Starting revs very high at random times

Discussion in 'Mech Tech' started by VDubvan, May 30, 2019.

  1. Hey, can anyone advise on this please?

    This happens probably every 10th time I start the bay up, the engine just roars for a while and then either sorts its self out, or needs turning off again and starting again. Instantly turning it off then starting it again doesn't always resolve the issue.

    I've a video of it doing it but I don't know how to post it.
     
  2. Sticking throttle cable or linkage
     
    Wonty and mikedjames like this.
  3. No amount of bashing the pedal works, could that still be the cable?
     
  4. Yes. What’s happening at the carb end? Could also be a sticking throttle plate.
     
  5. What can happen when the bushings wear on the throttle spindle is that the throttle plate starts wearing a groove in the carburettor body, which it catches on and refuses to go home.
    Also the little stepped cam on the choke mechanism can get sticky and refuse to drop under gravity (or the choke wire is loose/choke heater faulty) so it keeps on holding the engine with the throttle set at 2000 rpm even with a warm engine.
     
    Skater 8 likes this.
  6. A weak carb return spring, combined with a slightly sticky cable and a slightly stiff pedal can cause all sorts of malarky! That and the auto-choke being misaligned or sticky... Are you on standard Solex carb?

    Sent from my EML-L29 using Tapatalk
     
  7. ...or a strong return spring wearing a groove and catching? Like in the photo.[​IMG]

    Sent from my MotoG3 using Tapatalk
     
  8. Not the technical answer here, but I've found the problem, the arm thing on the left of the carb is coming too far around, so on the odd occasion it happens I jump around the back and flick it back up manually, any idea on a longer term solution than the old reach around?
     
  9. In the picture the spring should be hooked through the hole. The way it is, it will catch..
    .
    If the hole is worn out, attach a metal plate to cover the hole. and drill a small hole in the right place. i.e..JB weld on a bit of metal that hooks over the top..
     
    Skater 8 likes this.
  10. The arm is meant to move 90 degrees as you press on the pedal.

    When it sticks: if the throttle cable is slack the problem is in the carburettor mechanism. If it is tight then its the throttle cable or pedal mechanism.

    Adding bigger springs or upping tension on a stock carb is masking a developing wear problen.
     
  11. Do I need to adjust my spring to match the photo above? Screenshot_20190621-105656_WhatsApp.jpg
     
  12. Yes this means it wont catch on the outside, but the fact that it was moved means something is catching.

    This could be the throttle cable fraying and catching or the pedal mechanism sticking in the front - that would show as a taut throttle cable when it sticks.

    Or the bushings on the carburettor throttle spindle are worn. This causes an air leak - if you waggle the nut that secures the lever to the carburettor from side to side without rotating it, the hissing will change if there is a leak.
    The other thing that happens in this case is the brass oval butterfly inside catches on the throat of the carburettor eventually wearing a groove. This then acts as a shelf for the butterfly, locking the throttle open until you stamp on the pedal. Replacing the throttle spindle bushings stops some of the sideways movement that lets the butterfly hang up.
    If your carburettor is a Solex then you can buy bushes on eBay.

    If its a Brosol then the bushings on eBay are too small to fit in the holes around the spindle . Find a friend with a lathe.. ( but creative use of a pillar drill to control alignment and depth of hole can make a hole to fit off the shelf 6x8x10mm oilite bushes). Or using small amounts of JB weld to hold the Solex bushings in can work, but it can risk gluing the throttle spindle (use thin smear of grease on spindle )
     
    Last edited: Jun 22, 2019
  13. Yep, the spring usually goes through the hole :)

    Sent from my MotoG3 using Tapatalk
     
  14. Your spring looks fine. Is your throttle damper (the thing with the saucer and shaft pressing on the throttle linkage) actually doing anything? Sticking? If so, just take it off.
     
    Valveandy likes this.

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