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1981 idling issue

Discussion in 'The Garage' started by jwin, Sep 8, 2020.

  1. jwin

    jwin XS400 Member

    Hey everyone,

    So I bought an 81 that was left in a garage for 2 years with fuel in the tank. I've cleaned the carbs (which were already pretty clean), flushed the tank, cleaned the petcock, unseized the engine (pouring MMO in the plug holes and turning the crankshaft), changed the oil and filter and cleaned the charred plugs. There are no cracks in the manifold or air filter element, the air filters look clean.

    I've been running the bike for 2 15 minute intervals and, in addition to the terrible smelling blue-grey smoke and new charring on the plugs, I've noticed it idles at about 2000rpm. When I turn the throttle up to ~3500rpm, the idle climbs and sits at about 2500rpm. I haven't done any tuning or syncing yet. Is it just a matter of letting the bike run longer to burn off any gunk in the system, or should I be adjusting the fuel/air mixture screw on the carbs?

  2. amc49

    amc49 XS400 Addict

    Those screws do not control idle speeds at all other than killing it.

    If you haven't been running bike at speed and simply idling it sitting still you are burning the engine up at 15 minutes, it's aircooled and will severely overheat.

    Hanging rpm is either a vacuum leak or the throttles are being held open. There is an idle speed screw, use it.


    Howdy, Jwin. As AMC mentioned hanging RPMs can be the result of a vacuum leak. Your mixture screws can create a lean condition which mimics a vacuum leak. You could start by richening the mixture or setting the mixture screws at two and a half turns out from being lightly seated.... do not over tighten.
    Make sure your air cleaner filter box is fully attached as this will make a big difference in the tuning.

    The mixture screws can and will affect RPMs at idle. There is a sweet spot that you will be looking for when it comes to fine-tuning them. You find The Sweet spot by listening to the change in idle as you adjust the mixture(you will be looking for nearest highest RPMs to get you in the range. There are millions of write-ups on this you should refer to.

    So, try richening up your mixture and see if that takes care of it otherwise you could spray some ether around your rubber boots and if you have a change in idle then you definitely have a vacuum leak.
    Hope this helps and good luck.
    Last edited: Sep 17, 2020


    if you have not done so already, be sure to check your idle adjustment screw. the one with the spring on it, as well. This opens and closes the butterflies and only controls RPMs. This should be one of the last things you do after fine-tuning is complete, but there's a possibility it could be screwed in way too far.
    Scotty4 likes this.
  5. amc49

    amc49 XS400 Addict

    To do it right you constantly look to be able to lower idle speed with the speed screw while playing with the mixture screws, then you never get it too high. And if the carbs were off to clean them you can spot the throttle butterflies in relation to the beginning of the transfer slot or first hole to know already that the butterflies are as low as they can go without trouble.
  6. jwin

    jwin XS400 Member

    Thanks all. Just an update on where I'm at: I've been mostly doing electrical work and have managed to get the electric starter working again. I let it run until warm and did a compression test - 138psi in the left, 140psi in the right. I've opened the idle mixture screws about 2 1/4 turns from all the way in. My idle sits at about 2300rpm.

    I've noticed the spark plugs get a little blackened and are a little wet when I take them out after running. The exhaust isn't blueish anymore but it's light grey and still smells awful. I haven't taken it out for a ride yet as I've only just finished the electrical work. According to the manual my compression is a little low (should be between 140-170psi). Is this so low that a vaccuum leak is likely the issue, or is it something else? And is the stink and exhaust smoke a sign I'm burning oil?
  7. amc49

    amc49 XS400 Addict

    Lower your idle speed using the SPEED screw not mixture one. Too high will run rich and add to the smell. Compression has nothing to do with a vacuum leak.

    Ride the bike, the only way they keep from loading the plugs up which is what long idling does.

    If the engine was stuck you may have permanent damage to eat oil. What long corroded cylinder walls do. Stuck ring(s) from it do even more. Riding might free things up.
  8. jwin

    jwin XS400 Member

    Adjusted the idle speed screw, now it's idling at about 1500rpm. I've taken it out for a ride twice in the past week. If I let the bike idle for more than a few seconds, the RPM drops until the bike dies. It doesn't seem to matter what the idle speed is set to, it will still drop in RPM until dead. The plugs after riding are a little wet and smell and gassy, with some blackened bits. I've noticed I've got some loose rust in the tank floating around. When I disconnect the fuel line from the carb and turn the petcock to prime, the flow doesn't seem impeded so I'm not sure if that's an issue (I have an in-line fuel filter and didn't notice any rust getting through either). I've got about 1/4 tank of gas in it, could that be the issue or is the dropping RPM most likely an air leak issue?
  9. amc49

    amc49 XS400 Addict

    Air leaks make engine lean, wet black bit gassy plugs are rich.
  10. jwin

    jwin XS400 Member

    Is it possible it's a combination of the idle mixture screws being too lean and an air leak?
  11. amc49

    amc49 XS400 Addict

    Not with wet plugs smelling of fuel. And rich can slowly kill the engine too.

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