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    Plastic Main Jets?

    I took one of the bowls off of my 1978 GS750 to get an idea of the condition of the carburetors. First thing I noticed is that the carburetor was immaculate inside. Immediately after I realized that the main jet is plastic. Not the pilot jet just the main jet. The bike has an aftermarket exhaust so I assume someone rejetted the carbs and the plastic has a 120 stamped into it. Are these okay or should I go back to the brass. Also when re-jetting for an exhaust is it usually normal to move the needle as well? The bike runs super rich and I'm looking to get it running a little better. It currently gets about 28 mpg which I know it can do better. Lastly I plan to re-wrap the air filter with new foam but the bike has about a 1/4 spacer/gasket made of hard foam between the filter and the plate above it in the air box. Is this factory or should I leave it out?
    The current garage:
    1978 GS750
    1975 GT750M
    1984 CB700SC
    1982 XJ650 Seca Turbo
    1975 RD250 - 350 conversion

    #2
    I used to use plastic jets. They work fine, but that was also in pre-oxy fuel. Let people know were your jetted now, including needle number, pilot main jet sizes and someone will probably chime in with some good baseline settings to start from
    S
    Current Rides: 82 GS1100E, 00 Triumph 955 Speed Triple, 03 Kawasaki ZRX1200, 01 Honda GL1800, '15 Kawasaki 1000 Versys
    Past Rides: 72 Honda SL-125, Kawasaki KE-175, 77 GS750 with total yosh stage 1 kit, 79 GS1000s, 80 GS1000S, 82 GS750e,82 GS1000S, 84 VF500f, 86 FZR600, 95 Triumph Sprint 900,96 Triumph Sprint, 97 Triumph Sprint, 01 Kawasaki ZRX1200, 07 Triumph Tiger 1050, 01 Yam YFZ250F
    Work in progress: 78 GS1000, unknown year GS1100ES

    Comment


      #3
      Thought I'd revisit this thread rather than making a new one. I have since determined that the main is 120 and the pilot is a 15 but I am not sure of how to check the needle markings without taking them out. I also do no know how to take them out without disassembling the whole carb assembly. The bike has an aftermarket exhaust (Not sure which one) and a uni foam filter in the stock box. The other day I rode about 100 miles and the bike was running great. Just before I got home I decided to fill it up and choose to give the non-ethanol fuel a shot considering the plastic jets and all. The next few days and about 15 miles of just running around town (so very short trips) and on the last one the bike started running super rough, not sounding good, terrible throttle response and no power. I took the plugs out and they where very black. I've since only been about 20 miles and put another gallon of regular 87 but nothing has changed so far. Any thoughts?
      The current garage:
      1978 GS750
      1975 GT750M
      1984 CB700SC
      1982 XJ650 Seca Turbo
      1975 RD250 - 350 conversion

      Comment


        #4
        If the bike was running great at any point it won't be jetting. They don't change sizes on the run. Maybe some dirt or other issue.

        Comment


          #5
          I think I would be checking your tank for rust or a failing tank sealer someone may have put in the tank. Check the petcock and rebuild the carburetors. Something broke loose and is blocking fuel flow. VM carburetors have a lot of tiny passages and I can tell you from experience they don't like rust or debris in the fuel system. You can tell absolutely nothing from the condition of the bowls, It's what you can't see that causes the problems. I have seen these carburetors look pristine and immediately discolor new carburetor dip within minutes of putting them in the solution. After a 24 hr. soak, you may not be able to see the carburetors below the surface of the dip.
          Last edited by OldVet66; 05-24-2017, 12:45 PM.
          '78 GS1000E, Dyna-S ignition, Dyna Green Coils, K&N pods, Delkevic SS 4-1 exhaust, Dynojet Stage 3 jet kit, Russell SS Brake Lines, Progressive suspension, Compu-Fire series Regulator 55402 and Advmonster cree LED headlight conversion.

          Comment


            #6
            It could also be an ignitor/weak spark issue resulting in gunked up plugs.
            '83 GS650G
            '83 GS550es (didn't like the colours in the 80's, but they've grown on me)

            Comment


              #7
              '78's were old school with points unless someone changed the ignition.
              '78 GS1000E, Dyna-S ignition, Dyna Green Coils, K&N pods, Delkevic SS 4-1 exhaust, Dynojet Stage 3 jet kit, Russell SS Brake Lines, Progressive suspension, Compu-Fire series Regulator 55402 and Advmonster cree LED headlight conversion.

              Comment


                #8
                Yes yes check your points, then I would start adjusting the mixture and get it right. Black plugs, your running plenty rich.

                V
                Gustov
                80 GS 1100 LT, 83 1100 G "Scruffy"
                81 GS 1000 G
                79 GS 850 G
                81 GS 850 L
                83 GS 550 ES, 85 GS 550 ES
                80 GS 550 L
                86 450 Rebel, 70CL 70, Yamaha TTR125
                2002 Honda 919
                2004 Ural Gear up

                Comment


                  #9
                  I'm actually thinking it may be ignition related. Judging by the sound out the exhaust it sounds like it may be mis-firing or down 1 cylinder. What would be a good course of action on the points? Just clean them? Something like 200 miles ago or so I advanced the timing back to the factory spec as it was out by about 10 degrees and checked the points gap. I guess I'll re check to make sure nothing came loose. As for the carburetors my thinking is that any sort of blockage would make it run leaner rather than richer correct? It has an inline filter and the tank looks really nice inside. Last night I rode a little and pulled the number 1 and 4 plugs. 1 was actually wet with fuel and 4 looked as though it was starting to return to normal. I'm headed out the pull 2 and 3 now because it was too hot to do after my ride.
                  The current garage:
                  1978 GS750
                  1975 GT750M
                  1984 CB700SC
                  1982 XJ650 Seca Turbo
                  1975 RD250 - 350 conversion

                  Comment


                    #10
                    1/4 going out together is highly suggestive of dropping a coil. Be careful, as you could hydrolock a cylinder in this situation
                    '83 GS650G
                    '83 GS550es (didn't like the colours in the 80's, but they've grown on me)

                    Comment


                      #11
                      Dyna-S. Get rid of the weak points (pun intended). http://www.ebay.com/itm/Dynatek-Dyna...-/380382026185

                      $115 is way cheaper than a tow in the middle of nowhere.
                      sigpic
                      09 Kaw C14 Rocket powered Barcalounger
                      1983 GS1100e
                      82\83 1100e Frankenbike
                      1980 GS1260
                      Previous 65 Suzuki 80 Scrambler, 76 KZ900, 02 GSF1200S, 81 GS1100e, 80 GS850G

                      Comment


                        #12
                        1/4 don't seem to be going out, it seems to be all of them I just happened to only pull those plugs the other night. And I would really like to put the newer coils and electronic ignition but I'm stubborn and want the fix the old stuff first . When checking the plugs yesterday the threaded insert came out of one of the boots and stayed on the plug, so I'm going to take them all off and test the resistance and hopefully get the one back together. I've found a few threads on people assembling the resistors but if anyone has a better resource feel free to share

                        Edit: I got the boot apart, the resistor is long gone and the hole that the threaded insert goes into is egg shape and won't let me re assemble it. So I know I need a new one, but my question is my bike has 2 different length caps 2/3 are longer and 1/4 are shorter. Are these different lengths available in the aftermarket like NGK? I can't seem to find lengths listed anywhere
                        Last edited by The1970's; 05-25-2017, 11:51 AM.
                        The current garage:
                        1978 GS750
                        1975 GT750M
                        1984 CB700SC
                        1982 XJ650 Seca Turbo
                        1975 RD250 - 350 conversion

                        Comment


                          #13
                          I think I've finally figured out that it's a spark plug that went bad. I moved around a known good pug cap and the problem remained but the problem would move around with one spark plug. The question now is why did it go bad? It had the wrong plugs in it when I got it so I put in the correct B8ES plugs maybe 200 miles ago and one already went out. I think maybe due to the bad connection in the cap?
                          The current garage:
                          1978 GS750
                          1975 GT750M
                          1984 CB700SC
                          1982 XJ650 Seca Turbo
                          1975 RD250 - 350 conversion

                          Comment


                            #14
                            bad cap is an option.

                            Fake plugs is another.


                            I blew 4 B8ES fakes within couple of hundred miles, drove me nuts because that
                            had not happened once in 25 years of using NGK spark plugs..
                            Rijk

                            Top 10 Newbie Mistakes thread

                            CV Carb rebuild tutorial
                            VM Carb rebuild tutorial
                            Bikecliff's website
                            The Stator Papers

                            "The thing about freedom - it's never free"

                            Comment


                              #15
                              You are correct in thinking a bad spark plug is a rarity. But, if it wasn't firing correctly, it can foul pretty quickly
                              1978 GS 1000 (since new)
                              1979 GS 1000 (The Fridge, superbike replica project)
                              1978 GS 1000 (parts)
                              1981 GS 850 (anyone want a project?)
                              1981 GPZ 550 (backroad screamer)
                              1970 450 Mk IIID (THUMP!)
                              2007 DRz 400S
                              1999 ATK 490ES
                              1994 DR 350SES

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