holley jet size

[markzepp820]

I have a 79' chevy truck with a 350. The motor is pretty much stock, it has an aftermarket performer intake, headers and a holley 600. I think the stock jet size in this holley is 66. What smaller jet size would be good for my 350?
Also, what are the benefits of reducing the jet size?
thanks

[VileZambonie]

Ok yes a smaller jet can be put in but that will change your a/f ratio leaning it out. Is it running rich?

[markzepp820]

yes, I think it is running rich. The inside of my tailpipes are black and when I drive off, friends say it smells rich.


thanks

[13Bre_TT]

The best way to tell if your jets are the right size is to shut the engine off while cruising at 35 mph or so, and then pull a spark plug and see what it looks like. Sounds strange but if you kill it while cruising then you'll know how the jets are for normal operation and not idle, or when the secondary's are working. ive been trying to get the right jets for a couple weeks (havent been that hard charging at doing it but though). Jets have a huge role in the a/f ratio and have a lot of aspects to getting it right, outside temps, and elevation play a role as well. They are cheap so get i just get a few sets around where my jets are now usually going in 2's. Just my .02

[Fairlane514]

I can't imagine a 66 is too rich on a 350.  Is it rich at idle? Try leaning up the idle mixture screws.......turn them in CW...........a good starting place is 1 1/2 turns out.