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'79 bouncing idle. I'VE TRIED EVERYTHING (kinda)

Discussion in 'The Garage' started by JessJGMiller, Mar 19, 2020.

  1. JessJGMiller

    JessJGMiller XS400 Member

    So this has been a year and a half worth of trying to figure out what is wrong with my bike. I bought this fully running about 3 years ago I think, and rode it around with no problems for about a year and a half.

    Here's my problem now: My idle will bounce around a bit and "searches." After running for a little while the idle will rev or stall, or pulse. No telling which one it will do. It's always random. Sometimes after I rev it it comes back down to idle fine, and sometimes it comes down slowly. When I coast to a stop while riding it will die on me every time unless I keep giving it gas. Riding isn't a problem, but I want this to run right. A lot of what I have read suggests it's running lean, but I have pulled my plugs and they were black as can be.

    Here's what I've done: Carbs have come off and been cleaned like 50 times. There is no blocked pathways, good looking jets, etc. I have replaced the carbs and cleaned those multiple times. I have replaced and checked the floats, I have made sure there are sealed gaskets, I have adjusted mixture screws. I have replaced the boot intakes between the carbs and the engine to ensure there is no air leak. I have replaced the spark plugs multiple times, and I recently adjusted the mixtures until the end of my plugs were more of a golden color (indicating a good mixture). I have gotten new air filters and changed the oil, and I have replaced the filter in the fuel line. I have cleaned the inside of the tank out to make sure I wasn't getting a plugged petcock. Nothing should be wrong mixture wise or with fuel delivery at this points since I have changed out so much and continue to get the same exact problem. I have checked the coils with a meter but didn't really understand my results.

    My next plans once I get the chance are to take out the engine and check the valve clearance and just to make sure everything is working properly in there.

    Any ideas?? Please help. This has been going on for a year and a half and it's so incredibly defeating. I have been close to selling it multiple times.
  2. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Next up: valve clearance and then sync the carbs. Id want to be sure of timing also to make sure its fully advancing.
  3. xschris

    xschris a lifestyle not a trend Top Contributor

    Post some pics of the bike so we can see your setup.
  4. JessJGMiller

    JessJGMiller XS400 Member

    What kind of pics are you looking for?
  5. xschris

    xschris a lifestyle not a trend Top Contributor

    Of the overall bike.
  6. JessJGMiller

    JessJGMiller XS400 Member

    IMG_20160731_163610.jpg IMG_20160731_163615.jpg
  7. xschris

    xschris a lifestyle not a trend Top Contributor

    Do you have the crank case hose hooked to the H-pipe? Hard to see one there. You need that. Those mufflers can cause issues, also any exhaust leaks to them. Your plug caps look to be the old stock ones used from the factory. Check the ohms on them they should be around 5-7k. Replace with 5k ones if they are. What type plugs are you using? If you have non-resistor plugs use the resistor caps. If they are resistor plugs use non-resistor caps. Have you synced the carbs with a manometer yet? I you adjust the valves you will need to do the sync again. Compression check? Stock air boxes and filters? What jets are you running in the bikes? Float height? Diaphragms are good and pass the seal test? ( hold the slide up with one finger and then put your thumb over the venturi hole. Let go of the slide and the slide should fall very slow if at all. If it falls fast there is a vacuum leak or the diaphragm is bad.)
  8. JessJGMiller

    JessJGMiller XS400 Member

    The crank case hose is attached. This is a pic of when I bought the bike and was doing some work so it may have been off at the time. I know they aren't the stock pipes. They were what were on when I purchased the bike. They are the factory plug caps and the spark plugs are whatever the manual told me to use. I'll have to check on whether they are resistor or non-resistor plugs. Do you happen to know if the stock plug caps are resistor or non? I plan to sync the carbs again after this is done. Original stock air boxes and recently replaced stock air filters. The jets are stock and have both been cleaned and replaced with new stock. Float height has been set to stock settings. I believe I have checked diaphragms but won't hurt to do it again!
  9. MoonClipper

    MoonClipper XS400 Enthusiast

    Just a thought, have you checked to make sure your floats aren't sticking?
  10. JessJGMiller

    JessJGMiller XS400 Member

    Yes as far as I can tell. I even recently trimmed up a gasket on the bowl because there was a chance that it could rub on it.
  11. xschris

    xschris a lifestyle not a trend Top Contributor

    Floats set to 26mm without gasket on carbs? 42.5 pilot jet, 132.5 main (stock sizes for the 79), needle jet set on third clip. Make sure you have real mikuni jets. The ones with the square within a square stamped into them. The aftermarket ones are junk. Your pilot mix screws should be set around 3 turns out from a very light seat. Also check the tips of them to make sure the tips have not broken off into the carb bodies. This is very common with these year carbs. What do your plugs look like? You need to check ohms on those caps as they are most likely out of spec if they are the original ones.
  12. Gra900

    Gra900 XS400 Addict

    Here’s a possibility. Mine start off ticking over fine, but as the engine heat up it gets more random. I’ve tracked it down to the butterflies not closing fully. If I reach down to the throttle stop there’s a little more movement the springs won’t overcome to close fully. seem you had done so much it’s gotta be something different. I don’t actually have a solution to this right now, unless it’s adjusting the position of the butterflies by knats knadger.
    Good luck , nice looking bike,
  13. JessJGMiller

    JessJGMiller XS400 Member

    Your butterfly position changed once your bike was warm? Even by adjusting the idle screw while the bike is running (which adjusts the butterflies without taking the carbs apart) I can't fix the problem.
  14. JessJGMiller

    JessJGMiller XS400 Member

    All the parts in my carbs are stock. Again, I have changed the carbs out with all new stuff and have the same problem, so I really don't think this is a carb problem. Needles aren't broken. I think they are set to somewhere around 2 turns out right now. That's what they had always been on even before the bike stopped running correctly. The manual suggests 1.5 if I remember correctly.
  15. JessJGMiller

    JessJGMiller XS400 Member

    Finally had a chance to get into the engine. I found that the threads where my spark plug goes into the head was really stripped which didn't allow my plug to go all the way into the head. Because of this, I found that I was only running one cylinder consistently. Does anyone know what size the threads are inside the head? My dad and I plan to try to tap it tomorrow.

    Also, as you can see in the picture, one of my valves is really discolored. Why might this be?

    Next steps are to lap all of the valves and then reassemble!

    Attached Files:

  16. xschris

    xschris a lifestyle not a trend Top Contributor

    While it's all apart it's a good idea to replace all the gaskets and seals you can. It will save time in the long run.

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