Breaks Down under heat

Shipper

XS650 Addict
Messages
495
Reaction score
367
Points
63
Location
Omemee Ontario Canada
Some time back my ‘75 650xs started breaking down after 40 - 50 minutes riding. The coils were originals so I replaced with Mikes. Seem to run well after that but I was never on long enough rides - until yesterday. Same problem. Where should I be looking next?
 
Yamaha kept changing the valve clearance recommendations.
I'm sticking WIth 6 n 3 as best all around for all years. That .002 for the intake is just too tight.
 

Attachments

  • 72-03-20_S288-01_ValveClearanceChange.jpg
    72-03-20_S288-01_ValveClearanceChange.jpg
    58.1 KB · Views: 48
You need to figure a way to trouble shoot the problem when the bike is hot and not running. Even of valve clearances are way off and thermal expansion has reduced the clearance to nothing, the bike will still run. The bike will run even if the clearances are twice what they are supposed to be. When the bike quits, do you still have spark ? Are the plugs wet or dry ? Start there.
 
Well this is fun!! Quieter engine that pulls much better in all gears. However, haven't tested the long ride yet cause I can't get my idle down to a stable 1000+. Guess now its time to rebuild my Mikuni'e?
 
Yes, at only .002", not much oil is going to get in between the parts, and even less as things heat up and expand. I don't know why Yamaha changed clearance specs over the years, the motors didn't change much. As Gary said, it's become common practice to use 3 and 6 for all years.
 
Yes, at only .002", not much oil is going to get in between the parts, and even less as things heat up and expand. I don't know why Yamaha changed clearance specs over the years, the motors didn't change much. As Gary said, it's become common practice to use 3 and 6 for all years.
I have a "pretty sure" memory of the why part: the time-frame would go back to late '73 through '74-'75. Yamaha started the TX designation. The valve stem groove was changed around then. Anyways, it was common suspicion that the spec on tappet clearance was tightened mainly in an attempt to quiet the motor (like a Honda .002",.004"). Honda was #1 competitor of course. Honda's of the era also generally had smaller 5.5mm valve stems vs. 8mm on TX. Didn't turn-out well; within a year problems appeared from loss of compression, etc. BTW, oil film - in this case - was not a factor. The larger valves simply grew when hot to take-up .002", .004"and then some, causing loss of sealing.
 
Last edited:
And I'll bet that's what happened here. Those tight intakes (especially the one he found at 1.5) probably expanded so much they stopped sealing, lol.
 
Back
Top