Intermittent spark from 2k to 3k RPM

[SRosso]

Hello I have a 1980 XS650 SG the problem I am having is the bike will idle fine but once you give it throttle the bike Boggs but once the bike is over 3000 RPM it runs fine. Today I bought an inline spark tester and it shows good spark but once you give it initial throttle the spark dies out to nothing then comes back. I rode the bike last weekend stopped about 30 miles in then when I headed back out on the road the bike ran fine all the way home. But it is back to its old ways of running terrible under 3000RPM.
I have done a new wiring harness, magnet upgrade gluing one on for the pick-up coil, and also a new ignition coil.
I don't understand why it would run fine after 30 miles or why the spark is dying between 2000 and 3000rpm. Any help would be great.

 

[nj1639]

This is where a known good, spare TCI black box would be handy.

 

[TwoManyXS1Bs]

Never underestimate vibrations, especially with intermittent connections...

 

[gggGary]

2 stroke tune up? (new plugs, LOL)
Pics of the carbs, filters???
Stock 5K plug caps are known troublemakers.

 

[gggGary]

The pickup coil wiring near the drive sprocket has been found "hanging by a thread" on several Xs650's when inspected.

 

[ckahleer]

Could be a bad connection somewhere. Check voltage at the ignition fuse. Is it within 1 volt of battery voltage?

 

[SRosso]

It is not the carburetors I have tried two different sets and using the inline spark tester you can see the spark light go out. I have measured the voltage at the battery off, idle, and over 2500 rpm and it charges just fine. I might have to get my hands on another TCI as that is really the only thing left at this point in my opinion.

 

[nj1639]

It is not the carburetors I have tried two different sets and using the inline spark tester you can see the spark light go out. I have measured the voltage at the battery off, idle, and over 2500 rpm and it charges just fine. I might have to get my hands on another TCI as that is really the only thing left at this point in my opinion.

Like Gary pointed out in post five above, quick check under the left engine cover where the stator and pickup wiring harness travels around the front sprocket and chain, I've also seen where the wires have been chewed there a bit by not being secured....chances are good that it is the TCI unit but this check costs nothing but a little time.

 

[Jim]

You say it runs fine at idle and above 3K. Seems to me that's telling us that both the retard and advance triggers are working and getting to the box. It's the transition period where the little magic chip inside the box builds the advance curve where the problem lies..... where it transitions from one trigger to the other.... But that's just my weird way of looking a electronics...

 

[SRosso]

The wires look good with that factory wire loam on them still. I measured the resistance of the pick-up coil and it is within spec according to my manual

 

[gggGary]

Just yesterday me AND another member found cracked broken pickup wires, mine was down to one strand and it created an intermittent spark, bike ran fine on the lift but would break up badly under load The pickups ohmed out perfectly at 700 ohms each, then when I checked how it ran again, I had NO spark, some searching found the broken wire.
BS34 carb float setting plastic and brass floats

 

[SRosso]

So I bought another TCI unit and put it on the bike and went for a 40-mile ride and it has solved the problems I was having. I guess something in my old unit is bad. Thanks for the suggestions.

 

[Ozboy]

Open it up and check all solder joints. It maybe a weak joint a quick re solder and you have a spare.

 

[nj1639]

I believe there's a member that does some refurbishing on those units.......iirc.