Air Bypass screws on Weber

[Bpine]

I have 3 Weber 48 IDF’s on a Toyota 3.4L in a class 1 car. Each barrel has its own runner to the cylinder. Not a common plenum. I obviously need to rebuild them and I just received the kits today. So I Guess I have a few Questions. The reason I am rebuilding them is, ½ to full throttle is fine . It has a lean deceleration back fire and is a little hesitant off idle to 2500 rpm ! forgive me if these are lame questions!



1. Common problems to look for?

2. I noticed in the directions, it recommends to leave the air bypass screw closed. I see mine are currently about ½ to 1 turn out and my idle mixture screws are ½ to 1 also!
What is the Air Bypass used for? Bypass from/for what?

3. tricks of the trade?


Thanks in advance

Bpine

[phorensic]

This is a duplicate of this thread: http://www.race-dezert.com/forum/showthread.php?t=13129 in case anyone is confused.

[Bpine]

Reason for the repost, Still havent got my answers!

Still looking for that Weber guru ot there! :confused:

Bpine

[ChuckH]

You need a weber tuning manual and a synchrometer

http://www.carburetion.com/books.asp

Weber Carburetors Haynes (BK-H10240) or Weber Carburetors HP Books (BB-HP774)
Details everything you need to know to select, install and tune the Weber performance carburetor. Includes information for foreign and American engines. Covers all sidedraft and downdraft carbs, Weber design and theory, choosing the right model for your engine, synchronizing multi-carbs, troubleshooting and repair

a few places sell synchrometers
http://www.redlineweber.com/html/application_guide/redline_synchrometer.htm
http://www.google.com/search?hl=en&safe=off&q=carburetor+synchrometer

My guess is the air screws being out that the idle circuit is flowing to much, things could have been way out of balance also, get things balanced, and then start tuning, write everything down so you can go back to previous settings.

Good luck

[prerunner1499]

Ok first of all Im not a weber guru, I just run them.. anyway here goes.

the air bypass screws are for equilizing the air volume in each side of the carb at idle and low rmp (while the carb is within the idle jet circut) This is very impoirtant in a center plenum car (single carb situation) or when running duals.
Basically you need to have all carbs running the same amount of air at idle and low rpm or you get that pop here and there because 1 ventury of the carb is just a little out of sync. Do you have a carb sync tool? I assume you do as this is a MUST HAVE item if running webers or delortos.
Make sure each cylinder is pulling the same amount of air IE, Syncronized.
2nd--, if you have the right size venturies in the carbs you should not have to fiddle with the air bypass that much.
3rd-- it is possible that the affected cylinder has a dirty idle jet, or drilled out main and you might not know it. Make sure all jets are clean.
4th-- one carb might have a slightly bent butterfly valve which will cause a lean or rich condition as it is a little more open or closed than the others at all times.
5th, make sure ALL 3 carbs are opening at the exact same time. One might be a mm or so early or late upon opening, this causes major sync problems. (could be caused by #4)
6th-- do a spark pllug read and you will be able to tell which cylinder is not running like the others. It should be a little greyer or browner if is is running lean.
****I would start with a plug read to get an idea which carb is the culprit.*** This saves alot of wasted time fiddling with the other carbs and going nowhere.
Get than dang 0'le "webber superformance tech book" that Bob Tomlinson of CB wrote. It is THE BIBLE of webbers. You can get it through CB, Cip1, or just abpout any advertiser in Hot VW's magazine.
Hope this helps

[Bpine]

Thanks Prerunner,

I appreciate the Info, That is the info I was looking for !

Bpine

[DailyPedal]

Has it always been like this? If it pops on decel in may only have an exhaust leak. What cam does it have? It may have been set up with a high lift, long duration cam which moves the powerband up the range...well past 2500 rpm. I guess what I am really saying here is to make sure you have a carb problem.