alright, my aunt baught a 1968 mustang 390, and it had a 750 double pumper on it. she took it off and put a 450 on to save gas, and she gave the 750 to me for 80 bucks. i have a LO3, with headers and true dual exhaust, and i was thinkin since i have this double pumper, i was going to go ahead and get a new manifold and new distributor, and while i was in there, replace the cam with a better one. any reason why i should not do this? i understand that the 750 is quite big for my 305, but will it really have a negative effect besides bad fuel economy? right now, i don't care about MPG's, so 8 mpg is ok for me lol. but will it have a good power boost as apposed to my TBI, and also just so i can make this 1 post, whats a suggestion on a cam (i have stock heads). any and all input on this is greatly appreciated.

 

I think the bottom end of the RPM range will get "soggy" with a DP that big on such a small motor. With the really tight torque converter they used in the trans with a TBI motor and the (usually) tall rear gearing they also usually use in a TBI car you may well end up going slower, not faster. A similarly sized vac sec carb would work just fine, but you don't have one of them, I know. I'd save that carb for the non-emissions 350 you'll eventually build and put in there.

Hotrodding the little 305 TBI motor has been done- you can read all about it in the TBI section of this board. The bottlenecks are just about everywhere- exhaust (which you've taken care of), cam (really really small), heads (swirl-port design), intake system, chip tuning, not to mention it's got 45ci less than it should have.

 

If you want to sell that holley DP hit me up... ive been looking for one that doesnt need a rebuild.

 

you can make the carb work, but if you are not real farimulaer with carbs and carb tuning it will be a lesson, more than likly you wil need differnt booster, you can actuly make more across the board power with a 750 cfm than with a 600 cfm on that motor. I believe that hot rodder had a artical on it and how the air flows arround the boosters and wich work best for what types of applications- this is a crap shot, but if you are keeping it under 6000 rpms and using a stick, go down 4 jst sizes and use a slightly steeper ramp ange for the throtal blades, down leg boosters and a smaller needle and seat for both the primary and seconday sides. after this start the car and get a vacume reading and then install the proper power valve. Better yet find a carb guru to take you unser his wing who will do it up right

later and
GB

rk

 

Your going to have a hard time getting the off idle stumble tuned out if its a double pumper, you really need a vaccum secondary carb. As far as the jets go start with the jetting Holley used from the factory and decrease size according to your altitude, factory jetting is set at sealevel. You can find factory settings on holleys web site.

 

i had a 750 dp on an old 305 that went 13.0s at 103 and 11.70s on the pill. It can be done, but took a lot of tuning. Was worth .3 at the track over a 650 VS carb. Big stall and big gears is a plus

 

my experience with that large of a carb on the 305 is it has NO street manners. idles irratically (not in a good way), rich at idle, lean at WOT, off idle stumbling (and i have a 4000 stall speed converter and 3.73's), etc. a 650 double pumper is the way to go on my car, and it's still daily driven, gets about 12mpg in town and 21+ on the highway and has managed to click off a 12.31@108mph without n2o.

 

That 750dp is setup rich at idle. This will be your biggest issue. The way to correct this will be to drill the idle air bleeds or restict fuel at idle with some wire. That double pumper will want a higher than stock stall converter also. I would actually not use it for this application.