Starting and idle problems

ripcity

ripcity

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Hey guys, I need some guidance here. My 1980 XS400 is completely stock. I'm having trouble with the bike idling. I have cleaned the carbs twice, replaced the mixture screw o-rings (that helped get it running again), I have made sure the petcock is not leaking, I don't think there are any air leaks, new spark plugs and caps, ignition coils are set up correctly. The bike has trouble running when I let off the throttle, even with the choke all the way out. I also need to open the throttle just to start the bike. I have tried the mixture screw at various positions. The float height is about 29mm. The diaphragms are also good. I really want to be able to ride this thing so I can see what all the hype is about. So far this bike is like a bad relationship-consuming all my time and money but not giving me any pleasure.
 
float height is ideally 26mm.

what colour are the spark plugs?

what other mods have you done?
 
I would adjust your idle speed screw at the bottom of your carbs as well. It will allow your throttle to remain slightly open at all times to give your bike the necessary amount of air at idle and start up.
Believe me, you'll be the happiest camper when you get this bad boy going.
 
I have not done any other mods. I have tried the idle adjust screw all the way open, but it needs even more throttle to start and idle. As far as the float height goes, the manual says around 32mm, but on this site everyone seems to go with around 26mm. What's the reasoning?
 

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On the side of the carb look for a cable connecting to it. You should find it fairly easy, that cable attaches to a metal arm of some kind that is moveable(It will pivot on a point attached to the carb). On the bottom side of this will be a spring or some kind and something that stops it from movnig past a point.

The thing that stops it moving is what you want to adjust. Adjust it(there should be a screw to push it against the spring) so that it moves in the same direction as it goes when the throttle is twisted. Adjust it a bunch forward and start it. It should start and idle high, let the bike warm up like this, then back the screw down to a lower idle.

High fives.
 
ripcity said:
I have not done any other mods. I have tried the idle adjust screw all the way open, but it needs even more throttle to start and idle. As far as the float height goes, the manual says around 32mm, but on this site everyone seems to go with around 26mm. What's the reasoning?
Click to expand...

you are running too rich.

the reasoning that we run 26mm is that the bike's work at that height and fail at 32mm :wink2:
 
yeah, plugs look new and you're way too sooty already. First, you can brush them gently with a brass brush to clean them off. They'll work more efficiently. Gap them correctly, then solve the issue of where your leak is. You're running too rich so the bike is compensating for an air leak somewhere, trying to balance it out.

You can spray aerosol carb cleaner around boots, carbs etc to find the leak. The engine will increase alittle in rpm when it sniffs up the gas. That's the place you need to seal.
 
Haha..fair enough. Ok, I suppose I'll re-adjust the float height and the mixture screws. I'll get back later with results. Thank you all for the help.
 
If you can get ahold of a compression tester (mine was 20 at harbor freight) you can find out if you have leaky heads too. Sometimes that creates a fuel rich situation as well because the correct amount of air is not drawn into the intake before combusting, and there's too much fuel left to burn.
 
Good call Drewcifer. I've been meaning to get my hands on one of those. The bike only has 3500 miles, but has been sitting for at least a year. I know that's a major thing to check when getting a bike going after awhile.
 
A rich mixture would definitely explain why you need so much throttle to start. When you get your air leak sorted out, ensure that your idle mix screws are 3-3.5 turns out from fully seated. Running rich, you may have adjusted them to attain a better mixture, and thus have moved them out of spec.
 
drewpy said:
...the reasoning that we run 26mm is that the bike's work at that height and fail at 32mm :wink2:
Click to expand...

Exactly. My reasoning is that at 26mm I can start my bike first or second kick or with barely a brush of the starter button, and then have it idle reliably all day long. At 32mm, it flooded the carb.
 
Last edited:
So I finally did a compression test; 110psi on the left and 140 on the right. Could this be leaky heads? The rpms seem to jump up and the throttle seems to be delayed. Float height is around 26mm.
 
Yes, WOT is very important or you will not fill the cylinder with enough air. Without being at WOT you will be creating a vacuum (as you should be) when the engine tries to suck in more air than is available, reducing the pressure available in the combustion chamber at TDC. Try at WOT and then report back :)
 

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