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engine shudder and high revs

Discussion in 'Technical and Troubleshooting Torque' started by Devery, Feb 10, 2010.

  1. My Yamaha sr250 has developed this problem, I'm guessing it is either fuel related or electrical.

    In the high rev range the bike seems to miss a beat, its best describe as a small shudder. If I stay in this rev range it can be constant or it can be intermittent. Not too dramatic, but definitely noticeable.

    The two ideas I have;

    - Bad or blocked fuel path.
    - Some electrical issue, a bad contact somewhere.

    What do you think? Any suggestions?

  2. check the kill switch for corrosion (longshot)
  3. I do need to check my switches.... but;

    Just checked the spark plug. Looks like its running lean. What adjustment can I do to the carb to get it running richer without rejetting?

    If it is running lean, could this "shudder" be a symptom?
  4. If a lean engine were to shudder i would imagine it would be right before it seized or threw a piston out the side of the bore.

    Sparkies might be rooted if it has indeed been running lean, that will give you a shudder as the plugs start to fail under load.

    Cant help with the adjustments on the carbie though, i have no clue...
  5. You sure you're not hitting the rev limiter? Feels horrible but is perfectly normal and easily cured by shifting up:wink:.

    Modern plugs with modern fuels are always a bit white and aren't really a reliable indicator of mixture strength any more. If you are lean, the most likely candidates are a pod air filter, a hollow exhaust, a partially blocked jet, a partially blocked fuel tap/line or an air leak somewhere. If the fuel tap is the vacuum variety, a leak in the vacuum line (very common) gives you the worst of all worlds by both being an air leak and restricting fuel flow.
  6. pat.. someone has told me that they had a similar problem and it was the vacuum line that was faulty and staving the carb. do you have any suggestions for repairing this or should i replace the tap?

    the bike runs better on prime, but still a little glitchy. might be another small problem down the line. i'll look into it.

    thank you.

  7. Spark plugs break down under load when they are shot, which would give you an intermittent shudder at peak revs, Only a suggestion,
  8. The vac line is just a small bore rubber line from a nipple on the carb to a nipple on the tap. If it's original it probably looks like a Pharoah's scrotum by now. Take it down to Clarke Rubber or Repco or SuperCrap and see if they can do a replacement length.

    Vacuum taps themselves are fairly robust and don't go wrong much. However, if the vac line replacement doesn't help it would be worth enquiring with Yamaha about a rebuild kit.

    And Brian's suggetion about plug replacement is worthwhile. Plugs are cheap, and no matter how new the existing one is, it's worth tring a substitute. I've had a number of NGKs that were dead straight out of the box.
  9. Thanks for the suggestions. I've been a bit busy so I haven't been able to do a proper update with how its going. I'll take this chance to try and best describe whats going on and hopefully I can fix it!

    Okay.... It got worse. I was able to ride around with the fuel tap on prime and that seemed to help but then the same issue became apparent. I took the carb off the bike and had a look at the float bowl and it was clean and I couldn't get the top part open as the bolts were too tight. I opened up the air box and the air filter foam was a bit shabby so I've ordered a new one. Checked the rubber mounts of the carb for cracks or wear, no signs. So in this process the float bowl was emptied... I bolt it back on, making sure the connection is good and tight. Then it feels fine, no problems at all. The problem does come back though, and it gets worse and worse over time. This confuses me, but hopefully it will give someone a good idea where to go from here.

    I guess my next move is to have a look at the jets for blockage. It would seem to me that the fact that pulling off the carb and bolting it back on has isolated it to being a carb issue, but i dont know where to start.

    Any help would be great.

    Thank you,
  10. Check the vent hole in your fuel filler cap. A blockage here has been the cause of many a mystery fuel starvation symptom.
  11. As in the fuel tank cap? I can't see any blockages there.
  12. Oh well, only a thought. If the breather hole in the tank cap is blocked, you get a partial vacuum in the tank as fuel is drawn off, which restricts the flow to the carb. Traditionally, a first check for a fuel starvation problem involves opening the cap to see if there's fuel in the tank. This releases the vacuum and, hey presto, the bike runs normally again for another few kms, only to die again a short while later. Rinse and repeat until you give the bloody bike away in sheer frustration. Yes, I have seen it happen.

    However, if using the PRI tap position helped, I would regard the carb to tap vacuum hose as the primary suspect, with the tap itself a close runner up.
  13. Pat, that is damn clever.

    I'll take it out for a spin, when it happens I'll pull over and open and close the fuel cap and see if I notice a difference.

    It did eventually happen when the tap was turned to PRI, so this would support your theory. I'll let you know how I go.

    Thanks again.
  14. I had a look at the fuel cap...


    A small bearing fell out of one of the holes. Can't remember which one. Two holes. top one, not sure what it does, bottom one seems to have a small hole but Im not sure where it goes?

    What do I need to clean? what should I be looking for? Where does the bearing belong?

    I did notice that I can push the cap down a fair bit more once it is locked in place.

  15. Hmmm. Picture doesn't work for me. Probably something to do with being on my work system.

    Most filler caps have a degree of spring loading to keep the rubber seal in the cap tight against the rim of the filler hole so the springy extra movement is normal. Can't help with the bearing location I'm afraid, as I'm not familiar with Yamaha caps.

    What you're looking for is a clear path that vents the interior of the tank to atmosphere without restrictions. It is most likely one of the holes in the filler cap. Have a look at the rubber seal on the cap and visualise what area of the of the cap will connect to the interior of the tank without that seal being between the two, if you see what I mean. Then see if you can find a hole or drilling that passes from that area to the outside world without anything in the way. Vent holes are generally quite small and so are prone to clogging with rust, dust and general crap.

    There is a slim possibility that your tank is internally vented, via a tube somewhere in the filler neck that then passes through the tank to a drain or hose elsewhere, but I suspectthat SR250s aren't that sophisticated. Still, if you can find a hose or fitting anywhere on the tank that is not obviously to get petrol out or in, make sure that it's clear.
  16. I've got some doubts about the fuel tank vacuum theory. That typically leads to intermittent spluttering as the fuel flow starts to fail, rather than shudder.
    Shudder could be anything from restricted fuel intake, to fuel pump, to partial ignition failure or timing issues, to clutch weakness.
    Still, a bearing shouldn't fall out....
  17. Probably a better way to describe it actually.... i'll have another look now.
  18. I'm not saying it is the problem, but it's as easy to check as it is to overlook and has, historically, been the cause of many a mysterious, intermittent fault.

    My money's still on the tap vacuum hose.
  19. is this the hose from the inlet manifold to the fuel tap?
  20. Yep. At least, that's what I'd expect.