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

Idle RPM climbs at stoplight.

Discussion in 'The Garage' started by Myron, Jul 19, 2022.

  1. Myron

    Myron XS400 Member

    I have a 1978 XS400E all original. The problem is that when the engine is hot, it will not idle down unless i use the brake. I can be cruising along and take my hand off the throttle and it will stay at speed until I brake. Then the engine will slow down. And often waiting at a stoplight it will idle fine for 6 seconds then RPM will climb right up, like 3000rpm. I hit the kill switch and start it again when the light changes.

    I have adjusted the idle mixture screw out, and now I am about 3 turns from seated. I think that is a half turn too far. It's starting to bog down from that. That doesn't effect the RPM climbing thing.

    I suspect it has something to do with the secondary jet and that vacuum diaphragm jet that runs it. I don't understand how it works. Could the spring be too weak? Is the diaphragm have a vent to atmosphere plugged?

  2. JPaganel

    JPaganel XS400 Addict

    Most likely causes:

    Sticky vacuum piston
    Hole in the diaphragm
    Air leak somewhere in the intake
    Stiffened and leaking throttle shaft seals

    I had this with shaft seals - when hot, the hardened rubber would change shape and how much air it let in.
    Myron likes this.
  3. Myron

    Myron XS400 Member

    All good points, Thanks.
    I did look through the carburetor and got everything moving correctly. I found the problem was the carb synchronization.
    First I put a vacuum gauge on the right carb and found it was bouncing around zero. When I put it on the left carb it was about 30.
    I didn't have a tool to compare the vacuum on each carb. So with an idea from this website I made one.
    This was the problem. One carb was rich and the other was lean.
    I drove it to work this morning and it is fixed. It is still a little lean, but that is an easy fix.

    IMG_20220723_074807_resized.jpg IMG_20220723_074449_resized.jpg
    JPaganel and CaptChrome like this.

Share This Page