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Won't rev past 5,000rpm - carbs or spark?

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    Won't rev past 5,000rpm - carbs or spark?

    I have a '78 GS550E that I've been doing a lot of work to recently. I replaced the stock exhaust with a generic 4-1 header and muffler and replaced the stock airbox with foam pod filters. I've been trying to get the jetting right, but am running into an odd problem that doesn't seem to be carb-related.

    I used a timing light recently to set the timing, and found that when on the number 2 or 3 plugs, at around 5,000rpm or higher, the light stopped blinking. On number 1 or 4 plugs, it keeps blinking all the way up to redline. Suspecting a spark problem, I pulled one plug wire at a time with it running, stuck a screwdriver in there, and verified that there is a strong (easily over 1") spark on all cylinders at all speed ranges. So then I figured it must be my finicky digital timing light getting confused by something at higher rpms. At idle, pulling plug wires from #2 or #3 cylinders makes only small differences on engine speed, but pulling plug wires from #1 or #4 cylinders really makes it start coughing. It seems to rev in neutral just as easily with any one of the plug wires missing, though.

    With the bike in neutral, it easily revs past redline. But when riding it, I can't get it past 5,000rpm in any gear, at any speed, at any throttle position. Right around 5,000rpm, it starts losing power and making a light popping sound - not backfiring, just sounds like popcorn popping, and it's not loud. I first noticed this in higher gears at higher speeds and more open throttle positions, but have since verified that it happens in all gears - even in 1st gear at 1/4 or 1/2 throttle. At lower rpms, all throttle positions pull fairly well in any gear.

    I have the stock pilot jets, needle clip on the bottom position (2 below stock) and have 105 main jets I borrowed from a GS750 (stock mains on mine are 80), recently cleaned carbs and made those changes and did a vacuum synch after making the changes. Doing some plug reads has suggested that it's slightly lean at 1/4 throttle, close to perfect at 1/2 and quite rich at full throttle. There's some carb tuning work to be done for sure, but since this problem with a sort of wall at 5,000rpm seems to be spark-related, I want to make sure that's fixed before making decisions about jetting based on plug readings.

    So, help me out, please. Has anyone else experienced this? Does this sound like a problem with spark or with carburetion? Or might it be something like restricted fuel flow? The fuel line is not kinked, I haven't checked the petcock filter screen. Also, it didn't used to do this before I made all the changes, which I did basically all at the same time (pod filters, 4-1 exhaust, cleaned carbs, changed needle setting and main jets, set timing). Given that it seems to have this problem based solely on rpm, not on speed or gear or throttle position, and that the timing light stops blinking over 5,000rpm on #2 and #3 cylinders, it seems almost certainly a spark problem to me, but your suggestions are definitely wanted.

    Alex

    #2
    If the plug wires are long enought I would swap the coils side to side and try it again. It might be advisable to get another set of coils You can get some good deals on Ebay for late model coils

    Comment


      #3
      Anybody? I'm pretty stumped here and don't have money and time to just buy new coils on a guess. Has anyone else experienced this problem? Does anyone have suggestions for troubleshooting it? I've read everything my Haynes manual has to say about the ignition system and done all the tests and they all come up just fine. The only real symptoms are the sudden lack of power above 5,000rpm and the timing light stopping blinking on cylinders 2 and 3 above 5,000rpm, but since I have demonstrated strong spark all cylinders at all rpms, something doesn't seem right.

      HELP!!!

      Comment


        #4
        Ok, You've covered a lot of ground here. On the electrical side of things, does #1 coil supply cylinder's 1&4? And the #2 coil would supply cylinders 2&3? Because you mention the timing light cuts out when on these past 5000rpm and also pulling a plug cap off of 1&4 affects idle. This suggests that somethings not quite right with one of the coils. If you've tested the coils, check connectinos and the plug wires (if they're old they can lose conducivity when they heat up) You say you've checked spark with a screw driver. Pull plugs 2&3 , put them in the caps, ground against the engine and see what spark you get. Also check things like the plug gap and the overall condition of the plug (when in doubt new ones are cheap)

        As for the rich mixture. Rich at full throttle suggests main jet is too large. My old CB650 did this at 7000rpm when I changed to a 4into1 pipe. It was just like an electronic limiter, 7 grand and no further. Downsizing the main jet cured it right up.

        Hope this helps....Sam

        PS: I find it always helps to step, back take a breather and look at obvious stuff first.


        Originally posted by alexlockhart
        Anybody? I'm pretty stumped here and don't have money and time to just buy new coils on a guess. Has anyone else experienced this problem? Does anyone have suggestions for troubleshooting it? I've read everything my Haynes manual has to say about the ignition system and done all the tests and they all come up just fine. The only real symptoms are the sudden lack of power above 5,000rpm and the timing light stopping blinking on cylinders 2 and 3 above 5,000rpm, but since I have demonstrated strong spark all cylinders at all rpms, something doesn't seem right.

        HELP!!!

        Comment


          #5
          having the same problem...

          very interesting because I'm having the exact same problem. It's almost as if it has a limiter past 5000rpm.

          Thing is that I haven't changed anything on my bike (except new stator/electrifier). The only thing is that I have hardly ridden it in the last two years.

          85 Gs400E.

          keep the ideas coming.. the riding season is short up here in Canada and I don't want to miss any more!

          -Joe

          Comment


            #6
            Sam, I have nearly-new spark plugs with the correct gap. The weird 5,000rpm "wall" happens whether the engine is cold or warm just the same. So I don't think it's heat making the coils lose power or anything.

            Well, I played around with it a lot today, and found out a few things. I did a few electrical tests and found that the voltage was running around 15 volts, going up to 17 at high rpm. My roomate suggested the modification I did on the stator wiring (eliminating the wiring to the light switch, plugging in all 3 stator wires to the 3 rectifier wires, as suggested many times in the forum and detailed on the stator papers) so I put it back how it is stock, and that made it run at 13-14 volts when the engine's running. That helped a good deal, allowed it to run above 5,000rpm, but it still makes some popping sounds and loses some power above that speed, so it's not really fixed, just not as bad as it was.

            The timing light stopping blinking at high rpm isn't totally consistent - once in awhile I can get it to keep blinking all the way up the scale, but that's rare. I always have to wiggle and hold the inductive pickup just right for it to pick up the signal at any speed, so I think the inductive pickup is just finicky. Regardless...

            I finally got it timed right - it had been hard to time, and I took apart the ATU and cleaned and greased it, and that seemed to help make it easier to time properly. I popped on the coils from my parts-bike GS750 to see if it made any difference, and I'm still having trouble getting the timing light to blink consistently. For now, I'm attributing part of that to just the finickiness of the inductive pickup. It does seem to blink more consistently now, though. Anyway, riding it around with the GS750 coils on it and having it timed properly, it's running better and more powerful, but it still has this sort of popping and loss of power at 5,000rpm and above.

            I know the carburetors need tuning, I know my main jets are too large, which makes it run rich at 3/4 throttle and above. But I don't know if this is causing the sort of odd problem I'm having. I can now rev it above 5,000rpm, but it still has the same problem with popping and loss of power, although not as bad as before. I'm still suspecting some sort of spark problems, but I'm not sure if I can ignore that and try to get the jetting right and see what difference it makes.

            So, any other suggestions? Can I try to get the jetting right despite what still seems like a spark problem? Should I try to put on a set of coils from one of the many 1970's Honda CB750s my roomate has sitting around and see what that does? Any other ideas about why I'm seeming to lose spark at high rpms?

            Alex

            Comment


              #7
              Aside from jetting, the last thing I can think of are your points. Your '87 550 should hav points and condensers in it. Check these over to make sure they're set correctly (gap and dwell angle) also check the points themselves to make sure that they are burned. If they don't look perfect consider replacing them.

              Sam

              Originally posted by alexlockhart
              Sam, I have nearly-new spark plugs with the correct gap. The weird 5,000rpm "wall" happens whether the engine is cold or warm just the same. So I don't think it's heat making the coils lose power or anything.

              Well, I played around with it a lot today, and found out a few things. I did a few electrical tests and found that the voltage was running around 15 volts, going up to 17 at high rpm. My roomate suggested the modification I did on the stator wiring (eliminating the wiring to the light switch, plugging in all 3 stator wires to the 3 rectifier wires, as suggested many times in the forum and detailed on the stator papers) so I put it back how it is stock, and that made it run at 13-14 volts when the engine's running. That helped a good deal, allowed it to run above 5,000rpm, but it still makes some popping sounds and loses some power above that speed, so it's not really fixed, just not as bad as it was.

              The timing light stopping blinking at high rpm isn't totally consistent - once in awhile I can get it to keep blinking all the way up the scale, but that's rare. I always have to wiggle and hold the inductive pickup just right for it to pick up the signal at any speed, so I think the inductive pickup is just finicky. Regardless...

              I finally got it timed right - it had been hard to time, and I took apart the ATU and cleaned and greased it, and that seemed to help make it easier to time properly. I popped on the coils from my parts-bike GS750 to see if it made any difference, and I'm still having trouble getting the timing light to blink consistently. For now, I'm attributing part of that to just the finickiness of the inductive pickup. It does seem to blink more consistently now, though. Anyway, riding it around with the GS750 coils on it and having it timed properly, it's running better and more powerful, but it still has this sort of popping and loss of power at 5,000rpm and above.

              I know the carburetors need tuning, I know my main jets are too large, which makes it run rich at 3/4 throttle and above. But I don't know if this is causing the sort of odd problem I'm having. I can now rev it above 5,000rpm, but it still has the same problem with popping and loss of power, although not as bad as before. I'm still suspecting some sort of spark problems, but I'm not sure if I can ignore that and try to get the jetting right and see what difference it makes.

              So, any other suggestions? Can I try to get the jetting right despite what still seems like a spark problem? Should I try to put on a set of coils from one of the many 1970's Honda CB750s my roomate has sitting around and see what that does? Any other ideas about why I'm seeming to lose spark at high rpms?

              Alex

              Comment


                #8
                Originally posted by sralph
                Aside from jetting, the last thing I can think of are your points. Your '87 550 should hav points and condensers in it. Check these over to make sure they're set correctly (gap and dwell angle) also check the points themselves to make sure that they are burned. If they don't look perfect consider replacing them.

                Sam
                The points and condensers were new about 2 months ago, and as I've been doing work on it the past few days I've checked points gap and dwell angle several times. The points aren't pitted or burned or anything, they still look new. Thanks for your suggestion, though.

                Today I'm going to pop in my stock size 80 main jets and take plug readings just to see how it runs and how lean it is, and then make a guess on what size jets I should buy and run down to the only place in town that stocks them and buy them. Hopefully my first guess will be right, and hopefully that'll cure all my problems. We'll see.

                Alex

                Comment


                  #9
                  you might try widening your plug gap a little bit more than whats recomended
                  mine are currently set at .045 and running strong (d8ea)

                  Comment


                    #10
                    if you are losing spark to 2 cylinders above 5000 RPM, as indicated by the timing light, then you found your problem, it's ignition based.

                    you need to check the resistances of the coil, low side and high. it might have enough 'juice' to fire only up to 5000 RPM.

                    your best bet are Dyna Green coils. www.dynaonline.com

                    ~Adam

                    Comment


                      #11
                      Well, I put in the stock size 80 main jets, and although it was running fairly lean at high throttle ranges, it was running very well, all rpm ranges were pulling with lots of power and no funny popping sounds. So all my worries about electrical stuff were unfounded, I think.

                      I bought size 90 main jets and put them in, and it still runs well but doesn't have as much power (which is expected, running lean makes more power than running perfect) and it's a little rich at higher throttle ranges now, so I'm going to drop my needle (raise the clip) 1 notch and see if that compensates for the bigger jet size. I'm not sure, though, since even at full throttle it's a little rich, but we'll see. I can still buy size 87.5 or 85 mains and use those if I need to.

                      Again, thanks for your help and suggestions. I'm about to run outside and play with it some more, and see if I've got it figured out or not.

                      Alex

                      Comment

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