Timing

[STVR]

Hello,

So I have a 1975 XS650 and I have been jacking around with the timing. I first checked my valve clearances and there were a couple that were off, then I gapped my points and moved on to the timing. Started with the right side made sure I was in the compression stroke piston was at TDC. Then I busted out my timing light started with the right side put the strobe on my timing marks and the best I could get it was the T mark should I be setting the timing on the F mark? I adjusted the plate but the best I could get was to the left of the T mark (near the F mark) on both cylinders. She starts up first kick and sounds strong just want to make sure I don't have any future problems.

 

[5twins]

Yes, you should be on the "F" mark for idle timing. You're running a bit retarded, but that's better than being too far advanced. Have you checked it at full advance? If you're on the mark there then that's probably where you'll need to leave the idle timing. This would indicate some wear in your advance unit and it's advancing more than it used to when new. If you're retarded at advance as well, there's a few things you can try. You can play with the points gaps. Making them larger will advance the timing a bit so set them at the max end (.016") of the gap spec.

Running out of adjustment on your points plate can also be an indication that your timing chain has stretched and is in need of replacement. You can buy some time by filing the slots in the points plate longer to allow more rotation of the plate.

 

[STVR]

Hey 5 twins,

Thanks for the response. I will try and regap the points @.16 I don't think it should be my timing chain as I replaced that last year I rebuilt the top and bottom end. I'm guessing my advanced unit its worn, maybe I can clip the springs and bring some life back to it?

 

[5twins]

Clipping the springs won't change how much the unit advances. They control the speed of the timing advance and return it to the retarded position when the engine slows enough. When new, your advance unit was made to advance a certain set number of degrees, something like 25 I think. As the parts in the unit wear, the amount they allow the advance rod to turn increases. That makes the amount of advance grow. A small amount of wear no doubt occurs where the weights fling out and contact the stops but I think the majority happens here, where the small ends of the weights hook into the slotted disc on the advance rod .....

The corners of those slots on the little disc are very sharp and eventually start cutting into that tab on the weight. That allows the little disc (and the advance rod) to both turn open (advance) more and retard more. It's now moving more than it's original 25°. If you take a look at your advance unit, you'll notice a line etched on both the little disc and the unit, indicated here by the yellow arrows .....

Those marks should align. As you can see, mine don't and my unit is showing some wear. I'm still able to time it properly so it's not too far gone yet. Check yours out. If your marks are farther off than mine, then maybe your unit is shot.

 

[STVR]

Hey there 5 twins thanks a ton for the images and the info! I checked the marks on my advance unit and there about the same as yours. Think I will try to regap my points at .16

 

[STVR]

So I have a question started my bike today runs great. I am currently working on suspension in the forks and changing out front and rear brakes. While I was testing my rear I turned the bike off then started it about 5 min later and there was a back fire and my right carb blew off, is this a timing issue on the right cylinder?

 

[5twins]

It could be. Back firing through the carbs like that is a symptom of the timing being off a little. It's not terribly far off, just a little, but that's all it takes. Sometimes the reading you get when static timing the bike looks good but with an actual timing light, you'll see it wasn't exact.

 

[STVR]

Hey there 5 twins thanks for all the info. Can I ask you something about front brakes? So I took my master cylinder off had to change the brake pads. While I did that I hand sanded my rotor very lightly to get a bunch of crap off of it. Put everything back together and bled my brakes. Tested it out and there is hardly any braking power in the front. I assume the pistons aren't compressing all the way. There is a little wiggle room in my pads. Am I doing something wrong or is my master cylinder toast? I have done this before with no problems

 

[STVR]

Sorry where I say master cylinder I meant to say caliper.

 

[5twins]

Sounds like your brake components (MC and caliper) need to come apart and be cleaned. It appears that the caliper piston is sticking. A thorough cleaning should fix that. In particular, you need to clean the groove out that the square sided caliper seal fits into. It gets loaded with crusty, dried brake fluid over time. That forces the seal out more and eventually, the piston starts to stick.

 

[jd750ace]

Like this. Just done this over the weekend. Thumb through my XS library for pics of what I found and what it looked like when done.

 

[5twins]

Yes, very good illustration of the brown crust you'll find. Many times it's even worse than that and takes a good scraping to get it all out. I finish with the little Dremel wire wheel. You want that groove spotless.

 

[jd750ace]

I did mine with an annealed bronze wire "toothbrush" takes some work. Pine sol seemed to help.