Missfire on cold start

[Ivan Mihajlovski]

Hello everyone,
I own a 1980 Yamaha XS400SE. I've been experiencing a strange issue with it lately. The problem started with the bike running normally, but after warming up (about 20-30 minutes of riding), the right cylinder stalls. I replaced the ignition coils and contact breakers, and now I have the opposite problem. The bike starts only on the left cylinder (the right one stalls), and after 2-3 minutes of running, a large misfire or bang happens, fire comes from the exhaust (I have a straight pipe), and then the bike runs okay afterward. What could be the issue?
Cheers!

 

[CaptChrome]

Since you have only messed with ignition components and the problem changed somewhat, I would stay with the ignition system. The one item you haven't mentioned is the condenser that is used with the breaker points. They do fail and when they do they can cause misfiring, sometimes intermittently. I might try changing that next.

 

[Ivan Mihajlovski]

I was thinking the same, I have the original condenser so I am sure it may cause some issues.

 

[Ivan Mihajlovski]

I've also replaced the condenser (all the replacements are from the Yambits website) but I still have the same problem. Also, there is a little oil on the spark plugs. Can it be a carburetor problem or something more serious?

 

[CaptChrome]

Also, there is a little oil on the spark plugs. Can it be a carburetor problem or something more serious?

Oil usually means that either the valve guide seals are leaking or that the piston rings are not sealing. Have you checked compression on the cylinders? That would be the next thing I do before pulling the head to see what's up.

 

[Ivan Mihajlovski]

The piston rings were replaced about 3000 kilometers ago (~1860 miles) so I guess they shouldn't be the problem.
I will check for compression.