Hesitation 351C

hi all- begging your indulgence here. New to posting and not the most technical person. I've got a 73 mach one with a 351C 4 bbl carb. starts great, idles smooth. But it's got a flat spot when I accelerate. doesn't matter what speed, if I'm starting from a dead stop with a light pedal, i'm fine. if it give it more gas it hesitates and then kicks in just fine. If I'm driving along at 35 or 50 mph, then give it more gas it hesitates for a second then kicks in fine. My mechanic tells me the carb is fine, the fuel pump is fine and the tranny is fine.
I've put new plugs and wires in and advanced the timing. That helped a lot but not completely. I can't advance the timing any more as it then won't start. (it already is at a point where it's a hard start). I've got points in the car, should I convert to a high output electronic ignitiion? Will that allow me to advance the timing a bit more to maybe get rid of this hesitation? Any suggestions? I love the car and it's pretty quick when you get on it, especially when the 4bbl kicks in...but this hesitation is driving me nuts. Thanks

Is this a new car for you or have you owned it for a while? If you have owned it for a while is this a new problem or an old issue that has never been resolved? Now sometimes there is a hesitation while the 4bbl is opening (aka: "the doowa's"). Is the carb mechanical or vacuum secondaries. What tranny is in the car? Has your mechanic checked the fuel filter? A lack of fuel due to a clogging filter can cause weird hesitations. More info will be needed to better answere your question.

Thanks for responding. I bought the car about 3 months ago and the problem has been there since I bought it. It is better since we advanced the timing and put new plugs/wires in but there's still some hesitation.
I believe the carb is mechanical but I'm not sure. I can tell you that when I'm driving and floor it to open up the 4 bbls, it kicks in just fine. The hesitation is when you give it more than a little gas and less than stomp on it. The fuel filter has been checked, no problems there. It's an automatic transmission. I don't think it's the trans and neither did my mechanic as it's not at any particular speed. Once I'm maintaining at any speed, and then give it more gas, it hesitates for a bit and then goes up smoothly to the next speed I bring it up to, then if I give it more gas, it will hesitate for a bit and then go up smoothly to the next speed level. Advancing the timing helped a lot but I can't advance any more or it won't start. thanks again for trying to help.

Sounds like carb issues to me. Just need to get it tuned right. What carb do you have?

It's a 4 bbl Holley. Should I just buy a rebuilt one or try to get someone to rebuild it? My mechanic won't rebuild it. If I replace it, should I get another Holley or switch to another type? If I do switch, do I get into intake manifold issues?
Thanks

What is the part number on your holley?

Sounds like the accelerator pump. Rebuild the carburetor.

Sounds like the accelerator pump. Rebuild the carburetor.
Or squirter. I was thinking more along the lines of just the acc pump gasket or cam/linkage. But depending on the carb it could be more specific.

Thanks again to everybody for trying to help me out. I was checking on the Holley website and they described the exact problem I'm having in their troubleshooting section. (Bogging). They suggest putting in a heavier spring to control when the secondaries kick in. Apparently if they kick in too soon, the bogging will occur. They've got a set of 8 springs with graduated tensions. I'm going to try swapping out the spring this weekend on a trial and error basis and see if that helps. I'll let you all know. I'll also get the carb # then.

I had the same problem with the carb on my '69 Cougar. But someone had monkeyed with it and messed it up pretty bad. It was missing the needle valve, 4bbl linkage was screwed up, acc pump gasket was folded up when bolting on, ect. It's rebuilt now so we'll see in a few days how well it works. Its a Holley 1850-1 or something similair. 600cfm single feed vac secondary 4bbl.

Thanks. Well, here's where I stand now. After looking at the Holley carb site, they indicated the bogging I was experiencing was most likely caused by the secondaries kicking in too soon. They recommended putting in a heavier spring. I tried several different springs and that didn't seem to help all that much. Also played with the timing again. Advancing the timing certainly makes the biggest difference but I get to the point where the thing starts hard like a race car......and the hesitation is still there somewhat. Someone else recommended putting in an electronic ignition and a high voltage ignition coil and increasing the gap in the spark plugs. I'm going to try that. If that doesn't work it's time to try to find someone who can rebuild the carburetor.
I'll let you know how it goes......

Happy New Year to everyone. Sorry it's been so long but I've been away from work for a couple of weeks.

Here's the deal. Put in a new rotor and Pertronix electronic ignition, and flamethrower 40,000v coil. It didn't help the hesitation.

Finally gave up and bought a new Edelbrock carburetor to replace the Holley. No more hesitation! Finally! But it idles really high and if I back it off it runs like crap, especially under load. Looked for vacuum leaks, made sure the distributor vacuum hose wasn't on the wrong port, played with the idle and fuel/air mix screws and the timing, looked at the troubleshooting CD six times and think I've arrived at the best balance between idling high and running well. She's now fast and has no hesitation. Now I just have to wait for the warm weather.

If anyone has any advice on getting the idle down without impacting the performance, please let me know. Happy new year!

How high is it idling?

I don't have a tach in the car but real high. If i back off the timing or lower the idle it stalls when I put it in gear/under load. Right now if I put it in drive it would probably drive 8-10mph without touching the gas. I'm wondering if a vacuum leak of some sort?

Sounds like a good possibility to me. When I had the 302/fmx in my '69 cougar i had to set the idle high (1700) so it wouldn't die when I put it in gear. I had a vacuum leak.