Carb jetting

[Knanthrup]

Ok so this is my first time messing with the internals of my carb. I have a Holley 4160 750cfm carb. I think I have been needing to change the jetting because it has been running awful rich. Today i pulled the bowls off, I see two jets on the primary side bowl and none in the secondary (just a plate thing). This is a newb question, but are both jets in the primary bowl for primary or is one secondary? How does that work?



Also, the two jets were both 72\'s... what number jets should I switch to? It ran ok, but stumbled a bit when I floored it at low rpm in higher gears, also lined the combustion chambers of the heads with black sut as well as header ports.



To add to the equation, while the engine is apart im upgrading from the regular edelbrock performer to the performer RPM manifold as well as gasket matching and home-porting the heads. I don\'t know if this will be enough of a difference so that the engine will actually be able to use the #72 jets or if I should still use smaller??



Any tips?

[Midwestchopshop]

wise choice for the performer rpm intake, as for anything else i had not the clue

[Scotty_S-15]

I\'m no expert, but 72s sounds about right. Got much of a cam? Before going smaller on the primary jets, I\'d check the power valve for size and condition, check the float levels, and idle adjustments. But the first thing I\'d do is check the carb # at Holley site, and you can find out what jets & power valve came with that carb, and that\'s a good starting point. Most come with 72 to 75 primary jets, 6.5 or so power valve.

....... And yes, the two jets in the front metering block are just for the primary half of the carb. That secondary metering plate is pre-set for a particular jet size, its normally calibrated to work with most setups. You can buy a metering block (kit) to replace that plate from Holley, thru your speed shop, Jeg\'s, Summit, etc. Then, it would accept regular jets. Don\'t forget, if you get a metering block kit, and if you\'ve got a hard dual feed fuel line, it won\'t fit anymore, well, not without major tweaking.

[eigthmile]

The 72,s are correct jetting. Stumble could be to light of a spring in the secondaries. With the performer rpm being taller and you porting the heads I would wait and see if the engine wants the extra fuel now. If that doesn,t do it bring it down to 70,s and give that a try. In my truck 2 jet sizes is 35 degrees on my egt gauge which makes a big difference on plug color. Hope this helps.