hello,

I attempted to install a holley 750cfm double pumper carb on my 460, to try and squeeze some extra performance before i get the money drop my new motor in. anyhow, before it had a 2bbl motorcraft. now, with either carb, it will run if i have the pedal halfway to the floor, but if i take my foot off the gas, it will back fire (and sometimes the carb will catch on fire) and die. also, with the holley it backfires while its running at that high speed. i'm thinking its going to be a valve timing problem, but before i pull the front off the motor i'd like other opinions. thank you

 

Are you saying you changed intake manifolds ? Need more imfo.

 

i only changed the carb, the 2 bbl had an adapter to fit on a 4bbl intake

 

Always use the correct intake manifold or you will actually loose power. If that carb really is a double pumper it probably is a 4779 (look at the choke tower for the number). This particular carb should only be used with a manuel transmission and then only for racing. It will get awful fuel economy.

This carb was famous for "blowing out" power valves with resultant backfiring. Get a rebuild kit.

A better choice to make power and run well on a truck not always used for racing would be a good duel plane aluminum manifold with a vacuum secondard 750 CFM carb.

When we raced cars I was always amazed that a properly tuned vacuum secondary carb would be significantly faster than a double pumper of the same size. That was always true with any automatic transmission.

 

lorenr is right about the vac sec thing.... i have a 76 F150 with a 460 780cfm vac secondary, and my friend has a 78 F250 750 double pumper... both trucks run top notch but mine smokes him every time....anyways....change that intake to the correct one, and then u need to adjust your timing...thats why your backfiring...unhook your vacume advance and plug it... now get a timing light and set your timing between 12 and 14 degrees BTDC at 650 rpms...my factory sticker calls for 12 degrees... it just depends on if i want power(14 degrees) or economy(12 degrees)....but thats just a starting point you can adjust either way 2 degrees at a time between test drives to find what you like better....also.. when your engine is off and the #1 piston is at TDC turn the crank so the timing mark is on the degree you prefer and open the distributer...under the rotor bug there are 8 prongs sticking out from the center... one of them must be lined up perfectly with the electronic pick up inside... if it isnt move the distributer until it is...the duraspark II igniton in most ford trucks is not adjustable.. it only has one position that will work completly right..it took me a long time to figure that out...no mechanic anywhere that ive been knows about that... thats why alot of old fords run crappy and spew black smoke and backfire...just because of the timing..try that...i believe its the answer you need....it made my 460 smoke these new camaros off the line...and anything else for that matter

SwOkcOffRoader
1976 F-150 460 780 4bbl holley "Hot Rod"
1978 F-250 4X4 snow commander
351m 500cfm 2bbl holley 35"tires "Big Daddy"
1978 Ford B-700 bus 389 industrial block 10 speed

 

Just for the record I ran all kinds of different holly"s and got my best performance and fuel economy from a motorcraft 4350, believe it was 630cfm.on a 429,11:1, and I think cam was .510/.292. One backfire on a holly and you need a power valve,SwoKc. is right about timing,when the DS2 is working properly I don't think there is anything better up to 6 grand.

 

i just rebuilt it right before i put it on

 

my friend that put a double pumper on his 460 turned his rig into a tire
shredding beast..(after I made that timing adjustment i mentioned above for him)...the double pumper came of a methynol race car...
so that means that carb was built for wide open conditions...he has alot of problems keeping it running steady but when he hits the gas it comes to
life......double pumpers are great carbs but not on daily drivers...my 780 vacumm secondary runs consistant....till winter.. then a little warm up time is needed..but i never have to mess with the mixture screws...just be sure those mixture screws are not loose on your carb.. if they are u have to change the seal on it.......by the way.. what is this 460 of yours in??

SwOkcOffRoader

1976 F-150 460 780 4bbl holley "Hot Rod"
1978 F-250 4X4 Snow Commander 1 Ton axels
35" tires 351m 500cfm 2bbl holley "Big Daddy"
1978 Ford B-700 389 industrial block 10 speed

 

its in a '79 F150 2x4