A buddy of mine and I are going to start work (slowly) on his 76 Bronco. He just put a 4 barrel carb on what we both still assume is a 302 with 351 heads. I've seen plenty of carbed engines run nearly as smooth as an FI one. His bogs a little when the throttle is opened. It's like it just has a bit of hesitation. To me the truck doesn't idle as smoothly as it should. I just wanted to help rectify him of this problem because I think the engine, overall, runs smoothly. As smooth as it can. I just wanted to know some troubleshooting tips that might help me get on the right path to a more responsive set up.

We are going to completely build a new 302 for the truck later this year or early next. For now I just need to get the truck in as smooth running a position as I can.

Carb is a Holley 750 (?? He didn't remember. It's way too much carb for a practically factory 302 tappet) If the carb needs to be taken down a few CFM then I need to see if we can sell it. Bigger isn't always better, especially in the case of small blocks.

Let me know what you guys think!

 

I would think a 600 cfm carb would be good for it

 

accelerator pump often wears and without that spray there can be a lag or hesitation.

 

If the carb is a 750 Holley double pumper with mechanical secondaries, that's way too much carb for the street. A 780 with vacuum secondaries might work if you're running say a 4.11, 4.30 or 4.46 rear end. The original Boss 302 used a 780, but that high revving engine was set up to breath way better than the 302/351 Windsor heads. A 600 Holley with vacuum secondaries would be more drivable and have better throttle response for you. Even when the carb CFM is matched well to the engine CID and rear end gears, you can have a stumble or bog from too high an idle speed, not enough transfer slot tip in, and/or accelerator pump problems.

If you can confirm the accelerator pump is spraying a good shot of gas and that there are no problems with the ignition system, I would readjust the 2 idle mixture screws, starting with about 1 1/2 turns from gently closed, use as little of the throttle position screw as possible and bring up the rpms using the 2 mixture screws. final idle speed adjustment should be minimal with the throttle position screw. If the primary butterflies are set too open by the throttle position screw, it leaves you less transfer slot to use when you transition from idle to tip in to accelerate and up end up with a stumble.

Another resource would be here:http://www.bob2000.com/carb.htm
I fooled around with small block Fords and Holley's for about 20 yrs and this guy knows what he's talking about.

 

I know about half of what you said. I will look into it. I hope he still has all the information on the carb. I was doing a little research and a 600 cfm should be fine. Although I do plan on getting the aluminum GT-40 heads as well. I will check out that web site and see if I can find anything.

 

Single or dual plane intake? A dual plane intake is a bit more tolerant of a large carb since plenum volume is smaller.

A single plane on the other hand is letting each cylinder see that 750 cfm.

 

Factory, so I'm going with dual plane. I kind of assumed that the dual plane would be a bit more lenient, but I haven't really had time to play with it. Worst case I'll get Holly on the phone and have them help me out. It almost is a non issue since the engine will be replaced. I really wish he'd have gone with a 600-650 cfm with vacuum secondaries. I still don't know much about the 750 that's on it now.

I suppose first on the agenda is getting rid of the hard top shell he has and putting in a new roll bar with soft top attachments. That and really cleaning up the truck, get rid of the junk and useless crap that's in it right now.

Worse part is that it isn't my project and I have ideas for what I would do but I have to step back, it's not my toy. I might post some project pics and info in "other Fords" when I get a chance. Or we really get to working on it.

I think I have him convinced to let me have the current 302 that we will be pulling out of the Bronco for a Rat Rod project.