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Thread: Carb Jet Help

  1. #1

    Carb Jet Help

    Hey folks, new guy here. I recently bought a 1979 R100T from a guy locally here in utah. Its in damn good shape, but hasn't seen any real riding in a couple years. The first think I did was check out the points set up and sure enough the ole bean can was full of crud. Put it through the ultrasonic at work semi disassembled and rebuild it with a touch of the required grease, new condenser and points. Replaced the leaking alternator seal on the crank as well as the bean can seal. Also put new plugs in, set of new 6v coils and some fresh wires. Voila, new ignition system. Freshened up the alternator with new brushes and a clean because it was covered in oil. Then I got to thinking about the carbs, and since I don't know the milage or how much its been ridden lately, I pulled off the float bowl and found a couple of the passages blocked up. I decided it was probably time to pull it all apart and here I am. They are a set of Bing CV 40s.

    So I took everything back to the ultrasonic for a bath, and she's spick and span. But, Im wondering what kind of jets I should put back in it. I live in salt lake, so a majority of my time will be spent at elevation of 4-10K. The bike has the following in it currently
    Needle Jet-2.64
    Main Jet -160
    Idle Jet - 45

    What do people in the salt lake area or at elevation run with good luck?

    Looking on the Bing/ BMW reference they are calling for a 162 Main jet for above 4K. The clymer is calling out a main jet of 170 stock, with 2.66 needle and 45 idle at what I'm assuming is sea level?

    I figured that with the wear on the needle valve Im going to be in for a needle and needle jet, idle adjust screw is a wee bit rusty, and a gasket kit. Figured I may as well order jets while I'm at it. EME is going to love me.

    Thanks for your help in advance, I have always been astonished at the wealth of knowledge this website has.


  2. #2
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    Welcome to the forum, Austin! What are the carb numbers? Any chance the previous owner has some insight as to why he used what he did in the carb? If the bike is permanently at 4000+ feet, then you probably need to lean out the carb since you have slightly less air molecules in SLC. I might adjust for running in SLC but not worry about trips up into the mountains...the CV carbs do a pretty good job of adjusting for that.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    The left carb number is 94 / 40 / 107, not sure about the right as Iím not home right now. As far as the former owner goes, he only owned it for 1 year before deciding he couldnít get it on the center stand. The owner before that only had it for three months, so unfortunately I canít really dig into why it was set up the way it was. Do you think I need to jet another size down from 160? Or is it already in a reasonable ball park?
    Thanks for your help.

  4. #4
    Registered User lkchris's Avatar
    Join Date
    Mar 2003
    Albuquerque, NM
    You should install stock jetting.

    Bing carburetors are automatically altitude compensating.

    If air is thinner, the rubber diaphragm doesn't lift the main (needle) jet as much.

    Bing recommends yearly replacement of diaphragms. Bing also notes that needles are subject to wear due to friction caused by passing fuel/air mixture.
    Kent Christensen
    '12 R1200RT, '02 R1100S

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