Yamaha XS650 twin

After a recent bout of twin-bashing on rec.motorcycles, I dragged out my old
XS650 parallel twin last night and took it for a ride. The ride was good, I
enjoyed the sound from those skinny chrome mufflers even if the 650 corners
like a pig compared to the sportbike I usually ride. Could do without the
enormous vibration too, particularly in the footpegs.

When I fired it up, naturally it backfired a bit not having been run since
last year. I had drained the carbs, blah blah blah so no worries. The
backfiring seemed (and that's all it was, seemed) to be coming from the
right pipe only. I put my hands behind the two pipes, and there was WAY WAY
more pressure coming out of the left pipe vs. the right. Intuitively this
seems very wrong. First suspecting rusty headers (couldn't remember just
how loud it was supposed to be) where they sweep up into the mufflers I
checked them carefully... no holes. Both pipes got hot at about the same
rate near as I could tell without an infrared thermometer. The temp of the
exhaust coming out of the pipes "seemed" about the same.

Anybody have any thoughts? Bad spark plug?

BTW the bike ran great, enough power to remind me that I never did get
around to replacing the front sprocket this winter... the engine sounds fine
and seems to run fine once warmed up marginally.

Dave S

 

1980. Sorry.

I'll have a look at the points and check the gap after I stick a new plug
in. Never have replaced the plugs on that bike.

 

I mean I WOULD have looked at the points if the bike didn't have electronic
ignition. Oops.

 

I'd expect this, if the right hand pot wasn't running right.

Just an engine waking after a long sleep.....

So leave it alone. If it ain't broke, don't fiddle with it.

Lovely bikes, btw. I've owned four or five and want another.

 

<snip>

Your comment about the sprocket reminded me - US models run lower
gearing than European. Going up one tooth on the gearbox sprocket
makes them run much more relaxed at cruising speeds. Of course, you
lose the acceleration, but these days nobody uses an XS650 for drag
starts, do they?

The taller gearing improves the fuel consumption markedly, as well.

And if you want a neat tip, drill a hole in the sprocket cover,
immediately above the sprocket, and route the rocker box breather pipe
down to it. Obviously, make sure the hole is the right size for the
pipe to be a tight push fit in it. The oil fumes from the top end come
down and lube the chain.

God, I'm getting the yen for another XS650 now. <Scans Ebay>

 

Less volume out might mean less volume in, from a sticking carb slide maybe?
Or reduced pumping efficiency caused by leaking valves or stuck rings?

 

I bet there's still nothing wrong with it, now it's been used....

 

Do these bikes have a balance pipe or collector box?, it may be that the
silencer has corroded internally and it is partially choked. If it is one
header to one silencer with no link pipes, then the above does not apply.

Are the carbs in sync?, if you have a carbtune ( or similar ), these are
great for troubleshooting.

Can you access the slides through the airbox?, if you can, lift both slides
and then let go. They should drop together, and they should be damped. If
one slide seems to stick, you probably just have a small amount of
corrosion on the slide, easily sorted.

That you said it runs great doesn't point to any major mechanical grief.

Lovely bikes. I quite fancy a cafe racered one.

 

Hmmm... that really doesn't look that difficult to do... the only thing that
would likely confuse me is moving the correct hall-effect sensor magnet in
the correct direction. I've never really done any serious engine mods like
that but boy, it would sure make this bike a lot more rideable for me if it
had less vibration. It looks like the only machining that needs be done for
the whole procedure is the notch cut out of the crank cheek.

Has anyone replaced the rubber handlebar vibration dampers (p/n 13119-001)
with something stiffer to get the mush out of the steering? You'd have to
for any kind of handling gains... if I can tame the vibes maybe I'll think
street tracker.

Dave S

 

No.

 

And the camshaft?

 

Hacksaw, JB Weld.

 

I was waiting for someone to say 'angle grinder'.

Wrong ng.

 

message

Mmmmm, good point. Guess it might run a bit rough what with the valves
operating 83-degrees out of phase with the piston...

 

Thanks. I'm very intrigued, will contact them for pricing. Do they build a
new cam, or would I send mine to them for modification? Likely the latter
unless I want to spend $500.

 

You'd have to sort out the ignition, too.....

 

Oh I'm quite sure they'll do custom cams. Just not a big enough seller to
put into their catalog...