302 1977 Sport Nearly stock is bucking/hesitating at low rpm

[irclausen]

Whew where to start.

I've had this bronco for 17 years it was my first vehicle at 15 years old and have never had any major issues.

1977 Bronco Sport uncut and as far as I know stock.

302 engine with as far as I know the original carb (confirmed motorcraft 2150)

I'm a novice mechanic with a lot of interest in this vehicle.

Recently the bronco has started "bucking" at low rpms (like I have to press really hard on the brake, and she really wants to go). Upon releasing the brake it's pretty anti-climactic. If I hit the gas there is plenty of power, but when on the brake it feels like she really wants to go.

There is a lot of history in 17 years happy to share but will start with most recent changes:

Running terrible, messed with the carb mixture screws and idle (likely a bad first choice)

Then did/replaced the following:

-Seafoamed
-Spark plugs and cables
-Ignition Coil
-Ignition Control Module (ran much better after this likely combo of plugs and ICM)
-Removed totally rusted and leaking muffler and replace with a cherry bomb (thrush).
-Bought a vacuum gauge to try and tune the carb (started at 1.5 turns out on mixture screws and 700rpm, tweaked for max vacuum)
-Suspect vacuum leak on the choke pull down (waiting for part to replace)
-Removed choke pull down and plugged line

Still experiencing the "bucking", looking for suggestion on fixing.

 

[jrwyant]

Sounds like a good start. What else did you check for vacuum leaks? Brake booster, distributor, transmission vacuum modulator, EGR?

 

[jckkys]

I'm not sure I can find a synonym for "bucking" in common automotive terminology. A lean surge is the best I can find. "Wants to go with the brake pressed really hard" is a different matter. Is the accelerator peddle or linkage some how moved by the brakes being applied?

 

[73azbronco]

so is it your brakes or the motor? If it "wants to go", you have a good motor, right? So you have a brake problem? Automatic then?

 

[September 1972]

Restriction in main jets caused by vehicle sitting with a belly full of ethanol laced fuel or E-85 fuel which caused phase separation looks like egg whites is one possibility. Pull carb and take a peek..

 

[jrwyant]

I had a "surging" type of experience once, when my coil's plastic horseshoe connector had heated, backed-off, and the wire connectors (which had not fully seated but instead merely touched the coil's studs) would lose their connection... I would feel the loss of engine power (fading), and I would then react by pressing the accelerator more, and as the wiring/horseshoe connector contacts would momentarily re-connect long enough for the engine to pick back up, the engine would seem to "surge" due to the increased throttle. But, right after that, the connection would be lost, and the engine would begin to die again... overall, it felt like something you could describe as "surging".

(That took me a while to discover/resolve. For some reason, that particular aftermarket ignition coil's two brass studs were slightly taller than the prior one's, and when I would clip that horseshoe connecter to the coil, I thought it was fully connected/seated, when it wasn't.)

I'm not necessarily suggesting that that's your problem, I'm merely saying that I can picture what you're describing.

 

[ledslinger29]

Nothing wrong with the engine if you sea-foamed it. Look at the brakes.


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[blubuckaroo]

I'd try plugging off the brake vacuum booster hose to see if there's​ an improvement.

 

[jckkys]

I'm amazed that there are so many cures being recommended for a problem no one can define. The OP describes his Bronco "bucking", what ever that means, associated with applying the brakes. Then he says the truck "really wants to go" while braking. Does "really wants to go" mean the engine throttles up or nearly dies? Then he says that releasing the brake is "anti-climatic". What does he expect to happen? Was the truck supposed to accelerate? Until he provides a coherent articulation of the problem, there's no way to offer a solution.