Spread Bore vs Straight Bore Carbs

I would like to hear some opinions on the two types of carbs on a 351C 4V. I currently run a good size cam and a Holley 750 vacum secondary carb with a Ram Air assembly. Problem is that because of the cam creating low vacum and the Ram Air using what little vacum there is, the secondary's are opening early and overfuelling the engine. So I am going to change to a Holley 650 DP with mechanical secondaries. Should I look at a "Staright Bore" or "Spread Bore" for this application? I mainly use it on weekends to cruise around but I like to roast the tires and tach it up. I weclome any feedback.

You need to get a holley spring kit. No need to buy a new carburator for that issue you have. It has instructions on what spring you should use, and how. Maybe $30.00 for the kit.

Thanks, I will look up a spring kit. I was told that a 750 is too big for a 351C 4V, and that a 650 DP would be a better match, any thoughts?

Clevelands love to breath, especially the 4V. Can we get any more info than a "good size cam"? Im fairly confident that any decent running cleveland would be fine with a 750, just needs a little tuning. What intake manifold do you have?

Ok if you have LOW Vacume how can the vacum secondarys be opening early ? Maybe I'm not reading the post right but .....

the vac secondaries open when vac is really low because you wack the primaries open, and that would make the vacuum as low as it will ever be, and thats when you want the secondaries open.

Since he has such low idle vacuum the secondaries think it is under load, and he wants the secondaries open.

At least thats the impression i get, maybe im understanding this all wrong

You are exactly right. The vacum keeps them closed until the rpm come up and the vacum drops, allowing the secondaries to open. I may be wrong but that's the way I understand it. Here are the cam specs

Intake .503 valve lift 308 duration Hyd Intake .291 lift at cam 224 duration
Exhaust .507 vlift 314 duration Hyd Exhaust .293 lift at cam 223 duration
Max intake lift 104 ATC and 108 degrees on Lobe centers.
All of this sit's atop an Edelbrock "Torker"
Does this help?

Originally Posted by sharpshot71

the vac secondaries open when vac is really low because you wack the primaries open, and that would make the vacuum as low as it will ever be, and thats when you want the secondaries open.

Since he has such low idle vacuum the secondaries think it is under load, and he wants the secondaries open.

At least thats the impression i get, maybe im understanding this all wrong

You could but a new carb and have the same problems. As mentioned you could get a stronger spring for the secondaries and you may need to look at power valves with a lower vacuum setting. don't be afraid to jet it a little leaner.

What is your idle vacuum reading?

Usually, the small primary/large secondary carbs were used for emissions and economy.

I was thinking of using a crane H288-2 but you cam seem to have more duraration. I have the same set up also using a torker. what power rang? How do you like it? who is it made by? I would like to compare it to the crane maybe i will get the one you have