1. Having trouble logging in? Try closing your browser and re-opening it. If that doesn't work, try a different browser or device. The site will be updated soon to resolve these browser issues.
    Dismiss Notice
  2. Dismiss Notice
  3. Want an XS400.com t-shirt? Go to the store to order. Limited sizes and quantities available.
    Dismiss Notice

CV pistons not actuating

Discussion in 'The Garage' started by Hishman, Jul 27, 2013.

  1. Hishman

    Hishman XS400 Addict

    Like the title said i have a problem with the CV pistons. If that's not the correct nam i mean the large cylinder with a rubber diaphragm and needle on the end. I cleaned my carbs like crazy using works and carb cleaner. I still couldn't get my bike to run right. It will only run on choke and even then barley, and it crackles and pops when i turn off the choke and give it gas. I took the intake off to check out why it was back firing. I then noticed the cv pistons weren't moving at all. I can lift them with my finger pretty easy as well. Is there any way to diagnose why they arent moving?
  2. Could be a hole in the diaphragm. Also look into lubing the cylinders as well. I've never had to but I've heard of it, I'm not sure what job of lube to use. Hopefully someone else can chime in.
  3. arfstrom

    arfstrom XS400 Guru Top Contributor

    There are pic of bugs stuck, in some of those holes, blow them all out, best of luck. You know the space where the needle rides, is a brass thing,that can be glued to itself, (corn syrup fuel) (main noozle) part 32, listed on carburetor.
    Good luck
  4. jmd_forest

    jmd_forest Bldg 1980 XS400 Cafe Bike

    Most likely culprit is the carbs are still dirty. It is not unusual for new XS owners to clean their carbs 2 or 3 times before they are actually clean. They really need to be COMPLETELY stripped apart and cleaned, every piece and every orifice. Also, as MechNewb mentioned, holes in the diaphram is a common problem, as well as other deteriorated rubber bits and pieces in the carbs. Finally, check your ignition timing. IF the timing is off it can also produce similar simptoms.
  5. Hishman

    Hishman XS400 Addict

    Hmm I am certain the carb is clean as I completely stripped it and soaked it in acid. But there is no harm in cleaning it again. I'm just gana get really good at removing the carbs :p. But before I do that I'm going to check the timing as the PO did an engine overhaul. I will also double check the diaphragms again.
  6. arfstrom

    arfstrom XS400 Guru Top Contributor

    You know, some PO's will but the stuff in backwards. ie the coils could be reversed, and I remember something about timing being off 180. Its been 2 years since I have seen that one. I have been hollered at for doing my technique. I could have lost my CDI.

    I also had to resolve the oil case from smelling like fuel? and the fuel running out the air box.
    (I re-read your problem)
    If you haven't done the boiling thing yet, try it, Use a piano string, or similar soft non-metal, heavy filament fishing string to probe the various passages. make sure you pull out every jet, needle, Coat the rubber stuff with a silicone seal enhancer, It will help remove, and replace, and perhaps add a year or two to the life of the rubber seal. Every thing but the butterfly valves. If you spray carb cleaner, use eye protection, Mikuni made these carbs to spray fluid right into your eyes. With that said, the Idle circuit, when your spraying, We have maps to your carb, One of the passages should produce 3 small skinny streams next to the butterfly valve. Your Idle screw should have a straight long end on it. Sometimes those get broken off. Do a google search for Mikuni BS34 rebuild, you should have a huge hit for the XS400 website, and then the other ones. Mikuni sold carbs to other manufacturers So there is alot of support out there.
  7. jmd_forest

    jmd_forest Bldg 1980 XS400 Cafe Bike

    Although you mentioned you completely stripped the carbs before soaking them in acid, if you did not pull the throttle shaft seals before soaking, I wouldn't be surprised if the seals are shot and leaking large amounts of air where it should not be. Same could go for the enrichment circuit and the idle mixture o-ring (if your model has one).
  8. Hishman

    Hishman XS400 Addict

    Well after reading my manual it says my timing is not adjustable as i have an 80 with electronic ignition. The bike will idle smoothly with the choke at half way. So i am ruling out it being 180* off. The bike dies at full choke and no choke. If i give it slight gas(tiny) it will slowly rev up then at a certain point the revs will radically raise. if i give it more than a tiny bit of gas it will bog and start to cackle and spit out the carbs.

    Yes i did not remove the butterfly seals when i put them in works. I had the brass caps on my idle screws that i had to remove. One idle air screw had a rubber seal the other didn't. I think i lost it not knowing it was there. I didn't use the one seal i had.

    I am going to remove the carbs again. Is there anyway to check the butterfly seals? Also are the idle air screw seals that important?

    With that in mind why would any of this cause the CV valves not to actuate. The diaphragms looked great and i checked for holes as well.

    Again thanks for all the help. This website is great!
  9. Scorpio1963

    Scorpio1963 XS400 Guru Top Contributor

    Yeah there should be 2 rubber seals at the bottom.Sometimes they get stuck or worn to the point theres only pieces.Its important you find or replace them or your carbs will suck air and run lean.This will give you idle problems also.I found mine at a local bike shop when I built my carbs.
  10. Hishman

    Hishman XS400 Addict

    I went ahead and made a video
  11. Scorpio1963

    Scorpio1963 XS400 Guru Top Contributor

    Not working on my end.
    As for getting carb spray in the eyes thats actualy a good thing,it helps in removing cataracs:laugh:
  12. Hishman

    Hishman XS400 Addict

    Here this should work

  13. I don't see how the butterfly seals leaking air wouldn't cause the cv valves to actuate. It would just run leak which is a second problem.

    The hole in your diaphragm will be a tiny pinhole so make you've checked then thoroughly.
  14. arfstrom

    arfstrom XS400 Guru Top Contributor

    Okay, Flame coming out of carb, Still with Lou, enriching circuit probably needs alot of work. on the bottom of the idle screws, is 3things, 1 spring, 1 metal washer, and below that is the ity bity rubber washer.

    The flame i am inclined to think is the valve clearance.

    Do the float check. Get a clear hose, and run it up next to the float bowl. the fuel level should be with in a 1/8 inch from the float bowl seam. Here is what I did. I over killed this one, I picked up extra Tee tube connectors. Just a hose is all you need,

    Posted via Mobile
  15. Hishman

    Hishman XS400 Addict

    Just did all the valves. They are spot on
  16. arfstrom

    arfstrom XS400 Guru Top Contributor

    Look up "float level, not equal" you will see the difference with good float, bad float

    Posted via Mobile
  17. jmd_forest

    jmd_forest Bldg 1980 XS400 Cafe Bike

    I have an 80 with electronic ignition also, and believe me, the ignition timing IS adjustable. After purchasing, the first thing I did clean and rebuild the carbs. After that the bike reasonably OK at idle but had no throttle response at all. I set the timing and it ran smooth and strong throughout the throttle range.
  18. HoughMade

    HoughMade XS400 Guru Top Contributor

    When you lift the pistons up by hand and let go, do they drop down immediately or do the slowly fall? They should slowly fall. The exact rate isn't important, but it should take a couple of second for them to come back down.

    If the fall immediately, there is an air leak above the diaphragm which is not allowing any vacuum to develop. You may have a timing related problem in some measure, but it look like classic "lean" to me. The needle isn't withdrawing out of the emulsion tube so you aren't drawing near enough fuel through the jet.

    It's going to run somewhat lean without air filters, but it should run OK. You need to get those pistons working. As you surmised, something isn't right.

    - You don't have idle air screws. These carbs control the idle mixture by varying the fuel, not the air. Yes, the O-rings are important.

    Also, with all due respect, you have no idea whether your carbs are clean until you see streams of carb cleaner spraying out every orifice in the carb. How do I know? Mine took 5 cleanings including 2 boils and I would have sworn it was perfectly clean every time. However, having sprayed carb cleaner through every hole in the carb and personally having seen the unrestricted spray of cleaner out the other end of every passage, now I know.....and the fact that my bike has run pretty near perfect for 2 years confirms.

    The picture below has the idle mixture on the bottom rather than top, but you get the idea.

  19. Hishman

    Hishman XS400 Addict

    Alright here is an update. The valves are good and so is timing. I disassembled the carbs and noticed the diaphragms where folded wrong and confirmed that now they work properly by using a vacuum cleaner to test. I just finished boiling the carbs and jets in vinegar for 30 min and then let them soak over night. How do i confirm the jet streams? I cant seam to get them to come out the three small holes by the butterfly. Im guessing something needs to be assembled (idle screw) in order to divert the carb cleaner to those three holes. If you know of an article that explains the process that would be awesome!

    Thanks again guys and gals
  20. You can clean out the holes with guitar strings or fishing line if that's what you mean by "confirm the jet stream".

Share This Page