New addition to the garage

[bosco659]

Got most of the bike back together and running. Checked timing and at idle it’s bang on. With gradual increase of engine speed, the timing advances, but at higher rpm (didn’t notice exact engine speed but probably 3,500+), the timing advances well beyond the max timing mark. Engine speed reduced and timing goes back where it should be. Checked weights and rod and all seems to be free. Could it be the springs are weak? One looked a bit irregular.

 

[5twins]

The springs don't control the amount of advance, just how quickly and progressively it occurs. You may have a worn advance unit if it's advancing too much. New, they are set to give 25° of advance. As they wear, that amount can start to increase. You can compensate for a while by retarding your idle timing setting a little.

 

[46th Georgia]

Congrats, sure is clean.

 

[bosco659]

The springs don't control the amount of advance, just how quickly and progressively it occurs. You may have a worn advance unit if it's advancing too much. New, they are set to give 25° of advance. As they wear, that amount can start to increase. You can compensate for a while by retarding your idle timing setting a little.

I do have some spare bits. What are the wear items? Rod, weights ….?

 

[650Skull]

The weight end blobs that go into the slot on the rod wheel. The wear is where the sharp edge on the advance weights and groves out where it tapers just before the blobs and the blobs do wear as well, Get smaller and creates play so the advance is altered

 

[bosco659]

The weight end blobs that go into the slot on the rod wheel. The wear is where the sharp edge on the advance weights and groves out where it tapers just before the blobs and the blobs do wear as well, Get smaller and creates play so the advance is altered

I’ll have a peek tomorrow. Pretty sure I have a couple of spare sets but not sure of their condition too. Thx.

 

[Jim]

If your stash of parts are repops, they're prolly of questionable quality. If they're well used originals, they prolly won't be any better.
As long as it isn't a crazy amount of advance, I'd do what 5twins suggests.
A little retarded at idle won't hurt a thing if it brings full advance under 40° Set it for about 38° at full advance and see where idle winds up.

 

[TX650A Van Islander]

This was my problem as well. Even a replacement brand new mechanical advance would allow the timing to go past full advance. I solved that problem with BB ignition.

 

[bosco659]

If your stash of parts are repops, they're prolly of questionable quality. If they're well used originals, they prolly won't be any better.
As long as it isn't a crazy amount of advance, I'd do what 5twins suggests.
A little retarded at idle won't hurt a thing if it brings full advance under 40° Set it for about 38° at full advance and see where idle winds up

If your stash of parts are repops, they're prolly of questionable quality. If they're well used originals, they prolly won't be any better.
As long as it isn't a crazy amount of advance, I'd do what 5twins suggests.
A little retarded at idle won't hurt a thing if it brings full advance under 40° Set it for about 38° at full advance and see where idle winds up.

The stash should be original bits but I don’t know how old or worn the are. Is 40 degrees where the max timing mark is?

 

[bosco659]

This was my problem as well. Even a replacement brand new mechanical advance would allow the timing to go past full advance. I solved that problem with BB ignition.

How far advanced were you? My eyeball tells me I’m at least 5 degrees+ advanced

 

[TX650A Van Islander]

Same. At least 5 degrees, maybe close to 10 At first I didn't realize it being a rookie and was fortunate I didn't hole a piston as I have read much about since, on this forum. I tried peening the little ears of the weights to no avail but really hurt my thumb with a hammer and eventually settled on physically limiting the throw distance of the weights by welding spacers onto the mechanism to limit the weights' stops. It was frustrating.

 

[MaxPete]

Same. At least 5 degrees, maybe close to 10 At first I didn't realize it being a rookie and was fortunate I didn't hole a piston as I have read much about since, on this forum. I tried peening the little ears of the weights to no avail but really hurt my thumb with a hammer and eventually settled on physically limiting the throw distance of the weights by welding spacers onto the mechanism to limit the weights' stops. It was frustrating.

CAUTION the wisdom on the forum is to NOT try to bend the weights or any other part of the advance bob mechanism.

I had a look at mine way back and I’m pretty sure the parts are made of a light metal casting and they are very brittle and will simply break if you try to bend them.

I’d go with the solution above: retard the timing at idle to compensate - or simply replace the mechanical advance unit with an electronic ignition.

Pete

 

[bosco659]

I will go the route of retarding the timing as an electronic ignition install would be cost prohibitive for this bike.

Is this a common ailment of the XS? Surely there must be a “fix” of the mechanical advance system?

 

[5twins]

There is no sure fire fix that I know of. Some peen the little tabs on the weights to widen them out some. Show us a good pic of your advance unit and maybe we can spot something. Here's the one on my '78 and it shows some wear. The alignment slash marks (yellow arrows) are off .....

I think what happens over time is the sharp corner of the slot in the little center disc cuts into the side of the little tab on the weight. This allows the little disc to rotate more in the CCW direction, to retard more. So, when these units wear, I don't think it's so much that they advance more, it's the opposite, they retard more. But the end result is the same, the total advance amount grows.

Even though my unit shows signs of wear, the bike still times up fine. I set my idle timing a bit retarded, over at the right slash mark of the idle timing "range" .....

..... and that puts the full advance about like so, a degree or two below the full advance slash mark .....

So, if you have them available, maybe just swapping different weights on will fix it, ones with less wear on their little tabs.

 

[bosco659]

There is no sure fire fix that I know of. Some peen the little tabs on the weights to widen them out some. Show us a good pic of your advance unit and maybe we can spot something. Here's the one on my '78 and it shows some wear. The alignment slash marks (yellow arrows) are off .....

View attachment 208080

I think what happens over time is the sharp corner of the slot in the little center disc cuts into the side of the little tab on the weight. This allows the little disc to rotate more in the CCW direction, to retard more. So, when these units wear, I don't think it's so much that they advance more, it's the opposite, they retard more. But the end result is the same, the total advance amount grows.

Even though my unit shows signs of wear, the bike still times up fine. I set my idle timing a bit retarded, over at the right slash mark of the idle timing "range" .....

View attachment 208083

..... and that puts the full advance about like so, a degree or two below the full advance slash mark .....

View attachment 208084

So, if you have them available, maybe just swapping different weights on will fix it, ones with less wear on their little tabs.

Thanks. I was away all day today but will have another look tomorrow and post some pics.

 

[Jim]

Something else to consider....
The ATU plate is bent out 90° for a backstop for the weights. The weights contact those stops at full advance. A strip of tape added to the inside would limit the travel of the weights. As a guess, it would only take one or two layers to do the job

Get some heat resistant metal tape and add however many layers required to bring full advance down to about 38° or so. Since the tape will be inside the stops, centrifugal force won't try to fling them off.

 

[bosco659]

I could try that Jim. I think I have some of that tape “in stock”.

If this works, could one drill and tap the stop for a couple of small set screws that could be Loctite’d in place?

 

[Jim]

I could try that Jim. I think I have some of that tape “in stock”.

If this works, could one drill and tap the stop for a couple of small set screws that could be Loctite’d in place?

I don't see why not as long as it was both stops. Keep in mind though, the engine vibes would probably wear into the stop and the weights being as the force would be concentrated into a small area.... the screw tips. So they'd prolly need periodic adjustment.
Done properly... a nice clean surface for the tape... and it'd likely be just as good as stop screws.

 

[5twins]

Now there's an idea I never even thought of, and it would even be adjustable. But, have a good close look at it. See how your slash marks align compared to mine. Ideally, they should exactly point to one another (no wear). And like I said, if your parts stash includes some weights that aren't worn, swapping them on that would probably be the easiest fix.

 

[Jim]

If you do try the stop screws, make sure you put them at the edge of the stops (red arrows) where there'll be the least amount of force applied to them. Maybe a 4mm screw? And an allen head so you can get an allen wrench in there to adjust them in place.