Electrical trouble-shooting starts with the wiring diagram 
Frankenstein Build
Hey Ladies and Gents!
Long time forum stalker, first time poster…
First off, this community is great and has been extremely helpful in providing context and insight as I dive into a project I’m working on.
I recently acquired a 1978 XS750 with the objective of bringing it back to life and going full cafe racer. It’s been a project I’ve wanted to take on for a while and I’ve finally made the leap. I have a background in motorcycles as I’ve grown up around them my whole life, and I have a background in maintenance as a crew chief in the USAF. I know helicopters and motorcycles don’t directly translate but it’s better than nothing.
I am aiming to diagnose a few things before I invest too much time and money, ie that electricals are good, and then engine, carbs, etc are in good order. Currently there is no sign of life from the bike from an electrics perspective and I’m wondering if I’m missing something. I’ve installed a new battery, checked fuses, and ground wire and from what I can see, everything looks good. However there is one wire that I’m not sure what purpose it serves or where it should be connected. I’ve included photos in this post. It’s a wire that goes to one of the ignition coils and then has a disconnected end by the battery.
Any thoughts as to what this might be? And any general words of wisdom as to what might be preventing the bike from sparking?
thanks again in advance for the help and I’m looking forward to being a part of this forum.
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Long time forum stalker, first time poster…
First off, this community is great and has been extremely helpful in providing context and insight as I dive into a project I’m working on.
I recently acquired a 1978 XS750 with the objective of bringing it back to life and going full cafe racer. It’s been a project I’ve wanted to take on for a while and I’ve finally made the leap. I have a background in motorcycles as I’ve grown up around them my whole life, and I have a background in maintenance as a crew chief in the USAF. I know helicopters and motorcycles don’t directly translate but it’s better than nothing.
I am aiming to diagnose a few things before I invest too much time and money, ie that electricals are good, and then engine, carbs, etc are in good order. Currently there is no sign of life from the bike from an electrics perspective and I’m wondering if I’m missing something. I’ve installed a new battery, checked fuses, and ground wire and from what I can see, everything looks good. However there is one wire that I’m not sure what purpose it serves or where it should be connected. I’ve included photos in this post. It’s a wire that goes to one of the ignition coils and then has a disconnected end by the battery.
Any thoughts as to what this might be? And any general words of wisdom as to what might be preventing the bike from sparking?
thanks again in advance for the help and I’m looking forward to being a part of this forum.
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Seems like you are just winging it, download and save the shop service manual for free from the carl salter site, you can't possibly service what you need to without it. Read and understand the whole manual first, if there is anything you don't understand in it, ask
Save up your old tooth brushes for cleaning things, they work great.
Save up your old tooth brushes for cleaning things, they work great.
add to that if they have copper washers; your should 'anneal' the copper to reuse them, simple step, you just heat copper until it glows, let it cool slow and it makes the copper washer softer to use a second or third time
... if it was a gas powered automobile some other items you might rob some intake vacuum pressure to power include: cruise control, ignition timing advance, windshield wipers or air vents If it was a diesel engine you would never want to rob any vacuum off the intake, so you would add a vacuum pump onto the engine to power those things.
If it was a diesel engine you would never want to rob any vacuum off the intake, so you would add a vacuum pump onto the engine to power those things.
Did we forgot to mention you should not buy a used engine for parts if the replacement engine has not or can not pass a compression test In a decent scrap yard you will see numbers written on the engine to indicate the compression readings when it was parted, if not then the engine was probably DOA
In a decent scrap yard you will see numbers written on the engine to indicate the compression readings when it was parted, if not then the engine was probably DOA
When you can't buy unique replacement parts any more that makes it obsolete. Obsolete makes it old and possibly only one more crash or one more engine failure away from becoming scrap.
"I'm a millennial, so for me the 90s were about 10-15 years ago"
I'm a boomer, when did they stop teaching arithmetic
"I'm a millennial, so for me the 90s were about 10-15 years ago"
I'm a boomer, when did they stop teaching arithmetic
Multi-meter is great but it's just a tool, you have to learn how to use one and interpret the readings or it's useless. Use your multi-meter to set the gap and test the breaker points. Resistance measured through the points when they are closed should be zero. The moment the points open as indicated by the alignment of timing marks on your flywheel, test meter resistance should indicate infinity. If it doesn't work that way then you have things assembled incorrectly and that is a common error. If the breaker points meter test correctly then they are not the problem.
Condenser/capacitor is what makes the spark and it does that because capacitors store electric energy until they are grounded and then they release all of their stored energy at the same time. Just like when you walk across a carpet floor in your socks and then touch something that gives you a static shock in this scenario you are the capacitor. If the condensers are old just replace them, it's a relatively inexpensive part that is prone to failure. It's just a tiny metal can full of rolled up layers of paper and metal foil that can easy be damaged by heat, water and rust. Points and coils are easy to meter test, condenser not so easy to test but it is also the cheapest part in the entire assembly.
Condenser/capacitor is what makes the spark and it does that because capacitors store electric energy until they are grounded and then they release all of their stored energy at the same time. Just like when you walk across a carpet floor in your socks and then touch something that gives you a static shock
in this scenario you are the capacitor. If the condensers are old just replace them, it's a relatively inexpensive part that is prone to failure. It's just a tiny metal can full of rolled up layers of paper and metal foil that can easy be damaged by heat, water and rust. Points and coils are easy to meter test, condenser not so easy to test but it is also the cheapest part in the entire assembly.
If it turns out to be a defective spark unit, let me know before next week and I can shop for one for you while I look for one for that other guy. Maybe I can get a bulk discount on spark units
edit: cancel that offer, I just looked at the service manual, doesn't even show a spark unit so it can't be that. Roll with new condensers
edit edit: good recommendation by @Youtube_Zombie to test resistance on the spark leads and caps as per the manual & OP: Does the bike have a solid state or ancient points type voltage regulator, apparently they came both ways. If it is points type I would replace it with solid state.
edit: cancel that offer, I just looked at the service manual, doesn't even show a spark unit so it can't be that. Roll with new condensers
edit edit: good recommendation by @Youtube_Zombie to test resistance on the spark leads and caps as per the manual & OP: Does the bike have a solid state or ancient points type voltage regulator, apparently they came both ways. If it is points type I would replace it with solid state.
Did you buy all the Forrest Mims books too
still highly recommended reading. Should be mandatory study in grade school, but first you would need to make the teachers read them.
Thirty bucks on flea bay, this is what points should look like:
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Conversion to pointless electronic ignition will cost you somewhere around 300$ to 500$
Conversion to pointless electronic ignition will cost you somewhere around 300$ to 500$
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