new member 80 xs650

Welcome and $150 for a running bike is nice, but prepare for fixing years of neglect. Check out the tech section and it has a lot of answers and also searching can find a lot as well. There is a lot of experts on the forum but many get tired from questions that have already been answered multiple times. You can many times find used parts on here in the classified section for cheaper than most other places. Good luck can’t wait to see the bike.

80650xs said:

A smaller master cylinder huh? This bike has probably the worst brakes on any motorcycle I've ever rode,

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Yeah, my 80SG was the same. 'Bout all the front would do is slow your eventual arrival at the crash site. Instead of the smaller master, I converted to dual brakes on the front. Very happy with that. The switchover starts here.

80650xs said:

And you're right I need to order a front sprocket. I'm not a heavy guy (130 lbs.) So the bike shouldn't have a problem with the taller gears... I hope!

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I'm a little heavier at 160, but I run the 17/31 and I'm very happy with it. The engine handles it just fine and the vibes are not bad at 65-70.

Sounds like you've just hopped on this thing and are riding it around without a care in the world, lol. Dude, this is a 40 year old bike and is going to need some things. Chains just don't fall off unless they're very, very loose. I always thought the front brake was pretty good but when I rolled mine out for the first time, it already had a thoroughly cleaned out front MC, caliper, a braided stainless brake line, and a drilled disc. It stopped great. Today, about a dozen years later, it stops even better (smaller MC, aftermarket pads).

When I first started riding mine, it had that same minor oscillation or head shake in the 30 to 40 MPH range. It took several things to totally cure it, each helping a little bit until it was fixed. These included a fork overhaul, new steering head bearings, and a fork brace.

5twins said:

Sounds like you've just hopped on this thing and are riding it around without a care in the world, lol. Dude, this is a 40 year old bike and is going to need some things. Chains just don't fall off unless they're very, very loose. I always thought the front brake was pretty good but when I rolled mine out for the first time, it already had a thoroughly cleaned out front MC, caliper, a braided stainless brake line, and a drilled disc. It stopped great. Today, about a dozen years later, it stops even better (smaller MC, aftermarket pads).

When I first started riding mine, it had that same minor oscillation or head shake in the 30 to 40 MPH range. It took several things to totally cure it, each helping a little bit until it was fixed. These included a fork overhaul, new steering head bearings, and a fork brace.

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Riding it around without a care in the world? lol obviously not if I have a backpack full of tools waiting for something to go wrong. I'm not new to old bikes. the chain was very loose, and I know why. Thanks for your input on everything else, I do plan on overhauling everything down the road.

80650xs said:

A smaller master cylinder huh? This bike has probably the worst brakes on any motorcycle I've ever rode, wouldn't a smaller piston make that worse? And you're right I need to order a front sprocket. I'm not a heavy guy (130 lbs.) So the bike shouldn't have a problem with the taller gears... I hope!

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You obviously never rode a 1950s Britbike, eh?
Smaller M/C gives better leverage. Stock M/C is sized to work TWO calipers not the single caliper the stylists put on North American XS650s.
Typical upgrades include replacing hoses with a single wire-braid/plastic hose & either drilling the crap out of the disk
or fitting an aftermarket perforated replacement.
Note that XS650 sprocket & chain sets are available in #530 & #520.
I'd recommend #530. #520 costs less but wears out quicker and the weight-saving is negligible on a 480lb road bike.

I picked up an 80 that needed a bit of work. It did a very similar thing with the 30-45 MPH issue. It was so bad if about threw me off the bike.
The tires looked very good. Even had some of the molding tits left in the tread. Checked the date code on the tires and they were about 12 years old. I swapped the front wheel of my 75 into it and most of the issue disappeared. Swapped in the rear tire and it went away entirely.
I then ordered up new tires.
Do you know how to read a tire date code?
On the brakes, the line pressure is what controls how well the brakes work. Your hand exerts a certain amount of force to the lever. The lever multiplies this force some. This force is applied to the piston.
The size of the piston determines the line pressure. If you push on the piston with 100 lbs. of force and the piston is 1 inch in surface area you get 100 PSI of pressure. Is the piston size is 1/2 inch surface area the line pressure doubles.
This line pressure is then applied to the caliper piston. Twice the line pressure the piston squeezes the pads twice as hard.
On the dual caliper system the line pressure may be 100PSI but it is applied to two pistons. This gives you the same squeeze in the caliper but you have two calipers doing the work so you get twice the braking powder.
A different M/C with a 12 mm bore works much better than the stock 14 mm. Some go as small as a 10 mm bore.
I like the 12 mm bore myself. works great with great feel. Going smaller works but takes a much more prcise application of finger pressure to prevent locking up the break.
Leo