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1977 GS-750 re-jet. Here is where I am at.

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    #1

    1977 GS-750 re-jet. Here is where I am at.

    This is a follow up to my post from a couple of days ago. It stopped raining so I was able to get out for a bit today. My bike has Emgo pods and a DG Street Performance 4 into 1. Intake boots were fine but I replaced O-rings anyways. New O-rings in carbs. Carbs completely cleaned. Bench synced but not vac synced yet. Stock jetting except for 127.5 mains ( stock is and was, 100 ) and jet needle in 4th position from top. Float height set to mid range at 26mm ( manual says 25 to 27mm ). Petcock has been removed and checked. It's clear. Valve clearance checked and within specs. Plug colour seems fine around 1/3 throttle with clip in 4th position. Still bogging when I roll the throttle quickly ( not whacking it wide open ) wide open off the line. Engine will die out if I hold this and not release. Bogs in higher gears to lesser degree. Held at full throttle up a hill for about 30 to 45 seconds and did the kill switch/close throttle/pull in clutch bit. Bike wouldn't go past 120 KPH by the way but would go well past that on level road. The hill wasn't any particular big deal. Plugs are lean still. I wonder if you have got out on your bike yet, Cafe Kid, to see where you are at? I am wondering how big a main jet to go for now or would float height be an issue for WOT. Low speed stuff ( 60 Kph in 4th gear with maybe 1/8 throttle ) is maybe, maybe, a tad rich on 1,2 and 3 ( dark tan colour. No hint of black )while #4 is completely sooty coloured. At the time the fuel screws were 2 1/4 turns on 1,2 and 3 and 2 turns on #4. Have turned #4 in to 1 3/4. All air screws are 1 1/2 turns. Rock solid idle at 1100 RPM. Doesn't fluctuate and doesn't hang up when I rev engine. Might be able to get back out later. I'm here to learn so comments are most welcome.

    Tom
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    #2
    so you did a plug chop when it was bogging out? and the plugs were lean? but its a little rich every where else in the band....

    i would say try moving the needle one half or one full position richer and see how it handles after that....

    if its doing it enough to kill the engine i would try boggin it till it dies, then pull the plugs quick and look at them, if they are wet, too rich, if they are white or dry looking than too lean...

    just my thoughts.

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      #3
      I should mention that the bike has always bogged out like this with WOT inputs for as long as I can remember, and I've had the bike since 1983. Don't remember if it did it with the stock 4 into 2, though. I put the pipe on it shortly after I got the bike. I just put the pods on a few weeks ago due to the boots on the air box side being shot and not being able to get new ones. The bike bogged with the stock air box setup and also when I had a K & N in the stock air box. This was all with the 4 into 1 on it. Next time I get back out I will go somewhere secluded, roll on WOT off the line and let the bike die and stop and check the plugs.

      Tom

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        #4
        did moving up to the 127.5 main help at all?

        Comment

          #5
          Originally posted by 80GS750 View Post
          did moving up to the 127.5 main help at all?
          Yes. When I first put the pods on I did nothing to the carbs because I wanted to see where I was at. I did a plug chop going up a big, long hill close to home. If I tried to open it up past 1/2 throttle going up the hill it started to bog out, die and slow down. It was all the bike could do to maintain 80 Kph up the hill! Bike ran terrible. Took the carbs off and raised the jet needle one notch to the 4th position from top. Much better, but still not right. Tried 110 mains. Too lean. Put in the 127.5's which I believe were suggested by Cafe Kid awhile ago. Better but not there yet. As Cafe Kid has mentioned, there are no jet kits for this bike so we are on our own trying to sort it out. My bike actually runs great but it's the WOT inputs. I really don't have any issues with driveability at all.

          Tom

          Comment

            #6
            Seems to me that it's the mains that are too small. Don't mess with float height - leave it as per spec and work around it.

            What size mains to try next? You'll have to have 'a feel' for this, whether you work up by 2.5 each time or jump sizes. Could be a PITA and not cheap as I had a 750 that needed 140s (I think, may have been even more - it was a long time ago).
            79 GS1000S
            79 GS1000S (another one)
            80 GSX750
            80 GS550
            80 CB650 cafe racer
            75 PC50 - the one with OHV and pedals...
            75 TS100 - being ridden (suicidally) by my father

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              #7
              If the 127.5s are still lean, Id go to 130?s and try there. The 127.5s sound like they are VERY close. Maybe even try moving to the 5th clip on the needle first. Keep in mind, our needle jets are bigger than the later model VM carbs, so our jump isnt going to be quite as big... you could also try to bracket the size, knowing the 127.5s are too lean (if the needle move doenst help) try the step up from the 130 (132.5?) and if thats too rich, then you know 130 is spot on...
              Last edited by Guest; 06-12-2008, 06:26 PM.

              Comment

                #8
                Originally posted by hampshirehog View Post
                Seems to me that it's the mains that are too small. Don't mess with float height - leave it as per spec and work around it.

                What size mains to try next? You'll have to have 'a feel' for this, whether you work up by 2.5 each time or jump sizes. Could be a PITA and not cheap as I had a 750 that needed 140s (I think, may have been even more - it was a long time ago).

                I wish I did have a feel for this sort of thing, but I don't as this is my first time re-jetting. Not sure what I will try next but I will get a couple different sizes. Jets are only $5 Cdn each so it won't get really expensive, especially if you consider taking it to a performance shop with dyno. I am guessing that would run me $300 to $400?? Can buy lots of jets for that and I get to learn lots while doing it myself.

                Tom

                Comment

                  #9
                  i would move the needle first, then get bigger mains if you have too....

                  you never know, the 5th clip position may be just right!!!

                  Comment

                    #10
                    Originally posted by 80GS750 View Post
                    i would move the needle first, then get bigger mains if you have too....

                    you never know, the 5th clip position may be just right!!!
                    Yup...thats what i suggested

                    Comment

                      #11
                      I've been through this battle before. My experience has shown that if it can still bog the engine out completel when you open the throttle, needle position will not be engough to solve the problem. It sounds like you are close, 130 might do the trick.

                      Comment

                        #12
                        Originally posted by 80GS750 View Post
                        i would move the needle first, then get bigger mains if you have too....

                        you never know, the 5th clip position may be just right!!!

                        Okay. This is good stuff guys, but here's my question. At full throttle the needle is up out of the way and the main jet is supplying the fuel, right? Why would changing the needle position have any impact at this point? I appreciate all the input guys. Just waiting for a call back from a local shop to see what they have for richer jets. They had the 127.5's in stock so......

                        Tom

                        Comment

                          #13
                          Originally posted by TBC View Post
                          Okay. This is good stuff guys, but here's my question. At full throttle the needle is up out of the way and the main jet is supplying the fuel, right? Why would changing the needle position have any impact at this point? I appreciate all the input guys. Just waiting for a call back from a local shop to see what they have for richer jets. They had the 127.5's in stock so......

                          Tom
                          Well, without even having my bike on the road yet, i went ahead and ordered 130s and 132.5s based on your info. Lil help back my way and i appreciate it

                          Comment

                            #14
                            the needle may come out of the jet all the way, but it is still in the passageway and based on the taper of the needle, it still limits the flow.....

                            moving it one more position will pull the needle that much higher out of the way, allowing more room in the passageway....

                            Comment

                              #15
                              Originally posted by 80GS750 View Post
                              the needle may come out of the jet all the way, but it is still in the passageway and based on the taper of the needle, it still limits the flow.....

                              moving it one more position will pull the needle that much higher out of the way, allowing more room in the passageway....
                              But the smaller restriction, therefore the one which limits the flow is the main jet.
                              http://img.photobucket.com/albums/v5...tatesMap-1.jpg

                              Life is too short to ride an L.

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