Help please! 1970 st70 not starting, has spark, fuel and compression.

Discussion in 'General' started by BennyB, Feb 14, 2019.

  1. BennyB

    BennyB New Member

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    Hi from Australia all,

    Hoping to get some assistance figuring out why my st70 has stopped working. I’ve had it running beautifully for weeks and suddenly it won’t start again! It has healthy fuel, spark and compression but just isn’t firing at all. So I am wondering, could this be an electrical problem? All indicators, horn and brake light are working when ignition is in the on position, but am I missing something?

    Open to all suggestions, thanks again!
     

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  3. racerx

    racerx Administrator
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    Welcome aboard!

    Remove the plug, instill a few drops of fuel directly into the cylinder, replace the plug. If it fires &runs for a few seconds you've got a carb problem. If it won't fire at all, you may have a sheared flywheel key. If the flywheel has rotated on the crank, the timing could be out far enough to result in a no-start condition.

    Keep us posted, either way. You can always ask more questions, if need be.
     
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  4. BennyB

    BennyB New Member

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    Thanks for the reply racerx, yes I have already tried putting some starting fluid into the cylinder and it won’t fire at all.

    Would it be a matter of getting a timing light onto it to determine if the flywheel has sheared? And could this still be the problem if I can see a healthy spark throughout the range of kickstart?

    Thanks

     
  5. cjpayne

    cjpayne Well-Known Member

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    Sometimes a plug can get gas soaked and wont fire right. Have you tried a different plug?
    X2 on what racerx suggested. I would check the timing marks on the cam and flywheel and see if they're still lined up.
     
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  6. racerx

    racerx Administrator
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    An engine needs compression, fuel, and spark to run...and you believe that all three are present. Yet, it won't even fire. So, the next steps are figuring out if it really does have adequate compression, is the spark healthy and is the timing correct. Cjpayne has it right. You'll have to remove the flywheel cover, regardless. Removing the round cam cover is only one more bolt and less work than pulling the flywheel. If the timing marks both line-up as they should, then you will have saved yourself an extra step, or three. If not, you've likely found the problem.

    I'd check the point gap, while the cover is off. If it's too narrow, you'll get a weak spark and the timing will be retarded. Next question...how's the spark quality? It should be blue-white; if not, something's not up to par...could be a fouled plug (they can look okay and still be fouled enough to effectively kill the ignition), weak/dying condenser or even fouled point contacts. These old breaker point ignitions can generate spark quality that would do justice to contemporary CDI...when everything is working properly. The problem is that they degrade over time, requiring maintenance.

    As for the compression question, a healthy top end will typically show 150psi. 130psi is weak, much below ~110psi and starting may be extremely difficult, if not impossible. Not everyone has a compression gauge. Try the shadetree mechanic's compression test. Turn the kickstarter by hand, slowly. If compression is anywhere near normal, you should feel sharp resistance on the compression stroke. OTOH, if you're even a little bit unsure as to when the compression stroke has been reached, compression is weak.
     
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  7. Gary

    Gary Well-Known Member

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    As a teen I had a Triumph that would foul out plugs- you'd pull it and it would spark fine put it under compression and it would not. So do as CJ advises. If that does not work and it seems your compression is low check your valve clearances. My 50 had so many miles on it that it would not start by the kick starter any more,had to be push started. I live on a big hill so I'd put it in third kept the gear lever down until it got moving at a good clip let the lever up.It would then start every time. As long as it was warmed up it would then kick start. Top end rebuild cured it,the bad sealing valves were the cause
     
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  8. BennyB

    BennyB New Member

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    Thanks all for the troubleshooting, I ended up getting it running by adjusting the point gap on it today. It started first kick after adjusting but I think it could be fine tuned as it’s still running a bit rough. It’s got good compression on it so that’s not a problem.

    Is it necessary to get a timing light on it to fine tune the ignition timing?

    Cheers!
     
  9. darrel gunderson

    darrel gunderson Active Member

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    A car timing light is the only thing I use. They are not that expensive.
     
  10. cjpayne

    cjpayne Well-Known Member

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    I just set them to .016. I have a narrowed feeler gauge to make access easier.
     
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  11. kirrbby

    kirrbby Well-Known Member

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    If you don't have one yet, consider buying a flywheel puller. I would buy a set of points, and a condenser at the same time. Add a spark plug and you're set up...ready for a complete minibike tune-up.
     
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  12. cjpayne

    cjpayne Well-Known Member

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    X2. Oh yeah, your going to need one of those, clutch nut tool, and a handheld impact driver with the JIS bits.
     
  13. BennyB

    BennyB New Member

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    Thanks all, will organise my tools to be ready for next time. Cheers!
     
  14. racerx

    racerx Administrator
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    Cool biz!(y)

    I suggest monitoring the point gap until the miles start to accumulate and you see that they're not closing-up rapidly. The rubbing block will wear down, eventually and that's normal. With new points, the gap will usually shrink initially...again, normal. What you want to know is if the gap is going away abnormally fast; that would indicate either rust on the point cam or a problem with the phenolic plastic rubbing block.
     
  15. DaBulldozer

    DaBulldozer New Member

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    Have you checked if the exhaust is plugged?
     
  16. Chrisqa

    Chrisqa New Member

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    I AM RESTORING A 1981 Z50R. IT HAS A NEW 14MM CARBURETOR BUT THE INTAKE TUBE IT CAME WITH IS TOO BIG AND WONT BOLT UP TO THE CARB FLANGE. LUCKLY I HAD AN OLD INTAKE TUBE THAT FITS AND NOW IT RUNS GREAT BUT I AM GOING TO HAVE TO CLEAN THE PLUG AGAIN. I FOUND OUT THOSE LITTLE BITTY NGK PLUGS GAS FOUL VERY EASILY IF I LET IT RUN TOO LONG WITH A TOO RICH OF A MIXTURE. WHILE I WAS TRYING TO ADJUST THE IDLE MIXTURE I WAS TRYING TO FIGURE OUT WHICH WAY TO TURN IT AND IT RAN ABOUT 20 SECONDS TOO RICH AND FOULED THEN IT STARTED TO BOG SPIT AND SPUTTER. I PUT ANOTHER PLUG IN AND FOUND OUT TO SET THE IDLE MIXTURE SCREW TO LEAN BY TURNING IT OUT THEN START THE ENGINE AND SLOWLY ADJUST IN. NOW IT RUNS LIKE A TOP. IM USED TO BIG LAWNMOWER ENGINES THEY ARE BIG ENOUGH TO BURN OFF THE CARBON AND SELF CLEAN THE PLUGS AND ARE A LOT MORE FORGIVING. THOSE LITTLE HONDA ENGINES ARE CRITICAL LITTLE HIGH PERFORMANCE DEVELS.
     
  17. Anthony Campano

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    Hi check your 10amp fuse.
     
  18. Anthony Campano

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    Check your 10amp fuse. Must be electrical.
     
  19. Anthony Campano

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    Put a magnum spark plug in they burn off everything worked out for me.
     
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  20. Chrisqa

    Chrisqa New Member

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    Thanks Anthony I will try that next time it needs a plug.
     
  21. Chrisqa

    Chrisqa New Member

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    Old points will fight you for hours until you replace them with new ones. I try to save as many usable parts as I can by restoring them. I finally won the battle and got em clean enough with fine sand paper and 91 percent alcohol and now a nice big blue spark even with a slow turnover. Ugh. Gary is right about plugs they can make you believe they're fine but no go. I had champion plugs for a car that did that to me before. Sometimes you can start a stubborn engine by taking the plug out and putting it in a good running engine and get it hot then immediately put it back in the stubborn engine. If you're careful you can torch em red hot to get the carbon off the insulator tip. Do not use an oxy/ acetelene torch. Use a small propane torch or put it on a small pile of red hot charcoal. Careful not to burn yourself or ignite fumes. I don't want to read about you in the paper. Some of the things I try are quite dangerous and so far I have all my limbs and both hands and eyes.
     
    #20 Chrisqa, Aug 12, 2019 at 11:13 PM
    Last edited: Aug 12, 2019 at 11:33 PM

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