Carburetor jet sizes

[coupe32]

I have a question concerning carburetor jet sizes. I am running a 340 5- speed manual transmission with a performer RPM manifold and a 700 holley double pumper carb (4778-3) I have changed the jets and have gone up about 3 sizes on both primary and secondaries. Before I changed jets from the stock ones the engine had a bit of hesitation with the new carburetor out of the box but that went away after the jets were changed. Is there any way to figure jet size or is it just hit or miss, change them until you get it right. Maybe some of you are experts on this and can help. With the jets I am running now the engine is running too rich with black sooty exhaust and plugs. It seems to run okay but is really running rich. I guess I will back off on my jet sizes. Anyone have a better idea of how to dial in the proper jets other than to keep changing them? The stock jets for this carburetor were 69's on the primaries and 78's on the secondaries.

[peg leg]

Your doing OK, and you apparently understand how to cure performance issues. It's a science, and jetting is a tricky one. First advice; don't go more than one jet size up at a time. Remember, the diameter of a jet is only that. The hole volume goes way up with one size difference.
I suggest going one size from original, and try that, but only after changing plugs to new ones. That way, you get a good true reading of mixture.
Next, look at idle advance. Too retarded, and you bog. Try between 6 and 10 degrees.
Now, before you do any more jet tests, please please change your oil. If you are getting black smoke, you are washing the oil off the cylinder walls, and could ruin your rings.
Ron

[wilks3]

I would put stock jets back in and play with the accelerator pump. Make sure the clearance is right at the pump levers.
Holley makes different accelerator cams and squirters for this problem. You didn't have enough fuel squirt when the throttle is cracked open that is why the stumble, you went lean for a second and then motor caught up. The bigger jets just masked the momentary lean condition. The easiest first thing to change would be the pump squirter to a bigger size. By the plugs being black you are way too big on jets. It is not uncommon to go smaller on jet sizes to get right mixture. I would suggest buying a Holley setup manual and go step by step to dial-in the carb.

[rumblefish360]

You have the 340 cam in there right?

[Shaun]

Originally Posted by wilks3

I would put stock jets back in and play with the accelerator pump. Make sure the clearance is right at the pump levers.
Holley makes different accelerator cams and squirters for this problem. You didn't have enough fuel squirt when the throttle is cracked open that is why the stumble, you went lean for a second and then motor caught up. The bigger jets just masked the momentary lean condition. The easiest first thing to change would be the pump squirter to a bigger size. By the plugs being black you are way too big on jets. It is not uncommon to go smaller on jet sizes to get right mixture. I would suggest buying a Holley setup manual and go step by step to dial-in the carb.

This is true. Holleys are rarely 'runs good enough out of the box' carbs and more so,the DPs. It sounds like a good carb for the engine. New plugs,an oil change and drop the jets down to 69 and try 76 in the back.
Then if you want to really get it tuned for however you drive it,you drive it(like you want it to) and shut it off and check the plugs. One from each side preferrably. It should be tan. For example,for a 1/4 miler,you would get it all the way through the traps and bang her into neutral and off with the ignition,coast off the track and check 'em. You can do this at certain rpm ranges too,at idle and for highway cruising to get what you want.
Shaun

[coupe32]

Thanks everybody for all the info. The cam is a Racer Brown cam .450 .470 lift duration at .050 is 220 230