Thread: 780 Holley Double Pumper Carb for the Street??

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I should have got opinion's before putting a 248/248 Cam in my 402 BB.I just thought the bigger the Cam the faster the car would be.(inexpierence i guess)

well the bigger the cam the faster the car is true.... as long as you build the rest of the engine with the cam specs..... i undercammed my first few high performance builds just because i was afraid of over camming

Originally Posted by thesals

well the bigger the cam the faster the car is true.... as long as you build the rest of the engine with the cam specs..... i undercammed my first few high performance builds just because i was afraid of over camming

Even though I have stock ported oval port head's the car makes good power from 2500 to 6000 RPM'S.I think though that this Cam could be killing my gas mileage and today that's somewhat important.I never had a working gas gauge in it to see how bad gas mileage was with my 402 BB.Its not fun getting less then 10MPG if the car is only driven on the street!! I know a smaller Cam would give it even better street manners and I could get rid of the 4:11 Race Gear's especially since I don't have an overdrive for it.

my reccomendation would be same thing i did..... forget about gas mileage at all on your classic, you're never gonna get good mileage.... make it your fun car.. drive it maybe 2 times a week just to cruise or go out with a lady or such... and for the rest of the week buy a little beater thats dependable, and gets good gas mileage.... i myself bought an 89 nissan sentra.... its not pretty, and it only has 90 horsepower, but after tuning it the best i could.... i get 42 MPG

69....

I have run a 750 double pumper on a 70 Z/28 with fine results just as the other guys here are saying.

Question: how can some outfit turn a 650 double pumper into a 780 double pumper? I'll tell you how: they piled up a bunch of used parts that included the body of a 780 vacuum secondary carb. They added the bowls from your 650 DP, etc. The carb is probably a mismash of parts....did they know what they were doing? If the carb doesnt run right, then the answer is _______
(insert answer here).

mike in tucson

Well I don't no exactly what all they did to the(650DP to flow 780) Carb other then put a 4 corner Idling Circuit, because it was many years ago.I have tried 2 Holley Double Pumper Carb's but regardless of the tune my spark plugs alway's were carbon fouled.I had new MSD Ignition,Coil,Plugs,Advanced Timing at Idle.The one thing it did was run great with Nitrous the first time I used the NOS System.After that I had the carb sent out to Texas to cure the fouling of the plugs.Now with the Nitrous I get instant detonation luckily my engine parts are Forged.

Originally Posted by thesals

my reccomendation would be same thing i did..... forget about gas mileage at all on your classic, you're never gonna get good mileage.... make it your fun car.. drive it maybe 2 times a week just to cruise or go out with a lady or such... and for the rest of the week buy a little beater thats dependable, and gets good gas mileage.... i myself bought an 89 nissan sentra.... its not pretty, and it only has 90 horsepower, but after tuning it the best i could.... i get 42 MPG

I never really worried about gas mileage until these 2 last years.Only because this is not meant to be a race car.I would like to think I could get at least 10mpg with a 402 BB but with the rather large Cam I have who knows

The one thing I don't have is a choke on the carb.I wouldn't think that would matter on a 90 degree day though.Im afraid to take it out on the road until I can get it Tuned and Idling correctly.I could live with the big Cam even though it has a Lopey Idle.Just need to find the right Carb.

Hot day here in Texas is 105 degrees.So you all ready half way there before you even turn it on.

Originally Posted by DennyW

You wouldn't think a choke would matter on a hot day. How ever, it does. The engine is a combustion engine. That means it's designed to work, and work good When it's up to temperature. Did you know it takes approximately 1 hour of engine run time to heat the block to even temperature ?

The initial start up, and for the first 3-5 minutes require a little richer mixture(needs a little more fuel cold) until there is enough combustion chamber temp to ignite the fuel properly. Just some thoughts.

Thats probably the only thing thats going to help me is a choke.Taking 10 minutes to Idle is ridiculous.I can go out and buy a new Carb but I just will have the carbon fouling of my spark plugs again.Im not an expert tuner but this was the reason for having the 650 DP sent out to get rebuilt.Its a very touchy Carb with the 4 corner Idling circuit and when it does idle, it picks up Rpm's at Idle.It goes to 1000 rpm to 1500rpm's without touching anything.It sounds almost like it start's to run lean.