never really understood. No idea what they did, or how they worked and never bothered to look them up.
Maybe just spring loaded and there to look pretty?

A dashpot activated by vacuum would kick the idle up under predetermined conditions. Either there just to set the normal idle when vacuum was applied, while others were there only to kick in only when the engine reached a certain temperature and it was typical to need higher idle speeds to keep from stalling.
Other vehicles used them also to raise the idle speed when the A/C was switched on.

Many EB's at least up to '72 had an electric solenoid which would have been for anti-dieseling purposes so when the key is on it's what sets the idle, and when the key is off it closes the carb down to like a 400-500 rpm idle speed to avoid run-on.
Not sure what year Broncos used what type of auxiliary idle control devices. Is your '74 mostly original? Anything like that on the carburetor adjacent to the throttle lever?

It's normal for the idle speed to come down anywhere from 100 to 300 rpm, or maybe even a tad more. But if the idle speed in neutral is already too low, in Drive it might be beneath the threshold where it starts to run rough.

Paul

 

[DirtDonk]

Oh, and I don't remember if we mentioned it or not, but did you see the thread the other day showing the deterioration of one member's carburetor spacer? It's an EGR type spacer like yours would have to, if it's still original.
If yours is still using it(?) then it could be leaking and causing issues that will never be able to quite be tuned out. Not without fixing the actual issue.

In his case the hot and corrosive exhaust gasses have literally eaten through the spacer in a couple of places. Extremely common on the aluminum spacers, but even the cast-iron ones can't stand up to it forever.

Paul

 

[jckkys]

Dashpots were used in the '50-'60s to slow the throttle closing. They simply had a spring, a diaphragm, and a small hole to let air in and out. There was no external input. The later ('70s-'80s) idle solenoid was activated by the ignition circuit. Neither of these, working or not, would cause your problem. The '74 EGR spacers were, I believe, aluminum. That's the reason they fail by exhaust gases eating through the aluminum. Later perhaps '75 up EGR spacers were cast iron. These are easy to find on ebey or RockAuto. Cast iron spacers won't be a problem for decades. Any carbureted engine will have a significantly lower RPM in drive than in neutral. This is aggravated if the idle mixture is set in neutral. If it makes you nervous to stand in front of a truck in drive with the emergency brake holding it, have someone apply the brakes for you. Some air will always enter the carb by the throttle shaft. It doesn't mean the carb is worn out. Holleys are made to work on any engine the Motorcraft 2150 was calibrated for a specific engine, vehicle, and transmission. Here's a good example;https://www.ebay.com/itm/REBUILT-MO...288342?hash=item23be767016:g:f6sAAOSwdmRc-8~x if you needed one which I doubt. Either way the 2100 is a far better carb period.

 

[CopperRanger]

Thank-you guys. You gave me quite a few things to look into and think about. I will wait till I try the idle and fuel mixture in gear before jumping the gun and buying a new carb, but if I do I will consider the 2100. I will also look at the spacer again. This week will be busy so it may have to wait till next week, I will keep you posted.