1976 XS650C Restore

Well, shoot.
So I had the carbs off and double-checked everything and it all looked good. Checked everything for leaks, etc.
(throttle shafts were tight with new seals I knew I had installed!)
Double checked timing, sync, valves, exhaust, etc. and didn't find any issues.
In spite of my initial diagnosis I don't think my problem was an air leak after all.
I adjusted the air screws out a bit more and that helped but I'm still getting an occasional crack and pop when I rev it and let it drop, seems to be a little worse on the left side.
When I rebuilt the carbs they were soda blasted and I made damn sure every orifice and jet was clear.
New coil, caps, correct plugs, etc.
Plugs looks ok, maybe a tad rich but I haven't done a real chop with fresh plugs.
The bike will idle smooth down to ~800 or so and it seems to run well and pulls hard with the occasional pop coming off throttle.
I dunno, I'm starting to think I may be over thinking this!
Tomorrow I'll take it back out and see what happens.

TimeMachine said:
Gotta love those 76c's. Very nice looking ! The chrome rack is a nice piece .. RT
Click to expand...

Thanks, I have the back rest for it too.
 
cycleranger said:
Well, shoot.
So I had the carbs off and double-checked everything and it all looked good. Checked everything for leaks, etc.
(throttle shafts were tight with new seals I knew I had installed!)
Double checked timing, sync, valves, exhaust, etc. and didn't find any issues.
In spite of my initial diagnosis I don't think my problem was an air leak after all.
I adjusted the air screws out a bit more and that helped but I'm still getting an occasional crack and pop when I rev it and let it drop, seems to be a little worse on the left side.
When I rebuilt the carbs they were soda blasted and I made damn sure every orifice and jet was clear.
New coil, caps, correct plugs, etc.
Plugs looks ok, maybe a tad rich but I haven't done a real chop with fresh plugs.
The bike will idle smooth down to ~800 or so and it seems to run well and pulls hard with the occasional pop coming off throttle.
I dunno, I'm starting to think I may be over thinking this!
Tomorrow I'll take it back out and see what happens.

Over-thinking....yeah, perhaps. These are 40+year old machines that have no electronics, no closed-loop engine controls (i.e. the carbs have no idea what is going out the exhaust) and so there are no on-the-fly adjustments being made and the carbs themselves are actually pretty primitive devices.

Frankly, I’m surprised that these bikes run as well as they do, given all the vagaries of altitude, humidity, ambient air pressure and temperature - for which they really make NO adjustments at all. Additionally, these engines are air cooled which means that they run hotter on a hot day or in traffic than they do on a cold day while buzzing down the road. Modern engines measure all that stuff plus throttle position and load and then they adjust the air-fuel mixture, ignition timing, fuel injection timing and even the number of squirts per piston stroke - all automatically, on the road, several hundred or even thousands of times per second. That is why modern engines always start, seldom need any tuning and usually run several hundred thousand kilometres with no difficulty at all and certainly few if any engine repairs.

Anyhow, I’m pretty content with my old bikes if they start and go when I want them to and if there is a flat spot here or there or a small burb when decelerating....well, that is no more than I put out myself and I guess I can’t expect more of an old bike than I do of myself.
Click to expand...
 
Yeah Pete, you may be right. I think it may be I'm just not used to how this bike runs. I've got too many Hondas!
(When I got this bike I didn't even bother to get running before I tore it down for the rebuild.)

But there's good news.
I couldn't wait until tomorrow so I took it out to go around the neighborhood for a couple of laps.
That couple of laps somehow turned into about 50 miles all around the area. :)
The bike runs pretty damn good with only an occasional pop coming off the throttle or down shifting hard. (still seems to be more on the left side)
It cruised along at 70 without any effort at all.
I think I felt a bit of clutch slip when I got on it hard but that's no surprise, I'm pretty sure it's the original clutch and it sat for 10 years.
Nothing fell off either, (that I noticed..) including the tail light bulb socket I glued back on so that's good.

I did notice that the throttle cable is binding a bit under the tank and not letting it idle down sometimes.
I'm just going to replace the cable, it's pretty stiff anyway. I'm pretty sure it's original too or at least a period replacement.

So I guess I'm just being fussy. Or maybe there is some small amount of crap in the idle circuit. Whatever.
I stopped and gave it some better gas. Tomorrow I'll go shake it down some more. :)
 
Another ~75 flawless miles today. It likes the 93 octane.
There's nothing wrong with this bike, we just hadn't been properly introduced! :)
xs650c_outside_5.jpg
 
Thanks guys!
I wish it was easier to find ethanol-free gas around here.

I did find one little thing to complain about.....
I'm pretty tall and the mirrors are too short and can't adjust them to where I can see!
So I'm going to get some taller mirrors, which is fine with me since I don't really like the look of the existing mirrors anyway. :)
(I see Mike's has some taller "euro" style mirrors.)
 
Yes, the '76-'77 carb set is usually pretty good on gas, probably because of the small stock 122.5 mains.
 
50 mpg is getting better, but if your carbs are tuned better, you should be able to get 55 mpg.
My 1978 Special which has the largest stock main jet size, gets 58 to 60 US mpg on the highway, and 54 to 55 around town.
 
Yeah, I was a bit puzzled by by my first calculation being so low when by all accounts I should get around 50mpg.
I'll do some more riding and some more maths. ;)
Either way, I've done about ~300 miles so far this week with no problems.
 

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