Carbs: 750 -- 650 -- 600?

[droptop73]

What is your goal? What do you use the car for? There are way too many "undriveable" cars out there due to a poor selection of parts. 

A 750 will offer more power and performance, a 650 will offer better throttle response and drivability. A smaller carb, within reason, gets better "signal" from the airflow through the engine and will be more responsive to throttle opening. The larger carb, again within reason, allows more airflow and potentially better performance. 

A carb recommendation is not a generic "this will work best" proposition. With a mild cam and 4v heads I'd suggest refreshing the 750 you have unless the throttle shafts are worn out. On a 2v with a mild cam I'd go with a 650 ish carb. It all depends on the combination. 

 

[jscott]

just regular driving, nothing special.  Would like better low end response, right now a bit sluggish off the line so to speak. 

 

[droptop73]

That is likely a combination of no vacuum to the distributor and maybe not enough accelerator pump. I've never really been a fan of the Mallory distributors, I put in a Duraspark with an old Ford Motorsports (MSD) box and love it. I paid a friend with a distributor machine to reset the mechanical advance to provide 26* at about 3000 RPM.  That said it's hard to diagnose running issues over the keyboard. 

Is the existing carb in good shape mechanically? If so, working with the carb you have will be the most reasonable route. 

 

[jscott]

lots to consider

 

[Kilgon]

I can't speak from the part of having a 351c but I would think based on your info for what you have and are wanting in performance a 650 to 700 tops would work well.  Way to many people oversize their carbs and then fight issues with running rich and foul plugs.  I bet half the guys running what they think is the right size carb have carbon on their plugs whenever they pull them and are constantly fighting a rich mixture.    The video I posted provides good info and should be used as a guide but it is not the gospel.  I have a 460 that can easily handle a 850 dp but for my driving habits and use I am running a 750 with vacuum secondary and it runs great.  It's little easier on the gas also.  

 

[jscott]

Just put a new Holley 600 on and she started right up.  After a few adjustments idles a lot better.  Need to get a return spring and c clip to finish kick down part.  At the same time I pulled the steering wheel to put a new one one.  I had ordered one with a billet aluminum hub, they sent a black bub so I am waiting for the correct one before I take a drive.........Unless i really get the bug to see how it runs then in will pop the old wheel back on.   When i took off the old 750 it seemed to have lot of gas in it.  How much gas sits in the carb normally.?

 

[c9zx]

A somewhat too large (by 100-150 cfm) of a carb is not what makes the engine run rich and foul plugs. Running a carb calibrated for lots of camshaft overlap and the accompanying weaker "signal" (read lower vacuum) on a stock or near stock engine that has a stronger signal and therefore draws more fuel at idle cruise and WOT is what cause the rich condition. Only once I had a carb dead on out of the box, it was a fluke in my opinion. You might look into things like idle channel restrictors, power valve restrictors, idle air bleeds, hi speed air bleeds, and emulsion tube hole sizing. The cfm for the stock Boss 351 and 72 and 73 Q code was 715-750 cfm, depending on what source you believe. Please do not take that as an endorsement of the 4300D carb, I have hated them since the first time I tried to make on "run right".

The center pivot bowls hold about 4 oz. of fuel, some variables (float type, jet block or plate, setting). Chuck

 

[jscott]

well I could not resist so I popped the old steering wheel on and just took her around my development.  We live in the GA mountains in a VERY hilly development so not a lot of high speed runs but climbing some steep roads she seemed a lot peppier.   Being this carb was a freebie I can always go up to a larger one, it is one of the simplest things you can do to your engine work wise.  So will leave it on for a while until the car is done and see how it responds on the open road. 

 

[jscott]

Maybe I am too simple minded and not very mechanically inclined.  But when I got my 1968 AMX years ago I did the same thing.  I do not remember the size of the carbs I swapped on that 390 but again I bolted on the new one (Edelbrock)  and just drove.  Just put it in, few minor adjustments and it ran great for the next 7 years while,I had it. I remember a co AMX  owner, very mechanically inclined put an EFI on his 360 and could never go out without the tow trucks phone number handy lol.