1. Dismiss Notice
  2. Hey Facebook people... We've created a group for XS400.com members to connect. Check it out!
    Dismiss Notice
  3. Want an XS400.com t-shirt? Go to the store to order. Limited sizes and quantities available.
    Dismiss Notice

Bike idles fine but will not rev

Discussion in 'The Garage' started by moomaster, Sep 19, 2011.

  1. Hi everyone! I've just bought myself a 1980 XS400 Special for the princely sum of $400 off craigslist. Unsurprisingly enough, it has some issues.

    The most pressing one is that my bike will start and idle fine, but the throttle does nearly nothing. Anywhere from 1/8 of the way open to full throttle will make the bike rev to about 1800rpm, up from its idle of 1300rpm. I've had the carbs off the bike multiple times and they've been thoroughly cleaned. One of the carb diaphragms holds a vacuum, but the other does not appear to. Both cylinders are getting a clean spark.

    I don't know why it's acting like this. Initially neither diaphragm would hold a vacuum, but I had one extra that I switched in. That didn't make a difference. I figure that with at least one apparently fully working carb I would notice some sort of change, but there wasn't any.

    What could my problem be?
  2. bcware

    bcware Well-read Top Contributor

    Did you pull out the jets and clean the tiny pin holes in them? You may need to adjust your float height. Inspect your carb boots for cracks. Inspect the entire intake tract for leaks and cracks. Verify your fuel cap is venting.

    Did you visually inspect the diaphragms for cracks or holes?
  3. Ruined1

    Ruined1 82 Maxim xs400 Owner

    Inspect the throttle cable. Make sure it's actually doing anything on the side of the carbs. Sometimes it's the obvious things that stump me...
  4. Yeah, I've made sure every single jet and passage on the carbs are clean. I adjusted the float height to what it states in my manual. The intake boots looked to be free of cracks but I can take a closer look. The throttle cable is connected and working fine.

    I don't think it's something like an air leak because no matter what I'm doing with the throttle or what speed the engine is running at it runs smoothly
  5. bcware

    bcware Well-read Top Contributor

    Excellent. If you set the float height to the value in the manual it is very likely that you set it wrong. This is, of course, because the manual is wrong.

    Do you have brass or rubber floats?
  6. FlyingAnvil

    FlyingAnvil XS400 Addict

    No kidding, that is some crazy business. How so, can you explain?
  7. Ruined1

    Ruined1 82 Maxim xs400 Owner

    Could you please specify the appropriate float height for a stock machine? I'm going to be doing my carbs when my haynes manual comes in and if it's wrong in there too well... won't be doing myself much good =) :thumbsup:
  8. HoughMade

    HoughMade XS400 Guru Top Contributor

    26mm from the gasket mounting surface to the top of the brass floats (when upside down) seems to work well.

    However, it seems that the needles may be the issue here. The way the needles and diaphragms should work, with the carbs assembled, push the slide up, and it should descend slowly. If it can not be pushed up easily and falls smoothly, but somewhat slowly, there is a problem with the slides or diaphragms. What happens when you push the slides up?
  9. bcware

    bcware Well-read Top Contributor

    I have a '78 and a Hayne's manual; The Hayne's was wrong; it covers too many years. Don't trust the carb data they provide (jet sizes or measurements). I was beating my head against the wall for weeks trying to get my bike running properly and in the end it was an incorrect float height or float needle action.

    After dirty carbs these two things seem to be one of the most common problems. Like HoughMade said, start at 26mm for the brass floats.

    Also, look at the float needles once you're done setting the height. Hold the carbs right-side-up (as they sit on the bike) and move the floats open and closed with your hand. The needles should only be moving a few millimeters. Sometimes the tang that controls the pivoting action of the float needle gets bent and they are opening too much or too little. If they are opening to much you'll flood because they tend to get stuck. If they are opening too little you'll slow flow enough to starve the engine at higher rpms. I have photos of all of this if anyone needs it.
  10. Originally each needle would easily move up and down when I moved it by hand and if I covered the crescent shaped vent hole it wouldn't make a difference. I switched out one of the diaphragms and now that needle goes up and down easily, but if i lift it up and cover the vent hole it won't come down. I took that as a sign that the diaphragm could hold a vacuum.

    The bike's behavior was identical both before and after the diaphragm switch.

    Also the float height was at 26mm before I changed it and it didn't work then either.

    My next idea is to go start it and take off an air cleaner and push a slide up with a screwdriver while turning the throttle. Good plan?
  11. bcware

    bcware Well-read Top Contributor

    My slide needles fall down slowly. Without covering any vents if I lift them manually they take something like 1 second to completely close. Running the bike with no air filters will certainly let you see if they are even opening.

    Do you have the needles with notches cut out to adjust the needle height? If so what notch is the e-clip set at? Were there any other mods like headers, exhaust, or air intake modifications?
  12. HoughMade

    HoughMade XS400 Guru Top Contributor

    Agreed- they should fall slowly- no need to cover anything.

    I agree, take the filter off and see what is happening. It seems to me that you are not getting any more fuel (or air) as the butterflies open, which to me means the slides are not moving....assuming the butterflies are working well and the emulsion tubes are clean.
  13. lhaolpa

    lhaolpa XS400 Junkie

    I read a post on another site,maybe xs650,and he was explaining about the slides getting gunked up from deposits of gas.According to him,the slides and their bores can be improved by wet sanding using WD-40 and 400,800,and 1000 paper.The slides should fall quickly without the diaphragms sealed. Be easy on the sanding and clean well. lha
  14. Everything on the bike appears to be stock, and the diaphragms are moving as you guys described.
  15. bcware

    bcware Well-read Top Contributor

    I'm going with float needle, float height, or needle jet. The simplest explanation is that you're simply not getting enough fuel.

    My carbs had screens below the float needles; these became clogged frequently before I thoroughly de-rusted the tank.

    Is there an in-line fuel filter? Is it clean?

    Next, The float needle pivot tang (as I call it) controls how much the float needle opens and closes (see the photo). These tangs frequently become bent accidentally when adjusting the float height tang against a flat surface. The float needles can't be opening too much or too little.

    Lastly, are you measuring the float height correctly? I attached a photo, but ignore the height it is set at. For brass floats you're shooting for about 26mm. I believe the correct way to measure is from below the gasket, but since you shouldn't disturb the gasket unnecessarily you can subtract 2mm from your measurement (24mm).

    What I would do is keep getting more aggressive with the float height if this setting doesn't work; i.e. lower it when working upside down.

    Something else to check is gas cap venting and fuel petcock screen; either can get clogged and ruin your fuel flow.

    If your needle jets are adjustable check their setting. If they're too low they are not opening enough and thus starving you of fuel when you open the throttle.

    Attached Files:

    buztabuzt likes this.
  16. HoughMade

    HoughMade XS400 Guru Top Contributor

    Take the air filters off and while idling and opening the throttle, spray some starting fluid or WD-40 in there. If it revs up beyond what it did before, I'm afraid your carbs aren't as clean as you think or there is otherwise some sort of fuel restriction. If that still doesn't cause it to rev, check the timing and the only way to do that is with a strobe light on electronic ignition models.

    Do your carbs have plugs over the idle mixture screws? If so, time to remove them and adjust the screws to 3 turns out (from a very, very light seat). The first 15% of throttle is almost all the idle mixture circuit and it contributes up to about 35%. If that is not all clear and working correctly, you'll never get past 2000 or so rpm to get the main jet on line.

    from trial and much error, I have discovered that getting this circuit completely clean is difficult. I had my carbs apart and together at least 5 times before I got it clean enough to work right. Here is what I finally discovered to clean and verify that the circuit is clean:

    1. With the carb off and the idle mixture screw in 3 to 3.5 turns and the enrichener (choke) in, use aerosol carb cleaner (I like Berrymans...and it will burn on your skin and melt any plastic in the area) and with the straw on, spray cleaner through the air jet at the carb inlet. While doing this look through the engine side of the carb. You should be able to see s strong stream coming out where the idle mixture needle is. If you don't, keep spraying until you do.

    2. Once that is clear, turn the screw in and seat it lightly. Now spray the carb cleaner in the air jet again. It should not come out the needle hole anymore, but there are 3 tiny holes right above the throttle butterfly. You should see 3 distinct streams coming out the holes. Not trickle, not dripping- thre strong distinct streams. You may have to hold the butterfly open a bit to get a good view. If you can't, spray until you do.

    3. With that done, turn the carb over and if the float bowl is not off, remove it now. Remove the plug on the idle jet tube. It is a smaller tube built into the side of the main jet. Then, with a small flat screwdriver, remove the idle jet. Make sure it is clean. the hole through it as well as the holes on the side should be clean. Use carb cleaner if you have to, but do not stick anything harder than the brass it is made of in any of the holes.

    4. With the idle jet out, repeat the same procedure that you did spraying through the air jet, but this time spraying up from the bottom where you just took the idle jet out. With the screw in at 3 to 3.5 turns, you should see the strong stream coming out from the hole at the idle screw. With the screw lightly seated, again, you should see the 3 strong streams from the holes above the butterfly.

    5. Re-install the idle jet and plug. Set the idle screws to 3 to 3.5 turns- reassemble everything else.

    - if you do all this and actually see the streams of carb cleaner each time you should from each location, your idle circuit is guaranteed to be clean. If you do not actually see the streams where they should be when they should be, you can not be sure.

    Finally, as to the main jet, you can look through it with the slide out and see if it is clean, but unless you take the emulsion tube out, you will not know if the holes in the emulsion tube sides are clean. With a broad screwdriver, take the main jet our, the firmly, with a flat punch or something wide enough to span the center hole, push the emulsion tube up from the bottom into the carb throat- remove, inspect and clean if necessary. Heres the kicker- I boiled my carbs in vinegar water for about 2 hours- twice, but when I finally got around to taking the emulsion tubes out, the outside, the part you cannot see when they are installed, were caked with varnish.

    Good luck!
    buztabuzt likes this.
  17. bcware

    bcware Well-read Top Contributor

    Haha, our posts should keep him busy for a while :p
  18. HoughMade

    HoughMade XS400 Guru Top Contributor

    Indeed! But, oh how that bike will run when he's done!!!
  19. Okay, I set the float heights to 26mm and its now dripping gas onto the ground. The float bowls are tightened on, it's coming out of the throat of the carburetor. I'm guessing that the float is too low now. Once it's stopped doing that I'll spray out everywhere you said and see what happens.

Share This Page