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Xs400 weird idle issue

Discussion in 'The Garage' started by Sam mcd, Sep 10, 2019 at 4:29 PM.

  1. Sam mcd

    Sam mcd XS400 Member

    Hi all,

    I've been fixing an old xs400 (1978 xs400f) I got a while back and finally have it registered and warranted (NZ) however I've had a constant issue that I just can't seem to fix. The bike runs well and has good power, but the revs will hang when you touch the throttle (not always) once you load the engine up by say putting it in 1st and letting out the clutch they drop back to a normal idle. Carbs have been thoroughly cleaned new diaphrams, and throttle shaft seals and new intake boots. I've checked for leaks with starting fluid and I can't find any. It gets worse as I go leaner on the pilot jet and better as I go richer but to get it to stop I have to be so rich as it'll foul the plugs If left idling for more than 2 mins. At the moment it's slightly rich but not enough to cause it to foul plugs. I can however see exhaust gasses out the exhaust so definitely still rich. The bike has pod filters and custom made stainless headers 38mm pipe 1.6mm wall that I welded. And aftermarket mufflers. Here is a few photos before and after what I've done so far. Any suggestions? Thanks! 20190707_183116.jpg
    20190911_090413.jpg 20190911_090406.jpg 20190911_090423.jpg
     
  2. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    The engine will never run well enough with those air filters. You need to use the H-pipe and preferably the factory air boxes, but you can find ways to mate up other filters.

    Another possibility could be the mechanical advance mechanism sticking on full advance.
     
  3. Sam mcd

    Sam mcd XS400 Member

    Okay, I have the H pipe so I might fit that tomorrow with these filters (all I have at this stage) and see how that goes. I did a plug chop on the highway and I'm a tad lean so I'll go up a size on the mains too. I've checked the advance mechanism and it seems to be ok? It springs back into place smoothly. Anything else I should check for? Thanks!
     
  4. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Try installing the H pipe before doing jets. Having pods may make the bike seem to run lean, or rich..

    Modern fuels will usually always make plugs look lean.

    I would leave the carbs jetting/settings all stock except for idle screws turned out 2.5 turns, go from there.
     
  5. Sam mcd

    Sam mcd XS400 Member

    Hi,
    Thanks for the reply, have done all of that and still similar result, I've uploaded a video of the issue here, it seems to be slightly intermittent. But happens more often than not. Any load applied to the engine drops it back to idle and if you cover up the idle air jets on the carb mouths up with you fingers briefly it drops back to idle too, which I thought was weird? Maybe not.Thanks!
     
  6. got20z

    got20z 1977 XS360

    I had a very similar problem on my xs750 a few years ago with those same filters. It was just still very lean and once I got the jetting right it helped clear it up. Good luck. It seems impossible to get these bikes to run worth a shit even when you aren't running aftermarket parts on them.
     
  7. Sam mcd

    Sam mcd XS400 Member

    Yeah weirdly it's a touch rich at idle but still hangs up like it's lean? It's very confusing
     
  8. xschris

    xschris a lifestyle not a trend Top Contributor

    Bad diaphragm maybe? Did you sync the carbs with a manometer? You may have broken off tips on the pilot mix screws in carb bodies. Black foam uni pods on stock H-pipe works good. Not as good as stock setup but ok. Any changes from stock intake and exhaust with these bikes will make tuning always an issue and will never be perfect.
     
  9. Sam mcd

    Sam mcd XS400 Member

    Diaphrams are sweet, brand new. Synced carbs with a digital manometer so they are balanced, and I've checked the mix screws and they are good too. I could try new air filters but I can't see how that would affect it? Thanks
     
  10. xschris

    xschris a lifestyle not a trend Top Contributor

    These have vacuum operated carbs so how the air comes in affects them. Most pod filters cause poor air flow and turbulence making tuning very hard. Stock foam filters with stock boxes fixes that problem. Non-stock exhaust also causes issues.
     
  11. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    My bike did exactly as yours does, before I replaced the throttle shaft seals...But you said you did that already. I was constantly chasing the idle. I later found out that the throttle cable was sticking and not returning properly. The new cable made it perfect.

    Have you checked timing on both cylinders?

    Are you cold starting with the choke/enrichment? Do you have to use it at all to get it started? You should have to need it for cold starts. I always use it for cold starts and as it warms up, the idle will rise. If I snap the throttle it will hang around 2000 until I release the choke and its good to go.
     
  12. Sam mcd

    Sam mcd XS400 Member

    Hi,
    I've checked timing and adjusted it correctly throttle is returning correct starting with choke. I did a plug chop today while it was doing this and both cylinders were rich! I'm wondering if I have a worn needle jet I pulled it out and the needles look good no wear but I don't know if it's my eyes or not but the hole in the needle jets both look slightly off like a little oval almost? And I can see a TINY bit of fuel seeping past even at idle when the slides are right down if I take the filters off while it's running. Should the needles be a snug fit in the jets? Mine definately have some play! Wondering if I should get a carb overhaul kit and replace them and the idle mix screws while I'm at it?

    Cheers!
     

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