SL70 engine issue

Discussion in 'Tech Area' started by Old Guy Too Many Bikes, May 11, 2021.

  1. Old Guy Too Many Bikes

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    Hello all, I aquired an SL70 engine not long ago and just got it running on the bench. It idles ok, but just won't take any throttle. if I turn up the idle screw, it starts to increase rpm slightly, then starts to die. Carb is new, valves adjusted, points and timing good. I had this issue with a CT90 a while back and it turned out the issue was a bad condenser. But is seems like the SL70 condenser is not screwed on, but soldered in. Can it be tested without removing it?
     
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  3. CLEC66

    CLEC66 Member

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    You running it with an air filter? Had a z50 motor that wouldn't run on the bench without covering have the carb entry.
     
  4. Old Guy Too Many Bikes

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    I think that's a mixture issue. An engine running too lean would run better with a air filter if it offered some resistance and richened the mixture. Kinda like having an engine run better when you close the choke. This has no effect, it's like a weak spark, that can only provide so much. I had an ST90 that when you turned on the headlight, it sputtered, unless it was idling, but that was a non-magneto engine.
     
  5. lukelaw1

    lukelaw1 Active Member

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    have you tried lightly sanding and cleaning the points. also make sure the inner part of the magneto is clean.
     
  6. Old Guy Too Many Bikes

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    Yes, I cleaned and filed the points, set the point gap and timing, cleaned plug, advance moves freely, runs, but won't take any throttle. I tried raising and lowering the needle, no bueno. I have not removed the stator coils, they look pretty gunked up.
     
  7. JHminitrails

    JHminitrails Member

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    Have you verified your cam timing is correct? If it is one tooth off one way or the other that will cause you some grief. Something easy to check and eliminate
     
  8. lukelaw1

    lukelaw1 Active Member

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    This is a far stretch, but I have seen this once personally, also on a sl70 carb. Inside the carb body, one of the brass plugs that is used to block off a cross drill port was missing.
     
  9. Old Guy Too Many Bikes

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    Thanks for all the advice, I'll check the cam chain. One question for you experts out there. I know all the 90's need a battery to run well, but can a magneto engine like the SL70 run well without a battery? I also have a CT200 which I fired up on the bench with no issues. For it I used a 6 volt coil I had laying around and for power I used a 6 volt 0.5 amp power supply. It fired right up, black wire from coil to + on power supply, neg grounded on engine case. Green wire from coil to points on engine. Wired the SL70 engine the same way, but it will only idle. . I know that on the 90, as long as the coil is getting 6 volt power, it should run, but on the 70, might the stator leads need to go through the rectifier to provide power to the coil? It seems to me it should either run or not run, unless the spark is weak for some reason?
     
  10. OLD CT

    OLD CT Well-Known Member

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    The coil needs to be ''grounded well'' to provide a strong spark, not hanging in the wind. How does the spark look? On a ungrounded coil, the spark will be weak. A ct70 can run without a good battery/rectifier. It just needs the battery harness black jumper loop for the main harness with a dead battery/blown rectifier. Not sure about SL's.
     
    #9 OLD CT, May 14, 2021
    Last edited: May 14, 2021
  11. CLEC66

    CLEC66 Member

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    All 3 of my SL70's run great without a battery.
     
  12. CLEC66

    CLEC66 Member

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    Just to clarify I obviously need the black jumper wire at the battery plug in to run.
     
  13. OLD CT

    OLD CT Well-Known Member

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    I would also ''break down'' and buy new points. Sanding and cleaning wont help, if the rubbing block is too worn out.
     
  14. Old Guy Too Many Bikes

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    When I pull the plug and turn the engine over, there is a spark, but not real strong. Here's something I noticed. When I had the point out to clean, they worked fine, insulator good, continuity stops when points are open. However, when I put the points back in, adjusted the gap, set timing, when I check for continuity from the black points wire to ground, as I turn the engine over, expected continuity with the points closed which I got, but with the points open, I don't get an open circuit, ie, infinite resistance, resistance goes up, but not to an open circuit. Does this point to a bad condenser as I originally thought, or are the stator coils also to blame?

    Also, when I wire it up to run, I don't mess with the two other leads coming from the engine (the AC leads from the stator) since I figured the 6 volt power supply I wired to the coil would provide the spark.

    I remember back when my CT70 was stock, it ran fine with a flat battery, although my ST90 (non-magneto ignition) had issues.
     

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