4 barrell question. (jets)

[SU'dCrazy280]

Yo, I have a 1976 280Z with a Holley 390 cfm, holley red fuel pump, arizona zcar manifold, and fuel pressure regulator set at 4.

the car runs fairly well, starts on first try everytime and is much more powerful than the old injection already, but i was told by a mechanic that i need to play around with some different size jets on the two barrells closest to the valve cover (the ones w/out the butterfly over them). The engine is a 2.8L bored .040 over, medium racing cam, flat top pistons, and im on a tight budget for this vehicle. I am trying to tune the car for racing and autocross for the summer. The jets on the electric choke side are the correct size i guess, but the others aren't, and I don't want to spend a ton of time and money trying to tune this thing perfectly and I am headed off to college at the end of the summer, so if any of you know which size of jets to use, or have at least an idea, that would be awesome. Thanks

MarkyT

[Guest tony78_280z]

but i was told by a mechanic that i need to play around with some different size jets on the two barrells closest to the valve cover

This is great. The "mechanic" doesn't know holleys, does he?

 

I assume he is talking about the primary jets. How did he come to this conclusion? 95% of the time if someone suggest a carb needs tuning, he is right. What are you tuning for?

 

Main jets (primary) should be set for decent fuel economy. Power valve is tuned for the power circuit, your secondarys (vac or pump) should be tuned for WOT.

 

1. Nobody can magicaly pull a jetting size out of their arse.

 

2. Get a good book on your carb. Read it and learn it and understand it.

 

3. Read this stuff.. http://www.bob2000.com/carb.htm

 

4. Check out my webpage.

 

5. Get a book!

[Guest rick458]

a Holley 390 does not even have secondary jets you would need to get a conversion plate to run jets Or keep changing metering plates

[SU'dCrazy280]

thanks guys. I'm new to carbs, I had to take the easy route because i'm headed off to school and I just want to get the car runnin perfect enough as a daily driver.

MarkyT

[Guest rick458]

Get the Book Super tuning and Modifying Holley Carburetors by Dave Emanuel

it will tell you about the 4150 Double Pumper Mech Secondary

and the 4160 Vacuum secondary (of which the 390 is)

and has the jet sizes, orifice sizes, bleed hole drill sizes, metering plate sizes etc.. you may even find it at AZ or Orielley

I got mine from Summit

[Pyro]

4 psi is too low for a holley. Run 6 or 7 psi.

 

How do your plugs look?

Does it smell rich at idle?

Have you adjusted the idle mixture screws for best idle (highest vacuum)?

 

You need to pull the front fuel bowl off to see what jets you have. Then you can order smaller or larger jets as you need them. But get a set of gaskets for the fuel bowl before taking the old ones off.

 

Typically, holleys come jeted rich from the factory. This makes their carbs safe to just bolt on and go. You can gain 2 or 3 mpg and much better throttle responce with the right air/fuel ratio. I have found in most cases, 2 or 3 jet sizes smaller than the factory installed jets do the job. Every engine is different, so you must figure out the jet size for yourself, but 68's or 69's in the primaries is a good place to start.

[WHP]

I had the same set up on almost the same engine. The carb comes stock with #51 main jets. I ended up with #49. I am at 3000' altitude so the factory jets would have been dead on at sea level. I also had to change the pump cam to purple. These small jets do NOT come in the Holly jet kit assortment, they have to be purchased seperately. Like the previous posts said, get a good manual. I have the Haynes Holly Carburetor Manual and like it. BTW, my carb and manifold is For Sale in the classifieds here as I no longer need it.