Hi Guys,

The bike has been sitting in parts for several years. Started rebuilding it last year. Took it for some small rides till the battery was drained as the battery was not getting charged while driving. I charged the battery and it would start again. Narrowed it down to a broken R/R. Installed a new mosfet R/R last week and the rear cylinder misfired with low rpm's (high rpm's no problem). Accidently I switched the rear cylinder pickup coil wires (it would still run with misfires from rear cylinder). Switched the cables to correct order and found out that there was a broken cable on a pickup coil which corresponded with rear cylinder colors. Fixed this and it was running nicely for a couple of minutes on the workbench. So I thought lets take it for a spin today. However, once outside it would not start anymore (maybe a backfire sound or 'tsst'-noise from rear cylinder inlet).

Motorcycle specs:
Yamaha TR1 1981
Mikuni TM38 dual carbs
Anti-gravity 16cell lithium battery
M-unit + new wiring
Mosfet R/R

What I already did:
1. Check for sparks, both spark. And without the rear sparkplug the engine would easily start on front cylinder only. Also, only rear plug in, it would not start.
2. Checked for gas. Noticed that rear plug seemed dry, so checked the carb and cleaned it. With both plugs in, it would not start. Front plug in, it would start.
3. Checked ignition coils, switched them, still only starting on front cylinder only. With both plugs in, it would not start.
4. So maybe the rear cylinder valve timing or clearance is off? Checked rear cylinder valve clearance and was in order. With only rear sparkplug in, it would suddenly start. So I tried with both plugs in, it would not start. With only front plug in, it would start.
5. Mechanically everything seems in order. Compression in both cylinders and on each separate cylinder it will start but not with both cylinder/sparkplugs in at the same time.

My thinking:
It looks like an electrical problem. Where do you guys think the problem is? What should I look into? Is the TCI broken or ..?

Let me know!

never start with only one spark, you have to connect the 2nd to ground ! it will damage the TCI.
is the starter strong enough to turn the motor with both sparks an compression ?
if it is to lousy then start with one spark and other to ground and try to limt gas to minimum, if both zylinder have minimum gas (carburateur open 0,1mm) the start is best you can get, the screws at carburateur zylinder side open 2 turns.

good luck
w