New bike, The works

Calev

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Recently purchased a 1978 Yamaha XS650 Special (video of running) from a lad. The PO had purchased the bike and took it to a vintage motorcycle shop for repairs/replacements/upgrades. After they rode for a summer, the bike sat in the garage for a couple of years. I bought the bike non-running knowing that it was due to dead battery and gunked carbs. My hunch was right, after cleaning the carbs and recharging the battery, it ran first kick. But not well. I ended up tuning the carbs to the best of my ability, purchase a fresh battery, and replaced the spark plugs as one plug looked very fouled and oily. Since then, it runs okay and haven't had any major issues. It's not perfect as after riding for a couple 100 miles, I have started a list of repairs that I am going to do this winter. I'm also planning on tearing down the motor this winter too as the left cylinder is leaking oil causing the bike to smoke a littler on hotter rides.

The bike is mostly stocked except for the carbs, the drilled-out exhaust, and no turn signals. The PO had performance carbs installed by the mechanic. I have points ignition with original rectifier/regulator. With all that said, here is a list of things I've noticed after riding for couple 100 miles:

  • Bad gas mileage, 20mpg at most
  • Smoke out of left exhaust pipe
  • Small oil leak on left cylinder
  • Starter sometimes work when button is pressed
  • Bad rectifier
  • Stator is grounding somewhere
  • Very small amount of rust in tank

Right now, I want to tackle the bad gas mileage, could this be from my carb tuning being too rich?

Thanks!
 
Right now, I want to tackle the bad gas mileage, could this be from my carb tuning being too rich?
Could be any number of things...
Dragging brakes.
Huge rear sprocket.
Low compression.
Retarded timing.
... and yes, incorrect carb tuning. I'm sure there's other things I forgot, and it could be multiple causes, not just one particular thing.

Btw... welcome to the forum!!!
 
Few thoughts on your problems.
Starter sometimes work when button is pressed
If nothing happens when you press the button electrical problem. Not hard to figure out. If you get a horrible grinding noise and the motor doesn't turn over. Weak spring on starter gear or worn gear. Lots off stuff about both problems in the forum. Search function is your friend.
  • Bad rectifier
  • Stator is grounding somewhere
I have to ask why do you think you have problems with them?
Very small amount of rust in tank
Before you do anything with the carbs get that taken care of. If needed install filters in the fuel line(s).
  • Bad gas mileage, 20mpg at most
  • Smoke out of left exhaust pipe
20mpg you should have gas running out your exhaust pipes.
The PO had performance carbs installed by the mechanic.
As far as I know those are Chinese knockoffs. I would see if the PO still has the original carbs. They may not be "performance" carbs but if clean and set up properly they work well.
 
Could be any number of things...
I’ll check those items, I know brakes aren’t dragging as I put he bike on the center stand and spun the wheels pretty easily. Thanks for welcoming me here, I’ve been a lurker and been reading mostly chargin system posts.

Few thoughts on your problems.

If nothing happens when you press the button electrical problem. Not hard to figure out. If you get a horrible grinding noise and the motor doesn't turn over. Weak spring on starter gear or worn gear. Lots of stuff about both problems in the forum. Search function is your friend.
No grinding noise, I think it has to to with the button or a wire.
I have to ask why do you think you have problems with them?
I measured the resistance between the points on the rectifier, I was getting incorrect resistance reading for low idle charging And voltage at idle is 12.7ish. When rev‘d to 2000+ the voltage across the battery is 14.5. For the stator, I measure for ground between the white wires and it should be an open loop but was grounding to the frame, resistance was in spec.
Before you do anything with the carbs get that taken care of. If needed install filters in the fuel line(s).
I have two fuel filters for both carbs. The tank really isn’t terrible. Just little spots but could be better.
20mpg you should have gas running out your exhaust pipes.
It does back fire a little on deceleration. Maybe it drawing down the float bowl and I am not noticing, there isn’t any fuel leaks when the bike sits, a small amount of oil leaks from the left exhaust though.
As far as I know those are Chinese knockoffs. I would see if the PO still has the original carbs. They may not be "performance" carbs but if clean and set up properly they work well.
I am not a big fan of them as a one a Yamaha IT400 with original mikuni carb that would be a lot nicer than this pair. I’ll reach out to the owner to see if he has the original pair.

Thanks for responding. I’ve done quite a bit of research on the charging system as when I got the bike it wasn’t charging terrible but was kinda fixed after using some sand paper on the regulator.
 
And voltage at idle is 12.7ish. When rev‘d to 2000+ the voltage across the battery is 14.5.

That's describing a perfectly normal charging system.

No grinding noise, I think it has to to with the button or a wire.

I noticed in your video you have painted handlebars. Since they're part of the ground path, you might not be getting a reliable ground. When you push the start button, that grounds the circuit to the switch housing. The housing grounds to the bars, and then across to the left switch housing, which does the same. The ground then carries on out the black wire in the left housing to the headlight bucket where it connects to the ground wire in the main harness.
The horn also relies on that ground path to ground the switch and turn on the horn.

A little paint removal under each switch housing might just clear up both problems.
 
I measured the resistance between the points on the rectifier, I was getting incorrect resistance reading for low idle charging
You can't do resistance checks with the bike running. Resistance checks are done with no power on a circuit using a Ohm meter. Ok for the hair splitters out there you can do volt drop tests and then calculate resistance based on the volt drop. The very best way to check a rectifier is with a scope. That would be done with the bike running. A static test on a rectifier with a Ohm meter is usually good enough.
And voltage at idle is 12.7ish. When rev‘d to 2000+ the voltage across the battery is 14.5.
If the head light is on 12.7ish at idle is not bad to me. 2000+ with headlight on is spot on.
as when I got the bike it wasn’t charging terrible but was kinda fixed after using some sand paper on the regulator.
Solid state regulator is a easy and good upgrade. Also make sure your brushes are not worn out.
 
That's describing a perfectly normal charging system.
Maybe I should I test it again, this was a few weeks ago.
I noticed in your video you have painted handlebars. Since they're part of the ground path, you might not be getting a reliable ground
I’ve read about this and haven’t gotten around to checking if it has good ground. Probably the first thing I need to do.


You can't do resistance checks with the bike running. Resistance checks are done with no power on a circuit using a Ohm meter. Ok for the hair splitters out there you can do volt drop tests and then calculate resistance based on the volt drop. The very best way to check a rectifier is with a scope. That would be done with the bike running. A static test on a rectifier with a Ohm meter is usually good enough.

If the head light is on 12.7ish at idle is not bad to me. 2000+ with headlight on is spot on.

Solid state regulator is an easy and good upgrade. Also make sure your brushes are not worn out.
I’ve taken the regulator off and tested the resistance with a multimeter. I’ll double check my readings and the brushes are in good shape. I saw this post and was going to do for an upgrade.
 
Watched your video, good XS bike there.
Just to be street legal and actually safe it looks like you need mirrors & turn signals.
When bringing a new to you XS up to dependability it’s a good idea to loop around your garage, proving confidence.
Venture out cautiously up hill , not downhill in a precarious situation.
Once proven to be able to get it back home, start figuring out what it needs for tuning little at a time.
Cool 😎!
 
That's describing a perfectly normal charging system.



I noticed in your video you have painted handlebars. Since they're part of the ground path, you might not be getting a reliable ground. When you push the start button, that grounds the circuit to the switch housing. The housing grounds to the bars, and then across to the left switch housing, which does the same. The ground then carries on out the black wire in the left housing to the headlight bucket where it connects to the ground wire in the main harness.
The horn also relies on that ground path to ground the switch and turn on the horn.

(A little paint removal under each switch housing might just clear up both problems.)
Sometimes loosening the switch housing a little and rolling it back and forth clears th paint under the contact point.
 
Here is the maintenance report from the PO to get a sense of what has been done to the bike. I got the bike mid-October with around 29000 miles on the original odometer and have put around 200+ miles on it driving around town. I created that list from things I've noticed during those 200 miles. Since purchasing, I have replaced the clutch cable, gas cap seal, and battery. I've "tuned" the carbs to best of my ability; adjusted the clutch and cam chain tensioner. It's been riding well enough to where I don't question if it is going to start or stall.

I'm slowly purchasing parts for my winter teardown. I'm planning on keeping this bike as stock as possible with a touch of modern technology (LED lights, maybe replace gauges, electronic upgrades). I want to keep this bike for a long time and make it a classic beauty. I've attached a photo for inspiration.

I reached out to the PO to see if they had the OG carbs, turn signals, and mirrors. Unfortunately, the motorcycle shop took the carbs, and the PO lost the signals/mirrors. Any recommendations for sleek turn signals and mirrors?
 

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The video you linked to is a blue bike. The invoice says VM carbs installed... Your pic is a black Special with stock BS38 carbs.... :umm:
 
The video you linked to is a blue bike. The invoice says VM carbs installed... Your pic is a black Special with stock BS38 carbs.... :umm:
The Blue bike is mine. I called the mechanic shop because I noticed the carb discrepancy too. They couldn't give me a real reason why they put flat slide carbs on instead of VM34 as listed on the report. Since this job was done over 3 years ago, they can't remember exactly why. The picture of the black special is not my bike but inspiration for what I want to turn my bike into in the near future. Hope that clears it up.
 
Stock mirrors and turn signals are available reproduction. yamahaxs650.com has best price on those.

OE carburetors and air boxes aren’t too difficult to come by.
 
Okay folks, I had some time to check some things on the XS. Here is an update.

Charging System
  • Rotor: 5Ω between slip rings, no grounding between housing ✅
  • Stator: White-White 0.4Ω on all combinations, Yellow-White 0.2Ω, and White-Ground infinity ✅
  • Regulator: Ground-housing 0Ω, Green-Brown; low 0.2Ω, mid 9.2Ω, and high 7.4Ω ✅
  • Rectifier: All connections at 0.5Ω, current flow check is good, flows only one direction ✅
  • Checked brushes and they looked okay; I've attached a photo.
Charging Test:
  • Battery Voltage, no key, no lights: 12.83V
  • Battery Voltage, key on, no lights: Voltage drops fast. 12.3V~ after a 5+ seconds.
  • While running battery voltage is 12.8 at idle and 14.7V~ at 2000+ rpm.
I also took the right hand controls off to check if they started button was not properly grounding the handlebars and found there was it was making contact with a sanded part of the bar, I cleaned off the rust. I am still planning on changing the rectifier/regulator per that forum post, I just got the parts shipped here so that's in the queue.


I also checked the spark plugs, since I haven't checked since replacing and it is noticeable that something is funky with the left cylinder. When the bike is running, I place my hand about an inch from each exhaust to feel the pressure. The right side has normal pressure and distinct intervals. The left side has less pressure, quieter and not distinct firing intervals. Be way to describe it as a muffle mumble. These symptoms seem like low compression on the left side but am wondering what others might think. I found a person whose parting out a XS650 and am going to grab original carbs and some other parts.
 

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If you have not checked timing chain tension and valve clearance now would be the time to do it. If you have checked them recheck them.
Then do a compression test on both cylinders.
 
From the left plug color, it's too rich. Try adjusting the mix screw?
Your "muffler mumble" could be bad balance of the carbs, with the right butterfly opening sooner/more than the left.
 
If you have not checked timing chain tension and valve clearance now would be the time to do it. If you have checked them recheck them.
Then do a compression test on both cylinders.
I have adjust the tensioner for the timing chain but have not check the valve clearance. That is up next.

From the left plug color, it's too rich. Try adjusting the mix screw?
Your "muffler mumble" could be bad balance of the carbs, with the right butterfly opening sooner/more than the left.
I a bench top balance with carbs when I cleaned them out. They are also flat slide carbs so I used a slightly different method.
 
I have adjust the tensioner for the timing chain but have not check the valve clearance. That is up next.


I a bench top balance with carbs when I cleaned them out. They are also flat slide carbs so I used a slightly different method.
Bench top balancing is just a "ballpark" method... it just gets you close enough to run. To fine tune the balance you need either a manometer... or you can do fairly well by listening to the sound and feeling the mufflers as you already did. Adjust 'till the muffs feel and sound equal.
 
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