High Idle after initial warm up.

[l3ry4n3]

I know so little... My choke slides back in right away (wont stay out unless I hold it), would this cause this or is this just more of an annoyance?

 

[l3ry4n3]

Ok. So i taped up the carbs pretty well with electrical tape and sprayed some carb cleaner it doesn't rev. It seems like it likes to rev in 2nd going down to first (probably not the only spot, just what i noticed). I'm thinking it is probably not an air leak either. But I am not sure what to do next. I didn't have to use the choke tonight starting it. I did ride last night, but not at all today. Got the bike up to about 45mph then turned off onto a side road and it was already revving when slowed to take the corner. The bike really seems to be running great after I took it in for a carb synch, except for this issue. Guys there said that they thought it might be an air leak or plugged pilot circuit. Im not sure what this is though.

 

[l3ry4n3]

Hey guys found this on yamaha owners club. I just paid to have my carbs synchronized. What exactly do they do for a carb synch? Should I leave the adjustment screws alone at this point?

This sounds to be exactly the issue I am having.
http://www.yamahaclub.com/forums/topic/23413-xs400-high-idle/

"Posted 06 September 2010 - 11:19 PM
Thanks for all the advice and suggestions guys. You have all been a great help and I really appreciate it!

HoughMade, I tried exactly what you suggested. I set the idle mixture screws to 3.25 turns out, started the bike, yet it still was idling very high. Then I looked at the throttle stop screw and realized that it was screwed all the way in, to the point that the spring under it was completely compressed, hindering it from moving any more. So I then backed it out considerably, yet the idle was still high. So I then shut off the bike, restarted it, and lo and behold it was idling at a reasonable speed. I then continued to fiddle with the throttle stop screw until the engine speed was about 1200 rpm. I then took it for a 15 minute ride and everything seemed great. Whenever I pulled in the clutch the engine happily settled to 1200 rpm, so I happily putted home and then set the idle mixture screw at 3 turns out, and it still seems to be running great.

I was really fooling with the throttle stop screw yesterday, and must have turned it all the way in without realizing it.

The only thing that seems strange now is the choke. Before I cleaned the carbs pulling the choke out all the way would rev the engine to about 4000 to 4500 rpm. On the middle setting it would rev at about 3500 rpm, and pushed all the way in the bike would idle normally. Even after I cleaned the carbs the choke worked in the same way. Now all of a sudden, when I got the bike to finally idle correctly, the choke doesn't seem to have any effect. I can't start the bike with the choke lever pulled all the way out. If I put the choke lever in the middle position the idle speed will increase slightly to about 1500 to 2000 rpm, and if I push it all the way in, the bike will idle normally at 1200 rpm. But now if I pull the choke all the way out the bike will simply shut off. This seems really strange to me. Anyone have any suggestions? Maybe my choke gaskets are bad?? "

 

[Yarodigital]

l3ry4n3 Did you ever fix the Issue?! I came accross this thread, and it is exactly what happens to my bike. I replaced the carb boots and went through the carbs twice now. I am at a loss as to what it can be. I have a GS450 1981 which is really close to your bike.

Thanks if you can help me shed some light on this.

 

[l3ry4n3]

I never did figure it out. I got new carb boots and everything didnt seem to make a difference. I sold the bike a couple years back now so I have not been working on it. Hope you figure it out!