Joined
·
3,923 Posts
Just bought an '87, it's acting up and the P.O. asuured you that it just needed a carb cleaning? Or maybe your later model bike is just doing goofy stuff at inappropriate times, like stalling at high or low rpm's, or your bike runs like a scalded cat for miles, then quits. Maybe it won't restart. So you throw a few rocks at it, go in the house and numb your brain by watching Laverne and Shirley reruns er sumpin', come back later, hit the button and she fires right up. What gives?
So you've come on this site, explained your problem, then followed the excellent advice offered by others and quite logically checked the tank vent, intake boots, float level, etc. Maybe you've even done a total carb cleaning or a total rebuild (or 3). After a month of shopping for answers maybe FINALLY invest in a multimeter. You check the coils and do everything the manual tells you to do and find nothing. You finally cross your fingers, swallow hard, and order up that high-rent CDI unit. You plug it in and life is good. Or not. $300 up in smoke for something you weren't sure you needed. I call this "The '87 Dance" because when it comes to the TW those are the bikes where this occurs most often, but it can happen to any year bike of any make or model and the drill is usually the same.
What if there was a way to avoid the resulting court-appointed anger management. Diagnosing a failing, intermittent CDI has always been a pain in the butt. I have a low threshold for such agravation, so I did all of the above exactly once before I cooked up this method. It will tell you whether your issue is fuel related or spark related in advance, without going through all that.
This is a 12 volt inductive timing light. Some of you young'ns may have never seen one. Basically it's a strobe light of high intensity. It's meant to be powered from a car's battery via those two alligator clips. There's a little gizmo that clamps around the plug wire which senses a spark and sends a signal telling the light to fire. It's very instantaneous and is capable of flashing several thousand times a minute, following the rpm. They usually have a trigger like a pistol which turns the light on and off as the trigger is depressed or relesed.
![]()
They're reasonably inexpensive new and in the age of computer engine management and diagnostics they're fast becoming relics. Good used ones can often be had really cheaply at swap meets and yard sales. Since we're gunna modify it for use on a moving motorcycle that's where I'd get mine. For bike use get the smallest, most compact one that you can find.
They have a lot of other uses, but for our purposes here we just want them to read the spark and flash accordingly.
I'll take pics of my setup in use and add them to the explanation later so it will make more sense.
First we need to lose the alligator clips. We need a way to power the light in such a way that the leads won't come loose while you're riding. If you already have battery tender leads on your bike the absolute best way is to cut off the alligator clips and wire the timing light leads to a tender accessory plug, then just plug it in to the bike.
Next best is to bare the wire ends, loosen the battery terminals and hardwire them directly to the battery posts, then tighten. For one bike this method will get the job done as long as the connections are secure.
Next we need a way to secure the trigger so that it's always "on". I've bypassed the trigger switch on mine, but in a pinch you can just hold it closed with some rubber bands, zipties or electrical tape around the trigger.
Then we need a secure way to attach the light to the bike in such a manner that it can be seen while riding. The strobe will blind you if viewed directly, so I usually place it so the light bounces off the back of the fairing, then tape, bungee or otherwise secure it to the handlebars. Alternately you can place it off to the side and facing you in such a way that the edges of the light can be seen, but the direct beam won't fry yer eyeballs.
Now we're gunna snap the inductive pickup onto the plug wire, make sure your light is flashing, and go for a ride. We need a stretch of road on which we can hold the bike at wide open throttle for an extended period, but we don't need to be in high gear. Any gear will do as long as you can wind it out and keep then throttle wide open when the engine starts acting up. I usually find a place where I can at least hold it in third without being run over. Higher gears are better because it takes longer for the bike to roll to a stop. More time to monitor the light.
Regardless of whatever the bike may be doing, hold it at wide open throttle, in gear, don't touch the clutch and watch the light as it rolls to a stop.
Light "on" and flashing consistently with rpm = fuel delivery issue.
Light "off" at any point or flashing intermittently as the bike slows = CDI/spark/electrical problem.
It's a bit trickier if it's acting up at low or mid-rpm's. The best way to monitor that is to find the sweet spot, then either twist on it or back off of it until it until it hits the point of failure. Watch the light.
Your TW uses a wasted spark system. In other words it fires on every revolution of the crankshaft, including the exhaust stroke. It makes buckets of sparks. So basically if the wheels are turning and the clutch is engaged at no point should the light not be lit and flashing in cadence with the rpm, even if it isn't getting fuel.
Foller dat?
Same rules as above. No light = no spark, however briefly it may fail to flash.
At any rate whenever a bike hands me "mystery meat" I may futz around looking for the obvious for a short while but I'll drag out the timing light long before I start tearing into a carb or attacking a wiring harness with a meter.
If I had an '87 that was acting up I'd definitely take all the fun out of things by going to the light right from the git.
So you've come on this site, explained your problem, then followed the excellent advice offered by others and quite logically checked the tank vent, intake boots, float level, etc. Maybe you've even done a total carb cleaning or a total rebuild (or 3). After a month of shopping for answers maybe FINALLY invest in a multimeter. You check the coils and do everything the manual tells you to do and find nothing. You finally cross your fingers, swallow hard, and order up that high-rent CDI unit. You plug it in and life is good. Or not. $300 up in smoke for something you weren't sure you needed. I call this "The '87 Dance" because when it comes to the TW those are the bikes where this occurs most often, but it can happen to any year bike of any make or model and the drill is usually the same.
What if there was a way to avoid the resulting court-appointed anger management. Diagnosing a failing, intermittent CDI has always been a pain in the butt. I have a low threshold for such agravation, so I did all of the above exactly once before I cooked up this method. It will tell you whether your issue is fuel related or spark related in advance, without going through all that.
This is a 12 volt inductive timing light. Some of you young'ns may have never seen one. Basically it's a strobe light of high intensity. It's meant to be powered from a car's battery via those two alligator clips. There's a little gizmo that clamps around the plug wire which senses a spark and sends a signal telling the light to fire. It's very instantaneous and is capable of flashing several thousand times a minute, following the rpm. They usually have a trigger like a pistol which turns the light on and off as the trigger is depressed or relesed.
They're reasonably inexpensive new and in the age of computer engine management and diagnostics they're fast becoming relics. Good used ones can often be had really cheaply at swap meets and yard sales. Since we're gunna modify it for use on a moving motorcycle that's where I'd get mine. For bike use get the smallest, most compact one that you can find.
They have a lot of other uses, but for our purposes here we just want them to read the spark and flash accordingly.
I'll take pics of my setup in use and add them to the explanation later so it will make more sense.
First we need to lose the alligator clips. We need a way to power the light in such a way that the leads won't come loose while you're riding. If you already have battery tender leads on your bike the absolute best way is to cut off the alligator clips and wire the timing light leads to a tender accessory plug, then just plug it in to the bike.
Next best is to bare the wire ends, loosen the battery terminals and hardwire them directly to the battery posts, then tighten. For one bike this method will get the job done as long as the connections are secure.
Next we need a way to secure the trigger so that it's always "on". I've bypassed the trigger switch on mine, but in a pinch you can just hold it closed with some rubber bands, zipties or electrical tape around the trigger.
Then we need a secure way to attach the light to the bike in such a manner that it can be seen while riding. The strobe will blind you if viewed directly, so I usually place it so the light bounces off the back of the fairing, then tape, bungee or otherwise secure it to the handlebars. Alternately you can place it off to the side and facing you in such a way that the edges of the light can be seen, but the direct beam won't fry yer eyeballs.
Now we're gunna snap the inductive pickup onto the plug wire, make sure your light is flashing, and go for a ride. We need a stretch of road on which we can hold the bike at wide open throttle for an extended period, but we don't need to be in high gear. Any gear will do as long as you can wind it out and keep then throttle wide open when the engine starts acting up. I usually find a place where I can at least hold it in third without being run over. Higher gears are better because it takes longer for the bike to roll to a stop. More time to monitor the light.
Regardless of whatever the bike may be doing, hold it at wide open throttle, in gear, don't touch the clutch and watch the light as it rolls to a stop.
Light "on" and flashing consistently with rpm = fuel delivery issue.
Light "off" at any point or flashing intermittently as the bike slows = CDI/spark/electrical problem.
It's a bit trickier if it's acting up at low or mid-rpm's. The best way to monitor that is to find the sweet spot, then either twist on it or back off of it until it until it hits the point of failure. Watch the light.
Your TW uses a wasted spark system. In other words it fires on every revolution of the crankshaft, including the exhaust stroke. It makes buckets of sparks. So basically if the wheels are turning and the clutch is engaged at no point should the light not be lit and flashing in cadence with the rpm, even if it isn't getting fuel.
Foller dat?
Same rules as above. No light = no spark, however briefly it may fail to flash.
At any rate whenever a bike hands me "mystery meat" I may futz around looking for the obvious for a short while but I'll drag out the timing light long before I start tearing into a carb or attacking a wiring harness with a meter.
If I had an '87 that was acting up I'd definitely take all the fun out of things by going to the light right from the git.
