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Not a GS question. but Yamaha electrical

chuck hahn

chuck hahn

Forum LongTimer
Past Site Supporter
So I have been pretty busy and all of you know I am like as far away from understanding electrical stuff as anyone can be. So heres the newest symptom the 73 Yammy TX750 like to play.

Dead cold starts and runs great. Get it warmed up and the left side looses spark. I have changed the condensor and put in new points thinking these may be the problem..WRONG!

So now I am pondering whether the left side coil is going south when it gets heated up.
 
There are not that many parts in a simple system like yours.
If not any of the connectors, I would change out the coil.
You can probably use any 5 Ohm coil.
 
Someone posted once about using an OHM meter to check them..How do you do that?? I dont know if Matts meter will work but I can give it a shot.
 
Can you just swap side to side as a test?

It is common for coils to heat up and fail. When it fails, it the bad side coil hotter than the good side coil? If so it's likely shorting out inside.
 
chuck hahn said:
Someone posted once about using an OHM meter to check them..How do you do that?? I dont know if Matts meter will work but I can give it a shot.
Click to expand...
According to BassCliff's site: "To test the coils put one probe of the multimeter on each of the terminals on the primary side of the coil. You should get 3-5 ohms. Check the secondary windings by placing one probe into each of the plug wires coming from one coil. You should get 30-50,000 ohms for stock coils and 15-20,000 for aftermarket."
Although a different brand and different type, those readings should get you in the ballpark.
 
may check GOOD on all ohms test but real world it MAY ARC internally and poop out...
 
barnbiketom said:
may check GOOD on all ohms test but real world it MAY ARC internally and poop out...
Click to expand...
It could just as easily have a broken wire that loses contact when the coil heats up and expands.

Either way, get readings when the coil is cool and hot.

.
 
I havent taken the tank back off yet and started it to see if one coli is getting hotter than the other one. When this did it the first time I surmised the points on the left side had burnt so I replaced them and the plug had great spark so I put the plug in and it instantly started and ran on both cylinders so I took it for about a 15 miute ride and when I got home I thought it was running real rough and I pulled the left side wire again and it didnt change the sound..so I pulled the plug and again no spark.

This is why I am suspecting a heating up coil which fails.
 
Like already suggested - swap the coils around and see if the problem moves to the RHS. If it does, it's the coil.
 
Heat index was only 93 today and I had the energy to tinker with the bike. What i found was that one of the points was out of gap once I had used the timing light and set the timing.

Fiddled with the points again and also found that the timing light was having an intermittent skip in flashes on the right hand cylinder. Cut a little off the end of the wire and rescrewed the cap on and that symptom was gone.

Set the engine fans up and the slave bottle and let it run for as long as it wanted to see if one of the coils was getting hotter than the other to the touch and the both felt about the same, so I dont think I have any over heating coils.

Bike sat and idled for well over 45 minutes and not even so much as a sputter. And both cylinders stayed lit. I left the fans on for a good hour to cool it down like it had been sitting for a long time and it fired instantly.

So far ( fingers crossed ) it seems all the issues are sorted. Yeah I know, I just jinxed the heck out of myself now!!!
 
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