Total Motorcycle Community Forums

Hi,
I finally got the gas tank back on and started up my xs400. Now she idled well when I was just using a makeshift tank, all the way until the carb bowls ran dry. When I have the gas tank on now, all of a sudden idling is rough. I only have *very* little fuel in the tank, slightly over half a litre, and tried running on reserve to see if the petcock and everything was OK.

she idles roughly, and revs well only on the choke. (How long should I have the choke on before I can turn it off?) Another very interesting thing that happened only once. With the throttle butterflys completely closed, the engine revs started climbing all by itself. I remember a previous post mentioning runaway ignition cycles if the plugs fire too early, could this be the reason it happened?

So whata can I do about the rough idling. I remember dr.bob saying something about special cleaning of the idle circuit in mikuni carbs. To be honest I havent looked into that yet, but my carbs are fairly clean. could timing be an issue too?

Oh another question. How useful is the Lucas oil treatment for older bikes. I noticed a post on here about somebody using it on their bike, when their bike was diagnosed as dead, and have the engine actually run better.

It is safe or is it doubtful stuff like radiator leak sealers???

When it ran away on you with the butterflies closed, was it in the process of running out of gas?

Sometimes when you suck the floatbowls dry, you can pick up any crud or sediment that was hiding, in the into your idle circuit. Kinda like when you run the gastank dry sometimes.

I dont think it was running out of gas when it ran away on the revs. But i remember a pinch in the fuel line that I straightened out. How does it rev away when its running out of gas???

-Kartik

It's just a waaay too lean condition that causes it. It only lasts a few seconds and then your out of gas. Don't sound like that's what your problem was if pinching the line fixed it. Is the fuel line blocked? Is the interior swelling or coming apart from not being fuel-rated? Just throwing ideas around....

Im guessing that the petcock might be bad. I noticed that the inline filter didnt fill up with fuel when I turned it to prime. It happened everytime when I used the bottle as a fuel tank. I suspect the vacuum hose isnt so good at keeping a vacuum I think I'll rig up a syringe, or just a tube and suck air through it jusut to see if the petcock works fine. New fuel line wouldnt hurt I guess, I have some clear tubing lying around that should serve well, especially for diagnostic purposes.

So I'll go through the checklist and keep you updated about this, if I find something odd. What do you have to say about the Lucas-oil additive stuff?

The petcock sounds fishy alright. Prime should give you a nice steady flow of fuel.

The petcock is in perfect condition. I just checked it out this morning, removed the vacuum hose and sucked air through it with the cap on reserve and ON. Both result in a nice steady flow of fuel and I dont even have to have much suction at all.

So on to the carbs next when I get back from work. How does cooking them in vinegar and water help?

you are gonna have to put enough fuel in the tank to know that it aint running out--you cannot adjust carbs when they are trying to run out of gas--you need to get it up to normal operating temperature--carb adjutments on a cold engine are useless.

GAAAA!!! Im being plagued with problems.. I'm not even thinking about syncing the carbs right now. Here's a list of new problems that came up for the first time today:
There's enough fuel ini the tank, and the petcock is good.

1) Starter motor seems to slip, if I have to crank it a little longer, it sounds like its slipping. I dont do more than 5 or 6 turns.

2) Idling is rough when I dont touch the throttle. Even opening up the throttle a whisker and keeping it at 1000rpm makes it run like a dream. Moment I let go of the throttle, the engine goes to super low rpm, and just dies.
I havent touched the idle mixture screws yet, but adjusting the throttle stop screw doesnt seem to do anything to the rpm.

3) Immediately after the engine dies out I hear a small "whirr" for a half a second.. whats this???

4) Some problems starting when the engine is warm.

5) Does not run at all with the choke off

HELP!!! This is eating away at me and I cant sleep anymore