Hot Keihan pipes

Discussion in 'Airheads' started by mellor, Feb 25, 2012.

  1. mellor

    mellor learner

    Joined:
    Apr 11, 2010
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    London
    Hi everyone! I'm a long-time Old's Cool lurker and these forums have been a great source of advice and inspiration, but now its time to me ask for some advice :-)

    I just completed a top end decoke and rebuild on my '89 R100 GS. While I doing this I noticed that the exhaust collector box was in a bad way and decided it was time to replace that, after reading the forums it seemed that Keihan was the way to go. A couple of week later, a shiny new Keihan y-pipe arrived from Motorworks (UK) and on it went.

    My immediate observations were that the exhaust note has changed; slightly louder with a more prominent 'pop' on each cylinder firing while idle, the tone brighter and with a metallic resonance to it. Performance-wise, the throttle seems more responsive; acceleration is much more exciting and, it feels like the bike decelerates faster too with the throttle closed. Although this could be due to the cleaned heads and pistons, I'm sure the pipe is contributing. So, all good there! Very happy with the results.

    However, after my first run of around 15 twisty miles on a cool evening, I noticed different sounds to before as the bike cooled, instead of just the clicks of the pipes contacting there was a new squeaky plastic noise! I quickly discovered it was coming from the seat cover on the subframe, and the plastic exhaust fairing. The final muffler was much hotter than usual and was heating the seat and plastics around it as a result, even making the paint around the too-hot-to-touch panel mounting bolt soft, and separate from the plastic underneath...

    Has anyone else had noticed the exhaust running hotter after fitting a Keihan Y-pipe? OR is it possible that everything is running hotter as a result of the top-end rebuild? if the Y is not removing as much heat from the gasses as the stock system - is there a risk of it damaging my seat and plastics? Any advice appreciated!

    Jim
    #1
  2. disston

    disston ShadeTreeExpert

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    All that work sounds good but you don't mention if you regulated the carbs after top end work. And even with just a new pipe retuning the carbs would be called for. Lean running can cause hot temps.
    #2
  3. Velocipede

    Velocipede Been here awhile

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    Oddometer:
    507
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    Kingston Ontario Canada
    Exhaust valve clearances maybe a little tight?

    John
    #3
  4. Wirespokes

    Wirespokes Beemerholics Anonymous

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    That doesn't sound good to me. And I don't think it has anything to do with the Y pipe. First thing I'd check is the carb settings. Probably running way rich. It's my understanding a rich mixture turns the headers blue and makes for a hot exhaust. All that unspent fuel is still burning on its way out.
    #4
  5. Box'a'bits

    Box'a'bits In need of repair

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    Rob Farmer has also noted that the Y-pipes give the mufflers a hard time & that these tend to fail earlier than the pipes connected thru' collector boxes.
    #5
  6. Airhead Wrangler

    Airhead Wrangler Long timer

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    Also, air leaks when the system is installed can allow more air into help any unburnt mixture to combust. Check all the joints and make sure they're sealed well.
    #6
  7. mellor

    mellor learner

    Joined:
    Apr 11, 2010
    Oddometer:
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    London
    Thanks for the help everyone. There was indeed an air leak the system, one of the 'captive' nuts had wiggled free from its spot weld and is now lost forever inside the rear muffler :eek1 a crappy patch up repair with some exhaust bandage was letting in some air and reducing the back pressure even more.

    Still, with that properly plugged (still an ugly repair! does anyone have a suggestion how I could fix this properly? are blind rivet nuts a good idea?) it still ran hotter than usual.

    Clearances are all fine, so the carb tuning is my prime suspect. I concur with Disston is the whole systems is running lean and as the Bing manual it suggests that a less restrictive exhausts the engine 'breathes' better (I appreciate this is a ridiculously basic description of a complex system! but the theory is sound). It does't seem to be suffering from pre-ignition, but reading the plugs which are lighter colour than they've ever been its certainly on the lean side of perfect.

    So, I need to make the mixture richer. I've a stock 160 main jet (UK 40mm model, 90/40/123-124). Never having had to mess with this before I'm a bit of a noob, should I bump it up to 170, or 175? OR should I start with the needle setting first? Again, all advice much appreciated, apologies if this is well trodden turf...
    #7
  8. mark1305

    mark1305 Old Enough To Know Better

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    Probably 90% of the time the engine is operating at 1/4 to slightly over half throttle unless you really ride hard. (Mark the edge of the throttle grip relative to a fixed mark or the seam on the housing at idle and WOT so you can tell at a glance where it is at any given riding speed). So the carb circuit in play the most will be the slide needle position and that will be the most critical thing to tune. Remember - lowering the clip raises the needle which richens the mixture at everything between off-idle and near WOT. And check the diaprams for small tears.

    Stumbling during acceleration from idle or off-idle may indicate a need for a larger idle jet if the idle mix screws don't cure it.

    WOT mixture is where the main jet does its thing the most.
    #8