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Jetting '80 GS850

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    #1

    Jetting '80 GS850

    Hi Guys,
    I'm looking for some guidance here...(sorry for the long post)

    I just finished rebuilding and installing the carbs on my 1980 GS850GT. It is 29 Degrees F outside. The bike had not been run in over a year. I installed the carbs without the airbox (ran naked) as I will be installing UNI-Filter Filter Pods when I am completed.

    I flooded the bowls and verified that all 4 carbs were getting fuel by cracking the drains on each. The number 4 float valve did not seem to be seating all the way as there was a slow leak (1 drip every 5 seconds) from that carb. (I am not concerned about that yet...)

    I primed the carbs with starting fluid and WHAM, the bike fired right up and ran very smooth. Choke-On, the bike would die out after the starting fluid was all used up. I eventually got it to stay running on gas with the choke off, and keeping the RPM's over 2500 (by using a LOT of throttle manipulation...).

    When it was running, I would hold the throttle steady at about 1/3 throttle. The bike would oscillate RPM's between 2500 and 3500. I am quite sure that this is my leaky float valve, which I will be fixing. However, my problem is that the bike will die if I allow the RPM's to drop below about 1300. I think that it is an overly-lean condition that is causing this.

    SO, my question is this.
    I am going to have to at least fix the float valve, so while I have the carbs off again, what should my jetting be? (Clip Position, Air Screw Turns, Main Jet, Pilot Jet...)

    Here's the final configuration:
    Stock 4 into 2 exhaust and UNI-Filter Pods

    Also,
    Do you think that my low RPM running problem is NOT jetting? Why?

    Do you think that my oscillating RPM's was NOT caused by the leaky float valve on the #4 carb? Why?

    Thanks for the advice!
    Tags: None
    #2
    Your GS won't run properly without an airbox and/or some kind of air filter, especially in low temperatures. The mixture will stay too lean. I wouldn't think of rejetting before I had everything set up as it should be.

    Comment

      #3
      Re: Jetting '80 GS850

      The low rpm running problem is caused by the lack of the correct airbox which (if it was installed) would result in the intake getting the correct fuel and air mixture. It halfway runs with the choke because that supplies the needed additional fuel. As rpm increases, the choke becomes incapable of meeting the increased demand. The oscillating rpm's have nothing to do with the leaky float valve on #4 carb. The stock air cleaner must be properly installed and work in unison with the stock jetting.
      I would make no jetting changes until I had the bike running well in stock configuration.

      Earl

      Originally posted by Plymouth
      Also,
      Do you think that my low RPM running problem is NOT jetting? Why?

      Do you think that my oscillating RPM's was NOT caused by the leaky float valve on the #4 carb? Why?

      Thanks for the advice!
      Komorebi-The light filtering through the trees.

      I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. H.D.T.

      Comment

        #4
        I don't have the stock airbox, so my next question is what should the jetting be with that configuration...?

        Comment

          #5
          The cheapest and easiest thing to do would be to buy a stock airbox. One can often be found on e-bay, at local salvage yards and on the GS misc parts and bikes being parted out page. Chances are you can find what you need right here on the GS parts for sale page.

          (what should the jetting be with that configuration?)..............as a general rule of thumb, you would start out by increasing all jets by about two sizes, and doing run tests to check mixtures and burn colors for each stage of the jetting. Its very easy to be too lean though and burn a hole in a piston. Unless you have the experience to analyze the induction tuning, aproximate new flow conditions and calculate jetting for correct mixture ratios, you will be just guessing. The trial and error method/guessing will cost much more money, can be risky and will take FAR longer. If you are determined to rejet the bike, I would only buy pod filters from a source that would supply proven and accurate rejetting information for your specific bike. If rejetting, you will also need to furnish information on your exhaust system as intake, jetting and exhaust are all the same system. Forget about doing the engineering yourself unless that is your field.

          Earl

          Originally posted by Plymouth
          I don't have the stock airbox, so my next question is what should the jetting be with that configuration...?
          Komorebi-The light filtering through the trees.

          I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. H.D.T.

          Comment

            #6
            There's a stock airbox for a 79 GS850 on ebay right now for $10... I don't know if it's the same or not. I'm pretty sure the carbs for 79 and 80 are different.. but maybe the airbox is the same?? Just a thought.. JOE

            Comment

              #7
              For ten bucks, its worth the chance. It should fit betweena the frames OK, but you may have to modify the intake openings for the boots.

              Earl

              Originally posted by joeinglis
              There's a stock airbox for a 79 GS850 on ebay right now for $10... I don't know if it's the same or not. I'm pretty sure the carbs for 79 and 80 are different.. but maybe the airbox is the same?? Just a thought.. JOE
              Komorebi-The light filtering through the trees.

              I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. H.D.T.

              Comment

                #8
                I find it hard to believe that there is nobody else running filterpods that has already set this up...

                I really don't want the stock airbox. That's the whole point...

                Someone must have already been through this...

                Comment

                  #9
                  Well, its really very simple. Go to the manufacturer of your choice, use their data for your bike and buy the jetting and exhaust system they specify.

                  Earl

                  [quote="Plymouth"
                  I really don't want the stock airbox. That's the whole point...

                  Someone must have already been through this...[/quote]
                  Komorebi-The light filtering through the trees.

                  I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. H.D.T.

                  Comment

                    #10
                    Jetting an 850 without an airbox would be a nightmare. A lot of trial and error. But if you are determined to do so, at least get the pods on first. You would more than likely have to richen the entire range. Pilot and main jets, and raising the jet needle for midrange. Figuring out how much is the trial and error part. You will also get intake turbulence at speed. Good luck.

                    Comment

                      #11
                      79 air box is different!!!! 80-81 should work. As said it will not run properly with out the airbox Tuff to rejet, lots of trial and error. those who have tried ?? sometimes end up going back to the original air box. You could talk to one of the forum members??? Check with http://www.motorcyclecarbs.com Maybe they can give you some additional info and get you the parts you need. If I remember right these
                      people could help also Http://www.factorypro.com has a articall on tuning carbs on ther web site

                      Comment

                        #12
                        I recall we had a member on a previous incarnation of this forum who tried to get a GS850 to run without an airbox, he was a mechanic and could not do it . It ended up that somehow turbulence from the bike moving was just sucking all the air away from the carb area, and it would never run right, he gave up and returned it to stock.
                        Dink

                        Comment

                          #13
                          that absolutely mind boggling how the airbox create that much restriction...

                          Anyways, thanks all. I will try to reacquire my old airbox and return the filterpods.

                          So the bike should run THAT poorly without it?? I'm laughing to myself wondering what the engineers who designed this intake were thinking...

                          Comment

                            #14
                            I don't think it's impossible to get it to run decent wihout an airbox, but I think it would be far more trouble than it's worth. Most bikes have had airboxes for about 25 years. And for good reason. Particularly with the advent of constant velocity carbs, it is important that intakes be tuned to the needs of the carburetor. Just like with exhaust systems, some restriction isn't necessarily a bad thing. Some bikes do benefit from "opening up" the airbox a bit, but I doubt it's necessary in the 850's case. Engineers found out long ago, that creating "dead" airspace at the intake was preferable to turbulent air (despite what those "Tornado" adds claim). Come to think of it, I've seen plenty of K&N's at the dragstrip, but I've yet to see a "tornado"

                            Comment

                              #15
                              The engineers who developed these CV carbs were forced to make emissions the primary objective. If the emissions levels could not be met, the bikes could not be sold in America. That required CV carbs and a stable airbox environment. I personally feel that a properly running GS850 (stock) has one of the nicest powerbands in motorcycling - smooth, fast and responsive - and it can easily provide 100,000 miles of performance that way. One of the most memorable quotes from Joe Minton (mechanic guru/writer from Motorcyclist magazine) concerned people who messed with things before they really knew what was going on. He implied that they were going to the "hand-grenade school of high performance" - something to think about.

                              Comment

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