77 xs360 idle problem

[GroovyAardvark]

Hey guys,

I picked up a 1977 Yamaha xs360 the other day and I love it. I have put 45 miles on it and it ran GREAT.

Now it has a problem. The idle is very strange. I am hoping someone can diagnose this from the description. I would much appreciate any help!

Start the bike, will idle at 1400 (I like it a little higher) and then it will run for 5 - 10 minutes perfect. As soon as the engine warms up or it runs for a few minutes the idle slowly starts to die. It will go down to 1200, then 1000, then 800, then it will finally just die. This is what happens if I start the bike up and let it run for ten minutes.

If I ride the bike its even wierder. It will run fine, but then idling at stops it will feel like it wants to die sometimes, revs will go way down and I have to keep on the throttle to keep it alive. Last time I rode it however, it would idle at 3500 at stops! But then at some stops it will do the usual low revs and die.

Things I have done:

New plugs
New oil
Adjusted the mixture to the standard manual described settings.
Adjusted the idle up. (which is what explains the 3500rpm, but it doesnt explain why it is so low to beging with?)

Does anyone know what this is?

My plan is to take the carbs apart and see whats what, but just putting the feelers out there to see if this issue sounds familar to anyone?

Cheers,

 

[crazypj]

What do they plugs look like?
black?
white?
rubber plugs on vac take off missing?
Ignition/points correctly set?

 

[GroovyAardvark]

Checked the points and it was a little off.

Idle has improved a lot. Seems to run alright now.

The remaining issue is that it revs higher and runs stronger when the bike is on its side stand, as soon as i straighten it up the revs go down lower and it runs a bit crappier.

My current theory is petcock problem... (side stand and petcock are on the left side)
The guy I bought it off mentioned that it 'might need a new one'

 

[crazypj]

Uhhh,. NO
its not under load on sidestand.
If you were riding it along in a straight line leaned over at 20~30 degrees then it may point to a fuel flow issue. Its easy to measure, just disconnect fuel line and drain into a measuring container for a minute, should be around 600 cc or so.
Less than 400 will be a problem. Check venting first, then in tank fuel filter

 

[GroovyAardvark]

Thanks PJ, I will check it out.

 

[XS400D]

i know this is an old topic but my xs400 is having a similar problem. did you ever get it worked out?

 

[GroovyAardvark]

I ended up selling the bike because I moved to a place without a garage and 2 bikes inside was already too many.

The guy I sold it to, sent me an email a few weeks later and said he replaced the vacuum petcock with a standard manual on/off percock with new fuel lines and the problem is resolved and it runs great.

I bought a vacuum petcock kit and cleaned and rebuilt it on mine before selling it but it did not fix the problem. My tank and fuel filter were clean as a whistle.

My advice, replace the petcock with a non vacuum type, ASAP it is not worth the hassle.

I would also open up the carbs and see whats what. My bike had a bad float, which I replaced.

Good luck!