Carb rejet - The 1947 - Present Chevrolet & GMC Truck Message Board Network

z7199 · 10-12-2005, 11:36 PM

I have a BBC 402 pretty much stock.. I got a 850cfm Holley for it and I know it is to BIG but I was windering if I could rejet it to make it work. I plan on a cam and Heads eventually. Also any help on rejetting would be good also. I have never done or know how to do a rejet.

68Stepbed · 10-13-2005, 09:39 PM

First of all, BUY A BOOK on Holley carbs. It is a great reference guide to many tuning techniques. I don't suggest doing more than a jet change before getting a book.

Start with taking out the four bolts on the front bowl as pointed out in the first pic. The metering block may come off with the bowl or it may stay on the main body. If it stays with the bowl, just pull em apart. If it stays on the main body, you can change jets without pulling it off(unless you just want to).

The jets screw into the metering block as pointed out in the second pic. Make sure you use a WIDE flat blade screwdriver to remove and install them.

Just change em out and reinstall everything. Try the stock jets before changing anything. The cfm rating is the amount of air flowing through, not fuel. The jetting could be close out of the box. You don't know which way to jet until you get a baseline with everything stock.

Good luck and let us know how it's going.

68C15 · 10-14-2005, 07:36 AM

changing the jets will only affect the WOT fuel curve. where you spend most of the time is in the transition & idle. SA designs has a book written by David Vizard. get it!! I have the ISBN # if ya want it.

z7199 · 10-14-2005, 08:15 AM

Thanks Guys I will get that book first I guess. pics are great!!

68C15 · 10-14-2005, 12:46 PM

he has 2 books. "how to build HP" volumes 1 & 2
vol 1 0-931472-24-5
vol 2 1-884089-14-3
I think vol 2 would be more usefull unless you are a virgin at engines & tuning. otherwise start with vol 1.

72wascal · 10-14-2005, 01:02 PM

It is a bit more complicated than just changing jets. You have to consider what type of carb you have, the cam, the manifold type and total compression volume to know what type of carb will work. I would also read up on specific areas and types of carbs before tweaking anything. Usually a new Holley out of the box runs pretty well, and the 850 is a standard for most BB. I run a 750 on my 331 ci and it runs fine throughout the power curve. If your having problems idling, it would be more a different problem such as diaphram, idle circuit, or bowl settings.

As 68c15 said, the jetting won't matter until the WOT fuel curve comes into play. The secondary is not effective until this time. Without seeing the carb and seeing what the symptoms are it is difficult to determine what the cure is.

10-12-2005, 11:36 PM	#1
z7199 Registered User Join Date: Apr 2005 Location: Houston TX Posts: 76	Carb rejet I have a BBC 402 pretty much stock.. I got a 850cfm Holley for it and I know it is to BIG but I was windering if I could rejet it to make it work. I plan on a cam and Heads eventually. Also any help on rejetting would be good also. I have never done or know how to do a rejet. __________________ 99 nbs 5.3L x-cab Hypertech programmer True dual 1 chamber flowmasters magnaflow x-pipe 3" PA body lift 315/75/16 BFG All Terrian 15.3 @ 89mph 72' C20 402ci sc,lb in bed tool box, and new paint http://www.cardomain.com/memberpage/456511/1

10-13-2005, 09:39 PM	#2
68Stepbed Registered User Join Date: Mar 2005 Location: 10-Uh-See Posts: 5,609	First of all, BUY A BOOK on Holley carbs. It is a great reference guide to many tuning techniques. I don't suggest doing more than a jet change before getting a book. Start with taking out the four bolts on the front bowl as pointed out in the first pic. The metering block may come off with the bowl or it may stay on the main body. If it stays with the bowl, just pull em apart. If it stays on the main body, you can change jets without pulling it off(unless you just want to). The jets screw into the metering block as pointed out in the second pic. Make sure you use a WIDE flat blade screwdriver to remove and install them. Just change em out and reinstall everything. Try the stock jets before changing anything. The cfm rating is the amount of air flowing through, not fuel. The jetting could be close out of the box. You don't know which way to jet until you get a baseline with everything stock. Good luck and let us know how it's going. __________________ Matt 68 C10 stepside, LS1/700R4, TCI Engineering suspension system Last edited by 68Stepbed; 10-04-2007 at 10:31 PM.

10-14-2005, 07:36 AM	#3
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	changing the jets will only affect the WOT fuel curve. where you spend most of the time is in the transition & idle. SA designs has a book written by David Vizard. get it!! I have the ISBN # if ya want it. __________________ Man rule #77...if you own a 67 stepside with a caddy 472 you will never be in danger of loosing you man card

10-14-2005, 08:15 AM	#4
z7199 Registered User Join Date: Apr 2005 Location: Houston TX Posts: 76	Thanks Guys I will get that book first I guess. pics are great!! __________________ 99 nbs 5.3L x-cab Hypertech programmer True dual 1 chamber flowmasters magnaflow x-pipe 3" PA body lift 315/75/16 BFG All Terrian 15.3 @ 89mph 72' C20 402ci sc,lb in bed tool box, and new paint http://www.cardomain.com/memberpage/456511/1

10-14-2005, 12:46 PM	#5
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	he has 2 books. "how to build HP" volumes 1 & 2 vol 1 0-931472-24-5 vol 2 1-884089-14-3 I think vol 2 would be more usefull unless you are a virgin at engines & tuning. otherwise start with vol 1. __________________ Man rule #77...if you own a 67 stepside with a caddy 472 you will never be in danger of loosing you man card

10-14-2005, 01:02 PM	#6
72wascal Hawaiian Member Join Date: Oct 2005 Location: Maui, Hawaii Posts: 28	It is a bit more complicated than just changing jets. You have to consider what type of carb you have, the cam, the manifold type and total compression volume to know what type of carb will work. I would also read up on specific areas and types of carbs before tweaking anything. Usually a new Holley out of the box runs pretty well, and the 850 is a standard for most BB. I run a 750 on my 331 ci and it runs fine throughout the power curve. If your having problems idling, it would be more a different problem such as diaphram, idle circuit, or bowl settings. As 68c15 said, the jetting won't matter until the WOT fuel curve comes into play. The secondary is not effective until this time. Without seeing the carb and seeing what the symptoms are it is difficult to determine what the cure is. __________________ 72 C-10 ProStreet - 331 cu w/ T350 trans 9" shortened w/ disc brakes - Mild body modifications 3/5 drop - More in the works



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