Need help with my Vintage bike! (1978 Yamaha XS400)

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arm

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Hi all, I have a 1978 Yamaha XS400, and here's the issue.
The left cylinder wont spark, I replaced the spark plugs (no luck), replaced the original left coil pack to a newer one that matches the right coil pack (no luck), it has a upgraded ignition system from mikes XS which I replaced (still no luck), bought a new battery (no luck)

The bike, as it stands, doesn't spark at all, The lights and everything else still work its just an issue with spark. I can't seem to figure it out, and I'm convinced it's a wiring issue, but it's something I can't understand. if you want to read more about it check out this thread I posted on the XS400 forum

If anyone can give me tips id greatly appreciate it but as of now I'm looking for shops to take a look at the bike which is kind of hard due to its age, Rosey Toes has agreed to take a look at it but they are super packed this season and I don't know when ill be able to get my bike in. Does anyone know a bike shop near Misusagua/George town that can take a look at this bike for a decent price?

I appreciate any advice, I'm desperate to ride this thing 😅
 
arm said:
Hi all, I have a 1978 Yamaha XS400, and here's the issue.
The left cylinder wont spark, I replaced the spark plugs (no luck), replaced the original left coil pack to a newer one that matches the right coil pack (no luck), it has a upgraded ignition system from mikes XS which I replaced (still no luck), bought a new battery (no luck)

The bike, as it stands, doesn't spark at all, The lights and everything else still work its just an issue with spark. I can't seem to figure it out, and I'm convinced it's a wiring issue, but it's something I can't understand. if you want to read more about it check out this thread I posted on the XS400 forum

If anyone can give me tips id greatly appreciate it but as of now I'm looking for shops to take a look at the bike which is kind of hard due to its age, Rosey Toes has agreed to take a look at it but they are super packed this season and I don't know when ill be able to get my bike in. Does anyone know a bike shop near Misusagua/George town that can take a look at this bike for a decent price?

I appreciate any advice, I'm desperate to ride this thing
Click to expand...
I have heard good things about twin City recreation in Georgetown no personal experience

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I know nothing about this bike specifically, although I did have a '75 XS500 twin for a couple of years.

But you're saying the right cylinder sparks OK.

You've done a lot. I'd consider moving the components over from the right side one by one to see if you can get power to the plug. Start with the coil pack. No luck? Then switch over the spark plug boot and wire. If no luck, then what other components are specific to the right side can you try on the left side?
 
Do you have power at both leads going to the coils? I'd start there. Then if there was I'd do like others suggested and switch the components side to side to eliminate coil,HT leads and plug.

If you don't have power at the leads work backwards with a multimeter or a 12v bulb with two pieces of wire attached.
 
Breaker-points ignition?

Points good? Condenser good?

With key on, is there power to both sets of points? Rotate crankshaft -> do the points successfully interrupt power to the coil side of the circuit?

With points open, what's the resistance of the primary side of the coil all the way through the primary circuit. Should be single-digits ohms.
 
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Jack up the gas cap and slip a new R3 under it...J/K. As you know, old machines can be a royal pain. Could the ignition switch or side stand cut out switch, if equipped,, be suspect. How about the plug wire or cap?
 
arm said:
Hi all, I have a 1978 Yamaha XS400, and here's the issue.
The left cylinder wont spark, I replaced the spark plugs (no luck), replaced the original left coil pack to a newer one that matches the right coil pack (no luck), it has a upgraded ignition system from mikes XS which I replaced (still no luck), bought a new battery (no luck)

The bike, as it stands, doesn't spark at all, The lights and everything else still work its just an issue with spark. I can't seem to figure it out, and I'm convinced it's a wiring issue, but it's something I can't understand. if you want to read more about it check out this thread I posted on the XS400 forum

If anyone can give me tips id greatly appreciate it but as of now I'm looking for shops to take a look at the bike which is kind of hard due to its age, Rosey Toes has agreed to take a look at it but they are super packed this season and I don't know when ill be able to get my bike in. Does anyone know a bike shop near Misusagua/George town that can take a look at this bike for a decent price?

I appreciate any advice, I'm desperate to ride this thing 😅
Click to expand...
My guess would be spark plug cap with a burnt out resistor or the spark plug wire with the core burnt out
where it screws into the cap.
Points coils are different than CDI coils and that could be an issue.
 
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