Ignition Advance

[NewToXs]

I just got a new to me 1980 XS 650 as a cheap foray into "Classic" bikes to see if its for me.

It's in ok shape but does reflect its age. I've been riding it an notice it will occasionally fall on it's face from about 2000 rpm to about 3500 rpm. If it stumbles it will not pull out of it until it hits 3500 rpm. You can get it out of the stumble by either down shifting or pulling the clutch to rev it up.

This seems related to the timing/ignition to me and not fuel related as the stumble happens regardless of throttle position.

Does this sound like I'm on the right track and its related to the timing advance not consistently working?

 

[grizld1]

Strobe it and observe the advance. Although the electronic ignition models did not have an advance timing mark on the stator, there are many threads that show where the mark should fall on the case. Search and you will find.

 

[NewToXs]

Strobe it and observe the advance. Although the electronic ignition models did not have an advance timing mark on the stator, there are many threads that show where the mark should fall on the case. Search and you will find.

I intend on doing that but my only issue is it is not consistent and "catching" it during the process. It seems very specifically RPM related.

 

[5twins]

The advance is built into the electronic ignition and I've never heard about any issues with it. A big flat spot in the lower RPMs like you're experiencing, however, can be an issue on the later BS34 equipped bikes. The BS34s came set up very lean from the factory, borderline too lean, and any change to the intake and/or exhaust can make them worse, even a small change. Also, it's been reported than even a totally stock bike runs better if you up the mains one size to a #135. Have your mix screws been uncapped? .....

If so, set them to between 3 and 3.5 turns out. The factory had them set down around 2 turns out to meet E.P.A. specs.

Some have cured the flat spot by shimming the needle .....

You might also cure it by richening the idle circuit up a little bit. You can do that by decreasing the size of the air jet that feeds it .....

Go down two sizes to a #130 (#135 was stock).

 

[Jim]

from about 2000 rpm to about 3500 rpm. If it stumbles it will not pull out of it until it hits 3500 rpm. You can get it out of the stumble by either down shifting or pulling the clutch to rev it up.

This seems related to the timing/ignition to me and not fuel related as the stumble happens regardless of throttle position.

I had a TCI ignition box that did exactly that. Worked fine for about 10-15 min until it warmed up. After it was warm I'd have to upshift at about 2k until I got to 4th or 5th, then downshift until revs went over 3-3.5k, then it would run normally. Swapping the box fixed it. Swapped the old box back in to verify and problem returned.
Something you didn't mention that I also had was it would ( sometimes ) pop, sputter and fart on decel too until it dropped below 2k.

 

[NewToXs]

I had a TCI ignition box that did exactly that. Worked fine for about 10-15 min until it warmed up. After it was warm I'd have to upshift at about 2k until I got to 4th or 5th, then downshift until revs went over 3-3.5k, then it would run normally. Swapping the box fixed it. Swapped the old box back in to verify and problem returned.
Something you didn't mention that I also had was it would ( sometimes ) pop, sputter and fart on decel too until it dropped below 2k.

Don't notice a pop on decel and but that may not be saying much. I also notice this after it gets heated up and not so much in the first few minutes.

It's a new to me 40 year old bike so we shall see what's up as I get it in a little better shape. I'll pull the carbs and have a peak but I really don't think my stumble is fuel related. It just so pronounced at a very specific RPM range and easy to get out of the stumble simply by getting out of that RPM window.

The bike appears to be 100% stock aside from the fact someone repainted the tank and side covers with paint with some metal flake. It looks like it was a good job at one point but has subsequently started cracking.

 

[Jim]

I'll pull the carbs and have a peak but I really don't think my stumble is fuel related. It just so pronounced at a very specific RPM range and easy to get out of the stumble simply by getting out of that RPM window.

I remember when it was happening to me, I found a safe road to play on... got the engine into the rpm range where it was acting up and slowly applied a little choke. As the choke came on, it had no affect on the roughness until I was almost at full choke.... where the engine started to die.
In other words using the enricher (choke) had no affect on the problem. I think I'd give that a try before pulling the carbs apart.

 

[grizld1]

NewToXs, You're taking nothing for granted and describing engine behavior in detail. You may be new to the old twin, but you're no newb--welcome aboard!

 

[NewToXs]

I remember when it was happening to me, I found a safe road to play on... got the engine into the rpm range where it was acting up and slowly applied a little choke. As the choke came on, it had no affect on the roughness until I was almost at full choke.... where the engine started to die.
In other words using the enricher (choke) had no affect on the problem. I think I'd give that a try before pulling the carbs apart.

That would rule out a lean condition easily and it had no effect for me.

Really leaning to the CDI at this point. I went for a ride and it was chilly, in the 60s and didn't have anywhere near as much stumbling. I stopped and let the bike idle for a couple minutes and it obviously got hot. As soon as I pulled out of the parking lot the stumbling was horrible. This was a second time I hit the enricher circuit to no affect.

I'll add the CDI to my list.

 

[Jim]

I'll add the CDI to my list.

Just to clarify, it's not a CDI box. It's called a Transistor Controlled Ignition. I know that sounds nitpicky, but they are different boxes that operate in different ways.
You might want to do a search on this site for the "gonzo" box. It's a 20 buck Chinese copy of a Suzuki GN250 box. I've ran one in my bike and it worked great... as have many others. The only reason I'm not all in recommending it is because there's been a rash of reports recently of boxes bad from the supplier.

 

[sleddog83]

Hard to say for certain without swapping out the TCI box. Generally these boxes run till they fail, but you can also have situations where they run until they heat up, then fail, cool off then run again. After 40 years, the solder around some the components gets punky and resistive. Other solder joints break loose, causing intermittent connections. If you can't find a substitute box or a gonzo kit, then try reflowing the solder on the TCI board. It would probably be a good investment to get another TCI module. It is nice to have a spare for situations like this. Even if the Tci is not the cause now, it will eventually fail. I have three and a half modules at my disposal. All this said, I am leaning more towards carbueration issues. Both Jim and 5twins have good advice.

 

[NewToXs]

To update this and bring it to a close if someone is looking it was the TCI box.

Swapped in a different used TCI and my stumble is gone. My bike now runs better than it ever has since it been in my possession. That’s only about 3 weeks though.

 

[sleddog83]

Good to hear. Don't throw out the old one. It is repairable.

 

[jetmechmarty]

Don't throw the old TCI box away. There are people who repair them.

 

[jetmechmarty]

Good to hear. Don't throw out the old one. It is repairable.

Don't throw the old TCI box away. There are people who repair them.

Great minds think alike.