I inherited a '79 Yamaha XS400 (2 cylinder, 4 stroke, air cooled, SOHC, duel carburetors).

The timing chain had broken so I replaced it, setting the timing at the same time.

I cannot start it. The spark plugs are sparking and they get wet from fuel, but the engine does not fire. We managed to get a couple of little puffs but nothing else.

We removed the air filters and sprayed engine starter into the carburettors and this resulted in small flames shooting back out of the air intakes.

We also tried advancing and retarding the ignition timing.

Would anyone know what I could do? Could I have set the timing wrong?

  • 1
    I would double check the valve timing. Also, have you checked the carburetor floats and needle valves, etc? If you are getting wet plugs, it may be too rich and is flooding. In cases where carbs have sat for long periods of time with fuel in them, they get gummed up causing parts and pathways to get gummed up.
    – CharlieRB
    Feb 18, 2018 at 19:04
  • I would make sure everything was lined up properly when you replaced the timing chain. Did you adjust the valve tappet clearances?
    – Mysterfxit
    Feb 20, 2018 at 11:59
  • Hi @Mysterfxit no I didn't do the valve tappets. Hmm looks like I might have to check the timing again...
    – MeltingDog
    Feb 20, 2018 at 22:36
  • @MeltingDog if you didn't adjust the valves when you put it back together that's probably your issue. I would do that first.
    – Mysterfxit
    Feb 20, 2018 at 22:43

2 Answers 2


With a no-run scenario, think FASTEC:

  • Fuel

    You've confirmed delivery, but the resultant mixture could be too rich or too lean for the engine to run

  • Air

    Ditto on the reason: air-fuel ratios matter. Broken/disconnected hoses, sticky throttle bodies, the engine drawing in air from unexpected locations...

  • Spark

    Confirmed, shouldn't be an issue here.

  • Timing

    Is it firing at the right time? Are the valves opening and closing at the right time?

  • Exhaust

    There needs to be a path for the exhaust gases to flow out of the engine.

  • Compression

    Lack of compression can prevent the engine from running.

  • Your F and A are the same thing rich / lean is air-fuel ratio....
    – Solar Mike
    Feb 18, 2018 at 19:25
  • @SolarMike You're right that both contribute to AFR. The distinction between the two in this context pertains to where the issue might lie (fuel delivery, vacuum leak, etc.). I hope the edit clarifies this.
    – Zaid
    Feb 19, 2018 at 5:45
  • You could add clogged or damaged filters to both ...
    – Solar Mike
    Feb 19, 2018 at 6:09
  • I should also add that both air and fuel filters are fine. I am doing a mini-restoration on this and so have replaced many perishable parts.
    – MeltingDog
    Feb 19, 2018 at 22:57

So, first thought is did you change the fuel for fresh? If not, do so as old fuel is about as good as water...

Then, since you mention you have been advancing / retarding the timing, you should start from scratch and re-time it correctly and test again.

  • Yes, I should have mentioned, fresh fuel.
    – MeltingDog
    Feb 18, 2018 at 9:13
  • Old/stored/vented fuel(gasoline) is still flammable but not suitable for motorcycle engines. It only lost volatile molecules, which are much needed for flame propagation. Just add some premium fuel to stored/vented fuel before use. Mar 21, 2018 at 11:07
  • @SparKot so you can mix it in the tank easily enough but how do you get it to the carb : you need to drain carb and pipes...
    – Solar Mike
    Mar 21, 2018 at 11:09
  • @SolarMike Yes, good carburettors come with drain-screw for the float-bowl. Mar 21, 2018 at 13:45
  • And the cheap ones you just take the bowl off...
    – Solar Mike
    Mar 21, 2018 at 14:41

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