What size Main Jets for Colorado?

[airbur]

While rebuilding my carb I noticed that one of the main jets had cracked slightly where the slot is for the screwdriver. I went ahead and used it but this got me thinking....there are different sized jets....which one is correct for 6000' + for us here in CO?

http://www.carburetor-parts.com/Autolit ... /120-4.htm

Thanks.

[Zenzone]

I'm also wondering what guys are using. I have a Holley truck avenger 470 out of the box with stock jets. I have heard that most guys change them for altitude. Mine seems to run fine but I'm still curious. Hope there is a carb guru that can shed some light on this topic.

[Crazyone]

I'm also wondering what guys are using. I have a Holley truck avenger 470 out of the box with stock jets. I have heard that most guys change them for altitude. Mine seems to run fine but I'm still curious. Hope there is a carb guru that can shed some light on this topic.

For Holley 4 barrels, you want to downsize your jets for higher altitudes. All new holley carbs are jetted for sea level. For every 2000 feet of altitude, you want to down size your jets 1 size. So for our altitude, you want to go 2-3 sizes smaller for each jet. I have a 670 holley street avenger that came with 65 main jets and 68 secondaries. I ran it like that for awhile then checked my plugs. It was running a little rich, so I downsized 2 sizes (63 & 67) and the mixture seems about right.
The best thing to do is check the plugs, if they are black, probably a little rich. If they are a nice toasty brown color, your just about right. Hope this helps.

[Justin]

Do a search. Zilla posted up the suggested part numbers for the 600cfm Edelbrock performer.

[wezlsqezer]

I have a BG 750 and a Holley 600. Two different motors and they both are very happy with 65. I have gone as low as 58 above 11,000 ft. The big thing is the power valve. I use a single stage 2.5in HG. If the power valve is too big it will mess with what you think is your jetting. I have also used the power valve block off and just trusted the jets but it falls on it face when you need the extra burst of fuel. Don't over CFM. I have a 500in motor that actaully runs better around town on a 600cfm carb. If you over carb it doesn't have velocity and won't mix. I have a friend that runs a G.A.M. at Erie. The rules used to limit them at 490cfm. He still made 500+HP and instant throttle reponse.