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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
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
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