Correction: Should read "1993 Superlight Loses Power" After about a half hour into a ride, my bike suddenly lost power throughout the rpm range. Throttle response was very sluggish regardless of throttle position. The exhaust note was no longer "crisp". However, the bike ran smoothly with no misfires. Since this problem occurred instantly, I am a bit perplexed. I might add that carbs were rebuilt, air filter re-oiled (K&N), sparkplugs & fuel filter changed & fuel tank cleaned recently. So I am confident that I can eliminate these causes. I suppose that bad or dirty fuel might be the cause. But with no misfiring & the fact that the bike ran fine until the power loss occurred, I'm not confident that is the problem. Nevertheless, I will clean the sparkplugs & replace the fuel. If necessary, I will check the pilot jets since the power loss begins right from idle. Any other suggestions would be most welcomed. Thank you so much. Kit
First thought:- is fuel tank breather blocked/partially blocked? If you are quick enough, you can stop and open tank as if it is the culprit then the fuel cap will be reluctant to open. One common place for a blockage to form is within the short steel breather tube that emerges from the front of the tank but could also easily be the one-way valve or the flexible tube collapsing if aftermarket etc.
Chris: No blockage. Since the power loss was so sudden, that might point to something specific. Thank you.
stumped for now (nearly midnight here) as most things that come to mind would not affect both cylinders at the same time. Any other inline filters in fuel supply apart from main unit in tank? Is it anything like some form of starvation/how long before restored/do you think it's temperature related?
Chris: I have a list of things to check, as well as to replace the timing belts. If the problem persists, I will get back to the forum. If anyone has any other thoughts, please pass them along. Much appreciated.
Does it have one coil? This could be faulting to make a very weak spark, unlikely but possible... Or a two into one that has become blocked by a rusty collapsing baffle? Or it has slipped a woodruff key, if it has one, on the end of the crank running the belt drives? I have to point out that I am totally ignorant in regards to the belt drive layout on this bike but a mystery woodruff key problem sidelined me for months once before... Sorry if no help but just kinda thinking aloud...
Two coils. I plan to visually check the spark on each plug. Thanks for your thoughts. At this point, all possibilities are worth checking.
2 independent ignition systems, one for each cylinder, so unlikely to be ignition fault. Fuel supply problem seems most likely.
I wonder if you have 'lost' one cylinder. I have had this happen with both a 750ss and my 999. The motor feels surprisingly smooth and idles ok but is way down on power. Easy to check by disconnecting each plug in turn. In my case, it turned out to be caused by the alternator nut coming loose and the resultant wobbly flywheel took out one of the pick-ups
My money is on either the valve on the tank breather being faulty / fitted backwards or the pipe being crimped / collapsed after the point it drops through the headstock. Remove the valve and check it’s operating one way (and fitted to let air come in but not fumes escape). Blow down the pipe that’s connected through the headstock to check it’s not blocked. If all is well then you will know it’s not that.
Breather valve little arrow should face away from the tank, ie, towards the front of the bike. They can also get blocked, but don't be misled by blowing into it, and think it isn't working. You should be able to blow through it easily in the opposite direction to the little arrow ie; into the tank. Blowing in the arrow direction will feel restricted as it is designed to let go at about 3 to 4 psi releasing pressurised fumes from the tank, but not release earlier to prevent fuel coming out. Hence the squeaky farty noises your bike makes when in the shed as it gets warm. As Chris mentioned, the metal stub breather pipe on the tank can get blocked by paint; rust and or rubber from the hose. Push tough strimmer cable through it to clear it. Also check the tiny mesh filter in the fuel line T-piece between the carbs.
I assume that you are suggesting that (with the engine idling) I disconnect each sparkplug wire in turn & note whether the engine falters. Am I correct?
I will have another look inside the fuel tank. But I was not aware that there was a valve on the tank breather. However, I did remove the rubber vent line from the fuel tank nipple & blew into the tank through the nipple. Also, I blew through the rubber vent hose. Both were clear.
Agree, unless firing is intermittent, and no risk to excitor unit if OP was planning to remove plug lead while running. It’s never happened to me, but have read of people damaging a unit by running for long periods without a load. If I ever do a single cylinder test for setting up carbs et cetera, then I plug cap onto a spare spark plug and rest it on an earth.
