Dies at half throttle on the road

[Krafty]

Okay if you guys have been reading my other posts Im still working on the same problem, and I really need some input.

1969 429 thunderjet in 79

Carb is good, was rebuilt with bigger jets since the problem started, also tried a different carb. Changed the coil, changed the ignition box, has new plugs and rotor and cap, changed the fuel filter, I am going to remove the carb spacer tomorrow.

Now the problem

It used to just studder slightly at half throttle when on the road , then it got a little worse, and now the engine completely dies when at half throttle and studders a little before and a little after. if I let off the throttle a little the engine fires right back up and runs smooth and the same when I floor it.

Can't get it to do it when sitting in neutral or park. Or when the engine is cold.

HELP!!! This is my daily driver and Im really close to doing lord knows what to that thing

 

[Bronc76]

have you checked timing? did you check to make sure the plugs are gapped correctly?

 

[Krafty]

none of the settings of the timing or plug gap has changed since before it started dieing, the engine had been driving fine after I put the carb spacer in for about 3 months before the problem started, and the timing and plug gap for about 4 months before the problem started.

 

[Bronc76]

I dont think this is the problem but have you checked for vacuum leaks?

 

[Foul Al]

I dont think this is the problem but have you checked for vacuum leaks?

sounds like a fuel delivery problem to me. i swapped a 390 into my first bronco and i had a similar problem. the 390 was cammed and had a 650 in it. i went with a holly red electric fuel pump and put a gauge on the line to read my pressure. it turns out i was starving the motor at about 60 mph. i adjusted the flow a little and it ran like a ***** ape!! mine was backfiring a little and popping, mostly under the hood. i hope this helps.

 

[Krafty]

I put in bigger jests, and it was running fine with the setup it has for quite a while before all this nonsense started.

Would it not need more fuel at full throttle then mid? so the thing would be getting lots of fuel when I floor it cause it will run all day at full throttle.

how did you adjust your fuel flow? also could it be the fuel pump not keeping the fuel pressure high enough at the lower rpms?

 

[Foul Al]

I put in bigger jests, and it was running fine with the setup it has for quite a while before all this nonsense started.Would it not need more fuel at full throttle then mid? so the thing would be getting lots of fuel when I floor it cause it will run all day at full throttle.

how did you adjust your fuel flow? also could it be the fuel pump not keeping the fuel pressure high enough at the lower rpms?

i had a fuel pressure regulator on it with a gauge that i could increase or decrease the pressure. i also had a holly red or blue( i forget wich one) electric fuel pump on the inner rail back by the gas tank. good luck with the remedy, please let us know what you find to be the issue as i am curious.

 

[Krafty]

alright so I just got it back from the shop today.

Fuel line had a crimp in it, the timing was off, the timing advance was off, the carb was out of tune, and a case of beer and an IOU for 30 bucks later and I have my 429 back.

now I have a new problem. I got home pretty late so I could ask my uncle about it.

at wide open throttle at 40mph+ (I believe) it boggs initially but where it usually pulls out into the power band and winds out, instead of winding out it starts running lame. it sounds like its missing.

in my one hour long trip home of putitng it through the passes, I think its running lean cause I burned next to no fuel considering the bigger jets and the 429 cubic inches.

has anyone else had that top end problem? and what was the casue/ what was the fix?