Hello guys,
A few years ago when I switched my 78 from points to TCI I replaced all of the electrolytic capacitors in the ignition. These type of capacitors only have about a 10-15 year lifespan sometimes so its a good measure to replace them right at the start. After doing so the left cylinder would always fire dead on the timing mark but the right cylinder always seemed to wander +/- 3-5 degrees off the timing mark. The right cylinder will sometimes crackle on deceleration and miss at stoplights once warmed up. I've even had the engine die on me when coming off throttle to quickly slow for a turn. So i have decided to investigate. I put my O-scope on the ignition pickup inputs to the board and the coil drive transistor outputs to see if I could find any signal degradation and rule out the ignition pickup as the problem. (several years ago I swapped the coils back and forth and the problem was always on the right, so coils were already out of the picture)
The fun begins!
With the O-Scope I was having a tough time getting a good trigger signal, and the bike wasn't heated up enough to cause the idling misfires in the right cylinder, but the right cylinder was doing its usual +/- 5 degree timing issues. There was no difference in the ignition trigger inputs, so it is good. I did notice a slight difference in signal between the coil-drive output signals. So I then looked at the input signals to the two transistors and could't see any large, noticeable difference. So my main focus is on the coil-drive transistors. I have seen where the XS650 guys have had a lot of grief with the old coil-drive transistors and that replacing them makes a world of difference. I grabbed some ST901T transistors from Mouser, only $0.99 each, so its worth a shot. I believe I read a forum article where a XS1100 owner found that the ST901T is a VERY close replacement when it comes to transistors. The EDT41-C35 transistors used on the YAMAHA ignitions is obsolete and no longer in production. Another note, while just idling the old transistors became significantly warm, they do work pretty hard to fire the coils!
Components I have replaced:
So now thinking that the Old ETD41-C35's are bad or weak, I went ahead and replaced them. Got out my de-soldering iron and sucked some solder. A note, when working on these the boards are coated in an epoxy-like material, but acetone removes it quite nicely. If you do this you will want to remove the coating first, otherwise when heated it makes a heck of a mess and makes it difficult to solder in new transistors.
De-soldering iron:
Old transistors and heatsinks removed from the board:
Once I got the old transistors out, i took the heatsinks and cleaned them with acetone to remove the old coating. (notice the outline from the old transistors in the previous image) I wanted the new transistors to have the best contact possible for heat transfer. I also put a dab of thermal paste used for computer processors under each new transistor to aide in heat transfer.
ST901T's ready for insertion:
New transistors in place and ready to be buttoned back up:
After getting the ignition back into the bike, I warmed it up and then got out the timing light to see if it needed adjustment. The new transistors didn't affect my timing so no adjustment should be needed after this, but it is always good to be sure. I then went for a ride around town, is 90F out today, so it sure got the engine up to temp. I lowered my idle down to 1100RPM and the bike never had any misfires at stoplights so far, or right exhaust crackling. I will be riding it most of this week to campus and home to see how it fares. Will update in the future!
A few years ago when I switched my 78 from points to TCI I replaced all of the electrolytic capacitors in the ignition. These type of capacitors only have about a 10-15 year lifespan sometimes so its a good measure to replace them right at the start. After doing so the left cylinder would always fire dead on the timing mark but the right cylinder always seemed to wander +/- 3-5 degrees off the timing mark. The right cylinder will sometimes crackle on deceleration and miss at stoplights once warmed up. I've even had the engine die on me when coming off throttle to quickly slow for a turn. So i have decided to investigate. I put my O-scope on the ignition pickup inputs to the board and the coil drive transistor outputs to see if I could find any signal degradation and rule out the ignition pickup as the problem. (several years ago I swapped the coils back and forth and the problem was always on the right, so coils were already out of the picture)
The fun begins!
With the O-Scope I was having a tough time getting a good trigger signal, and the bike wasn't heated up enough to cause the idling misfires in the right cylinder, but the right cylinder was doing its usual +/- 5 degree timing issues. There was no difference in the ignition trigger inputs, so it is good. I did notice a slight difference in signal between the coil-drive output signals. So I then looked at the input signals to the two transistors and could't see any large, noticeable difference. So my main focus is on the coil-drive transistors. I have seen where the XS650 guys have had a lot of grief with the old coil-drive transistors and that replacing them makes a world of difference. I grabbed some ST901T transistors from Mouser, only $0.99 each, so its worth a shot. I believe I read a forum article where a XS1100 owner found that the ST901T is a VERY close replacement when it comes to transistors. The EDT41-C35 transistors used on the YAMAHA ignitions is obsolete and no longer in production. Another note, while just idling the old transistors became significantly warm, they do work pretty hard to fire the coils!
Components I have replaced:
So now thinking that the Old ETD41-C35's are bad or weak, I went ahead and replaced them. Got out my de-soldering iron and sucked some solder. A note, when working on these the boards are coated in an epoxy-like material, but acetone removes it quite nicely. If you do this you will want to remove the coating first, otherwise when heated it makes a heck of a mess and makes it difficult to solder in new transistors.
De-soldering iron:
Old transistors and heatsinks removed from the board:
Once I got the old transistors out, i took the heatsinks and cleaned them with acetone to remove the old coating. (notice the outline from the old transistors in the previous image) I wanted the new transistors to have the best contact possible for heat transfer. I also put a dab of thermal paste used for computer processors under each new transistor to aide in heat transfer.
ST901T's ready for insertion:
New transistors in place and ready to be buttoned back up:
After getting the ignition back into the bike, I warmed it up and then got out the timing light to see if it needed adjustment. The new transistors didn't affect my timing so no adjustment should be needed after this, but it is always good to be sure. I then went for a ride around town, is 90F out today, so it sure got the engine up to temp. I lowered my idle down to 1100RPM and the bike never had any misfires at stoplights so far, or right exhaust crackling. I will be riding it most of this week to campus and home to see how it fares. Will update in the future!
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