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Carby tuning advice, please

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Chairman, Oct 13, 2005.

  1. I have a nasty spot in my carby's fuel delivery that is causing the bike to bog down. Any suggestions most welcome.

    Some background. The bike is an SR500 single, running a Mikuni VM34SS carby. This is a conventional carby but has an acceleration pump that sprays a little extra when required. The stock airbox has been replaced with a K&H slip-on filter and the exhaust is virtually straight-through.



    I've never had the bike running in stock trim - it was built like this so I've been guessing the settings from day 1.

    Stock settings are needle at groove 3 (of 5) and a main jet of #300. The recommended adjustments to accomodate the slip-on air filter and open exhaust are - machine 50 thou off the bottom of the carb slide, move the needle clip one position richer and to increase the main jet to #330. These were done.

    With that set up I get bogging if I snap the throttle open from 50% to full. The bike pulls very strongly in all gears up to 50%, holds road speed well and, if I open the throttle slowly fom 50% to 100%, the delivery is smooth. But settling at 50% and then openning it fast causes power loss for a second or two, followed by a recovery and gradual speed increase.

    I had some trouble with rough running at larger throttle setting - the bike would backfire on a trailing throttle so I dropped the main jet back to #310 - this fixed the backfires. But the flat spot was no different. I then lowered the needle to the stock position - this was worse so I raised it as high as it would go. Some improvement, but not good enough.

    Any suggestions?
     
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  2. Sorry, I misunderstood the title and wasn't expecting a serious question in the thread.

    If anyone want's to know however, at 80kph the exaust note of a 1275 Morris engine in a Mini Moke will reach a perfect G. The speedo didn't work so I kept a chromatic tuner on the dash. All you need for that to work is a leaking engine pipe clamp. :D

    Show me a Moke without a dodgy engine pipe clamp and I'll show you a stunned dude. :shock:

    Edit: Typos, Sorry. :oops:
     
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  3. Chairman, one thing for sure is that I am no expert. It does sound however like your acceperator pump could be part of the problem (I am guessing here). Moving the slides up a little would mean a difference in vacuum. The larger main jet (#310) and the lifting of the slide would obviously mean more fuel, but lower vacuum. I say vacuum as this would obviously differ between how your applying the throttle and getting the bog down effect. It would be great to be able to see the air/fuel mix at where your problem exists. I would suggets backing off the accelerator pump a little. I am assuming your float is set correctly?

    Cheers
     
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  4. Last week I threw out a call for help - and now I have good news to report.

    I took the opportunity of the GP to do some "tuning runs" - figuring that most of Victoria's finest would be preoccupied patrolling Gippsland. After a lovely afternoon blasting the backroads of Balliang, the carby problem is almost sorted.

    I've raised the needle another notch (no grooves left, so the next change would be to machine a new one) and gone back to the #330 main. The flat spot is gone - silky smooth power delivery right through the range. At top end (WOT) the plug still looks a bit pale, so I have a few larger main jets to try. It might let me wring out an extra few km/h but I rarely use them around town so it isn't a priority.
     
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