Wont crank when warm

[G-FOURce]

I have a 1980 XS400 Special that is behaving differently today than it did just a few weeks ago. The only thing I have done in the interim between when it was behaving and when it started misbehaving is change the oil. The YamaLube 20w50, oil filter, o-rings and crush washer all came from my local Yahama motorcycle dealer.

Whats happening is that when the bike is cold, it starts easily on the second or third kick but after it has been run and is shut off it is hard to start (lots of kicking) and when it does finally start it will only stay running with the choke out. The RPMs jump straight up when it finally starta and when you push the choke back in the bike dies.

I pulled the plugs today (see pics). From the rider's perspective, the plug in the right cylinder (IMG 3886) smelled of gas and the lip around the plug hole (IMG 3887) was wet. The plug on the opposite cylinder (IMG 3880) had no unusual odor and the lip for that hole (IMG 3884) was dry. The plugs are 7s, if that matters.

Any suggestions on where to start? Should I be using 6s for plugs instead of 7s? Should I pull and clean the carbs? Do they need to be sync'd? Should I switch to 10w40? All of the above...?

Thanks!

 

[NewHavenMike]

welcome.

If you don't know when the carbs were synced, you need to do this. I check mine a couple times a year and always whenever the carbs have been removed.

All the maintenance stuff you mentioned with oil and all that, has nothing to do with how the engine is running now. Theres the possibility that you knocked something loose or moved something in someway that it is causing you bike to run poorly.

It sounds like the seat and needle in the carbs are no good and youre flooding out based on the look of those plugs or a stuck float. But you also stated that you use the choke to start it so.....

Unless you've been idling with the choke on recently with those plugs, Id say your idle is rich. The 7s are fine, I buy my plugs by the box.

(the rest of my post was lost when I reloaded the page and the auto save didn't save everything....)

 

[NewHavenMike]

I may use colder plugs because I run my bike a bit hard on my 45 mile highway commute. I would start by ordering a carb rebuild kit and replacing what I could. Then check valve lash and timing. Id probably get new carb boots as well and new vacuum caps and set the float height. Then its good idea to clean the carbs and get an external fuel filter. Adjust the carbs with the bike fully warmed up. It should stall when you screw the mix screws in all the way. Generally, the screws will be between 2-3 turns out to start off and shouldn't ever be much more than 3 turns out. Don't crank them down they are fragile.

 

[G-FOURce]

Thanks, guys! I'll pull the carbs, clean and rebuild them, throw in some new plugs, and see what happens.

 

[G-FOURce]

Update... put the new plugs in and the bike now starts on the first kick after its been warmed up and it is idling just fine with no issues. Im still going to do everything else anyway, but its nice having my little scooter back again.