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Bent exhaust valve

Discussion in 'Technical and Troubleshooting Torque' started by duncan_bayne, Sep 25, 2012.

  1. So, accelerating quickly from the lights today I missed a gear and over-revved Maggie (my '82 GS1100G shaftie). End result: she won't idle without a bit of throttle, and at ~ 4000RPM starts popping and misfiring, with the odd flash of blue flame from the exhaust.

    My psychic debugging skills lead me to suspect a bent exhaust valve, and possibly a corresponding dented piston :-(

    Anyway, I'm getting Mototecnic to come and pick her up in the van, strip her, and give me the bad news.

  2. Eh that sucks man! Did you clutchless shift?
    I did that on 250 once at slow speed fortunately. Apart from nearly pitching me over the bars i was quick enough to grab the clutch before the revs got too high.
    Good luck getting it sorted. Your deduction sounds about right to my somewhat uneducated brain.
  3. No - gentle experimentation at lower revs suggested that, unlike the Mito, she prefers a bit of clutch with her upshifts.
  4. Certainly sounds like a classic bent valve scenario. Whether there will be piston damage is dependent upon whether the valve(s) hit the piston or each other. Either is possible.

    The good news is that you'll probably get away with a new valve(s). It's unlikely that there would be enough of a gouge in the piston crown to cause problems. Just smooth off any sharp edges and you'll be OK.
  5. Here's hoping you're right - I need an expensive repair bill like I need a hole in the head right now.

    On a related note ... is it possible to fit rev limiters to older machinery?

    Edited to add: yup: http://www.dynoman.net/ignition/dynaRev.html
  6. When my '82 GT550 dropped a valve I parked it. Money pits, these old inline fours. You could get a late '90s Hornet 600 for $3k or less, that will go and go... Hmmm?
  7. maybe he likes REAL old bikes.
  8. Turns out it wasn't a valve :) The tank had some crap in it courtesy the previous owner and my over-revving shook it up. It then made its way through the faulty fuel filter into the carbs. Mototecnic just cleaned out the tank and carbs, fitted another fuel filter, and she's running well again.
  9. Well done.. I missed a gear on one of the gsx1100's i used to own.
    bent 14 valves, cracked 3 pistons, twisted the welded crank. broke the exhaust cam into 5 pieces, and broke the adjuster arms on 3 or 4 rockers... Funnily enough it didnt want to start at all after that..
    • Like Like x 1
  10. Thankfully it's often a simple issue after all.

    My Bullet has lost a lot of compression since a hard run last week. Here's hoping I'm in the same boat! :)
  11. Urgh, I'm cringing just thinking about that. What did you do, rebuild or new engine?
  12. rebuild, drag bike motor, on a budget after that so put some smaller cams I had back in it, sent the crank to QLD (from Darwin) to get repaired, put standard Katana barrels and pistons in it, replaced the valves and rockers, then a 9.99 first time back.. lost 4mph off the top end.
  13. The law of diminishing returns when tuning engines. It's often possible to go 90% as fast for 10% of the money :twisted:.
  14. You can only get the big cheap gains at the start, then its all cubic dollars.