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No Start (not even a cough) 1980 XS400 special

Discussion in 'The Garage' started by Chad Taylor, Jan 27, 2019.

  1. Chad Taylor

    Chad Taylor XS400 New Member

    Hi Everyone,

    I'm new to this site (actually to the whole "forum" thing entirely!) but I've been having trouble with my bike and would gladly take all the help I can get!

    So a little history: In a previous buy I got two yamaha xs400's (1977 and 1978 I think). The 77 had a title so I used that for a Cafe Racer build and robbed the carbs off the 78. Now I'm getting into a project where I'll need a power plant and that 78 engine was looking like the best candidate. But before I buy new carbs I want to know if the engine even works.

    I cleaned it off and I just need to know if it'll run at all, I'm just looking for any sign of life in this engine. So the name of the game is to wire up a battery directly to the coils (points ignition bike), spray carb cleaner (only thing on hand, no specific starter fluid in arms reach) into the intake right up against the valves and kick it over. If I hear just a small cough, sputter, single ignition then I'm happy and know the engine can run. This is the same approach I had on the 77 bike and it worked well enough then, so I figured it should be fine now.

    Of course it didn't work first kick, so I've run through and checked these things:
    -points gap (right distance, but I'm seeing small sparking across the points is that okay?)
    -timing (the mechanical advance is stuck in the no-advance position, but should be fine for just a kick over test. I wont be idling the bike. Also set the timing for each cylinder)
    -spark (I'm getting spark on both plugs and they're consistent)
    -compression (~115 and 125 psi on the cylinders)
    -battery I'm using is at 12.40 volts so it should be okay
    -valve lash is good on intake and exhaust

    So it seems like I have everything I need, spark, compression, fuel, timing but not even one smallest puff of combustion yet! Is there something I'm missing?
  2. xschris

    xschris a lifestyle not a trend Top Contributor

    The compression is very low. These have 150+ when good. Bottom is 125. You could try to swap the coils as they might be to the wrong side. But with compression that low it might not be even worth messing with. Post some pics of what you have. We like pics.
  3. I read about the teaspoon of ATF in reach plug socket as a hopeful way to boost compression . Believe it was to restore the rings and this seal better. Search forum for ATF compression and you'll find some discussion and hopefully gets you high enough to turn her over!
  4. xschris

    xschris a lifestyle not a trend Top Contributor

    This is a test for bad rings/pistons. After the oil is gone the bad compression will return. If you have stuck rings ATF or fogging oil could help to free them if you let it soak and use more than a teaspoon.
  5. Whoops... Thanks for the heads up. Guess that means with compression below 125...
    • double check valve adjustment
    • Check head gasket seal/ re-torque
    • or else a more thorough top-end job... rings, walls, pistons... (/ perhaps just buying new motor off parts bike)?

    Here's to hoping the 145 psi compression on my 'new' 400g will suffice. I performed with a cold test, not with a warm motor, and using kick starter so perhaps proper reading would've been higher.
  6. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Did you check compression with the carbs on or off? You need to have the carbs wide open to get a more accurate reading.

    I did my readings once and forgot to open the throttle, compression was very low, like 100lbs. When I opened it, I was over 160lbs.

    Even if the numbers are low, as long as both readings are pretty close, you should be alright. The lower the compression, the less power you will have and eventually the engine will be too hard to start and impossible to idle.

    I know it sounds counter-productive here, but I would try to get the motor to run first, then do a compression test. Theres nothing to these engines and they should pretty much always run or kick over.. If the engine has been sitting, theres probably no oil on the rings and its just not properly prepped to be a running engine. 120lbs should be enough to get it to kick over.

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