1987 YZ 125 Fresh top end, will not start
  • hboe7033
    Posts: 7
    I was riding for a couple hours, then my friend rode, I got the bike back and it wouldn't start.  Wasn't fouled plug, threw in a fresh one.  Then I checked compression, being low I rebuilt the top end.  After the rebuild, when push started, the bike ran like it was bogging down and sounded horrible and sort of clanky.  Then the next day I tried and it only will run for maybe 10 feet then die, back fire, run for 10 feet, and repeat while push started.  So far I have tried multiple plugs, checked spark (fat and blue), attempted without air filter, removed and cleaned carb 3 times, checked reeds (look great), cleaned out exhaust, adjusted carb needle position, and adjusted timing.  When I pull the plug after trying to start, it is lightly wet with fuel.  I run about 40:1 mix, and its fresh gas.  Based on how it acts I think its fuel, but I'm out of ideas, what am I missing?
  • markyrocks
    Posts: 1,292
    What is compression now? Why does it need push started? I think what ur missing is some kinda problem with the new top end. Id definitely open her up an look to make sure circlip didn't come off or something similar.
  • hboe7033
    Posts: 7
    I haven't checked the new compression yet, but it's notably higher by feel.  I have been push starting it to save my legs from a million kick starts.  Considering it ran on less compression with the old top end, I can say compression isn't the issue.  In the interest of saving gaskets, would you suggest removing just the head or cylinder too? 
  • daggitdaggit
    Posts: 9,487
    Have you checked for spark or just put in a new plug?
    Is the filter clean?
    Is the piston in backwards?
    Is the vent line on your gas cap clear?
    Never Too Old
  • hboe7033
    Posts: 7
    Yes I've checked for spark by laying plug on the head, filter is freshly cleaned but I have it off now to eliminate that variable, piston is in correct direction, I will double check though. How does the gas cap vent relate?
  • markyrocks
    Posts: 1,292
    Gas cap vent clog makes vacuum in gas can I.e gassed wont freely flow into Carb. My only question is did bike run ok before top end swap? BC if so only thing that changed was the rebuild.
  • hboe7033
    Posts: 7
    Before the top end, the bike would run fine, then you'd shut it off and it wouldn't start, only backfire, then after letting it sit for a while, it would start again just fine.  That makes me think its more or less flooding itself when running, but not enough to kill it, but when you shut it off it becomes flooded and letting it sit clears that up?
  • markyrocks
    Posts: 1,292
    That sounds like its not getting fuel fast enough you may have clogged fuel line or petcock. Try disconnecting line at the Carb and blowing compressed air up into the tank.(petcock on) If it fixes it then prolly not bad idea to thoroughly clean out tank an petcock.
  • hboe7033
    Posts: 7
    When I try to start it then pull the carb off, there is gas sitting above the main jet, I just noticed this.  I know its getting plenty in the float bowl and my petcock is clear, does that mean the float level needs to be adjusted? 
  • markyrocks
    Posts: 1,292
    Try running till it dies then immediately shut fuel off an then look to see how much is in the float bowl.
  • hboe7033
    Posts: 7
    It won't even idle or start for more than a second, so if I attempted to start, then pull carb, the float bowl is full and its very wet in the carb and just before the reeds.
  • markyrocks
    Posts: 1,292
    Ok well your saying before it would run for a lil while shut off an not start. Sit then start. Now it will only run for a second and die. If you think its fuel issue then lean out the jets an see what happens. One question. Have you taking spark plug out and spun the engine to see if it turning freely?
  • hboe7033
    Posts: 7
    When I lean it out it doesn't fire nearly as much, but also backfires nearly never.  When I richen it, it feels and sounds better, but also backfires a lot more, but at most on any setting it will only run for about 2 seconds with no revving. With spark plug removed it spins freely no problem.
  • markyrocks
    Posts: 1,292
    I don't know what to tell u. Sounds like u got air gas spark (I assume compression) . Only other thing it could be is a timing issue. Can you maybe post a video of what its doing? Have you checked all electrical connections? You said u cleaned exhaust. Did that include header and midpipe? When u rebuilt top end did u have cylinder inspected by a machine shop? Did u check it for warpage? Did u check crank bearings for play,?
  • NZriderNZrider
    Posts: 5,090
    sounds like a weak spark issue to me
    *********************
    Winning is 90% rider and only 5% bike, the rest is luck
  • daggitdaggit
    Posts: 9,487
    Open the plug on the bottom of the carb, turn on the gas, is it getting a good flow out of the bottom of thecarb?  If not open the gas cap, see if it changes.  
    Check and reset the floats. 
    make sure you didn't pinch any wires when you reinstalled the cylinder.
    Never Too Old
  • daggitdaggit
    Posts: 9,487
    If it starts after setting for awhile make sure all of the vent line on the carb are clear.
    Never Too Old
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