[1968KR]

Need help with a Holley carb, here's what is happening: The car runs and idles smoothly. It drives very well....but under "hard" acceleration (usually shifting into third gear around 70 mph) there is a hesitation. The car would recover after two seconds and "come back" to accelerate. I originally felt like maybe the carb was getting to much fuel (until yesterday) I got on the car and it hesitated (a feeling like taking your foot off the gas) again...this time no recovery. I backed off the gas and the car still did not recover. A gently pressure on the throttle and all I got was some coughing and sputtering. After that I never got above 50 on the ride home. When I checked the carb, I expected to find gas everywhere. Much to my surprise the two rear venturies were covered with a black smoky residue. What is going on? Is there any fuel in the secondaries at all?

 

[sharkmouth]

The very first thing I would consider since it happens at speed is fuel delivery (pump, filter, and pickup screen).

 

[Axnjxn]

Possibly the secondary springs or power valve are set too "tight" and are opening too late (at too low of vacuum). Maybe you can try a higher valued power valve or looser secondary spring. Just a guess.

 

[camachinist]

Perform a fuel pump pressure/volume test.

Kinda reminds me of when the supply pump (like your fuel pump) on my diesel pickup went south...I was driving back from LA at night and couldn't go over 55-60 or the engine would lean out. Evidently, the pump was nearly gone and the main thing that kept the engine going was the fact that I had nearly full fuel tanks and the vacuum created by the injector charge pump kept the fuel flowing....

In your case, you might have leaned out and gotten some mild backfires, which sooted the venturis. Be sure, once you diagnose the problem, that you inspect any power valves in the carb for diaphragm integrity. Backfires can damage these parts on some Holley carbs.

If you have any more details or an update to share, please post..

 

[sidewinder]

I would also check the float level and timing.