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Gs250t Vacuum Leak Bike wont catch

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    Gs250t Vacuum Leak Bike wont catch

    Hi all,

    I am quite new to being an at home mechanic, and I have ran into a problem with my gs 250t. A couple months ago, I noticed that the engine had a little difficulty starting up every now and then, I didn't think much of it, but looking back I also saw that the choke was very sensitive on the bike, and that the bike was ticking over above revs etc...

    As I said I didn't think much of it, and I thought, when I have some free time I can clean the carbs etc...

    Unfortunately over the course of 3 days in September, the bike went from push starting to not starting whatsoever. I didn't know at the time what a vacuum leak was, and I thought it must be a blocked jet/fuel line, bad fuel, or something along those lines. I spent a lot of time deliberating, I was getting good spark, the spark plugs were in good condition, but to be safe I replaced them. There was clean fuel, however none of it was entering the pistons, and the engine would not fire whatsoever, no catching at all... not even with starter fluid sprayed directly into the carbs. I thought it was an air fuel ratio issue I changed the air filter and tried again, still no luck.

    After some digging I learnt about Vacuum Leaks, and looked at getting the carbs rebuilt, which as it turns out, the turnaround time for this is quite long, a couple of months, and costly (I am a student, and I need the bike to go to work.)

    I am firmly convinced that the above issues are due to a vacuum leak, I dug around some more and found that a common issue for vacuum leaks in the manifold rubbers. Unfortunately there are no measurements I know of to compare them to, in order to determine if they are warped, however they are not cracked, and the rubber is still relatively flexible. The screws around them are fairly stuck in place, I am worried about destroying them If I try to take them off without an impact screwdriver, also very difficult to get at due to orientation with airbox.

    I had another look at the system this weekend and saw that the metal inserts on the airbox were loose, and also that the carbs were dirty where they connected to the air-box. I have seen on the forum, that pods etc... cause a lot of difficulty with the bike. The force needed to push the airbox into position is quite great, I'd be surprised if there was an air leak, but the dirt is quite visible. Do you think that a leak between the airbox and the carbs could prevent the bike from catching? The inside of the carbs are very clean, not a hint of residues or dirt, as though they had been rebuilt already. The pilot jet screw has been tampered with, as there are marks from a screwdriver from before.

    I wanted to know what you all thought about the issue above, if you think it is a vacuum issue, carb related or seal related, I have some industrial rubber silicone sealant, and vulcanizing tape which may fix the problem, I am going to replace all the hose clamps as they all are at their smallest adjustment, before I use any industrial products.

    I left it into a shop when it broke down, but no-one could identify the problem.

    I'm not in a place right now to pick up another bike and take out another policy, (Irish Insurance likes to price gouge massively) If anyone knows a way (even temporary) in which I can get the bike started and running decently for a couple months (until June) it would be much appreciated, I still need to take my full drivers test etc... hopefully I can do so before my policy expires.

    In case I cannot get the bike to start still, If anyone could refer me to somewhere to buy the correct sized manifold rubbers, or an adapter type unit just in case, it would be much appreciated.

    Does anyone know why absolutely no fuel would be able to get into the engine? I am stumped as to why the carbs would shut closed completely when there is no vacuum.

    Thank you in advance for your help.

    #2
    See if the vacuum line between the left side carburator is still attached to the petcock, the petcock requires engine vacuum to allow gasoline to flow into the carbs. You should be able to move the petcock lever to the "prime" position which bypasses the need for vacuum. A note of caution, in the prime position the float bowls could very well flood when you shut the bike off so make sure to return the petcock to either the run or res position.

    As to the other possibile leaks, they probably won't keep the bike from starting but will indeed cause it run poorly. If you need a carburator rebuild, Don't bother getting a kit, all you need is the o-ring set from cycleorings.com.
    1980 Yamaha XS1100G (Current bike)
    1982 GS450txz (former bike)
    LONG list of previous bikes not listed here.

    These aren't my words, I just arrange them

    Comment


      #3
      Hello Lab3

      Many thanks for your contribution, the vacuum line is indeed hookd up to the petcock, and I have tried priming, I emptied primed and tried again about 3 times lol... I'm surprised because I dont understand what could cause both carburettors to go at once... ???

      Thanks

      Comment


        #4
        If the fuel tap is damaged, could that cause problems in the vacuum line?

        Comment


          #5
          There's a rubber diaphragm inside the fuel tap, if the vacuum line isn't connected or is damaged then it wouldn't be able to allow fuel to flow. If fuel is still flowing in the prime position there may still be a problem inside the petcock, it's a very common problem on these old bikes. It SOUNDS like you've eliminated the petcock being the problem.

          All too many times what people think is a carb problem ends up being in the ignition instead. Fuel system problems usually don't happen all at once like you described, ignition problems often DO. You mentioned push starting the bike, is the battery good?
          1980 Yamaha XS1100G (Current bike)
          1982 GS450txz (former bike)
          LONG list of previous bikes not listed here.

          These aren't my words, I just arrange them

          Comment


            #6
            Battery is perfect, Its less than 3 months old...

            Comment


              #7
              I suppose even if the petcock was bad, the bike would still ignite starter fluid right? Its not catching anything...

              Comment


                #8
                I suppose even if the petcock was bad, the bike would still catch a little on starter fluid right?

                Comment


                  #9
                  Have you done any maintenance? Adjusted the valves?

                  To check spark remove the plugs, put the plug back into the cap, and lay the threaded portion against the engine and hit the starter while watching for a spark. It seems that you do have issues with sealing the intake system but there should be at least some coughing with starter fluid.
                  Ed

                  To measure is to know.

                  Mikuni O-ring Kits For Sale...https://www.thegsresources.com/_foru...ts#post1703182

                  Top Newbie Mistakes thread...http://www.thegsresources.com/_forum...d.php?t=171846

                  Carb rebuild tutorial...https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

                  KZ750E Rebuild Thread...http://www.thegsresources.com/_forum...0-Resurrection

                  Comment


                    #10
                    I replaced the spark plugs, checked their spark, and so did a mechanics... seemingly no problem with spark

                    Comment


                      #11
                      I didn't adjust anything to do with the engine... I changed the headlight and tail light out for led bulbs... but it ran fine after I did that... when I got the bike I did an oil change, I put on a new master cylinder, brake line, piston and brake pad, I also changed and regreased the bearings, and put on new tyres... and a new chain didn't touch anything else

                      Comment


                        #12
                        I'm with Nessism, you should get some kind of reaction using starting fluid. Do the spark plug check he described then check the valve clearences. Valve checks are important, you can also check for any possible timing issues since you'll have that cover off to rotate the engine.
                        1980 Yamaha XS1100G (Current bike)
                        1982 GS450txz (former bike)
                        LONG list of previous bikes not listed here.

                        These aren't my words, I just arrange them

                        Comment


                          #13
                          sorry, I also changed the battery, but there were still no problems then either...

                          Comment


                            #14
                            Ed

                            To measure is to know.

                            Mikuni O-ring Kits For Sale...https://www.thegsresources.com/_foru...ts#post1703182

                            Top Newbie Mistakes thread...http://www.thegsresources.com/_forum...d.php?t=171846

                            Carb rebuild tutorial...https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

                            KZ750E Rebuild Thread...http://www.thegsresources.com/_forum...0-Resurrection

                            Comment


                              #15
                              I'll check it, I also cranked the engine with some blue roll in the spark plug slot, there was no fuel on the blue roll when I pulled it out... How should I check for timing issues? I was told i'd need special equipment? I checked the new spark plugs when I installed them, their sparks were bright, I didnt check clearances, but I will do that. I need to pick up sheaths, they were falling apart (but still worked)...

                              Comment

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