Welcome to Netrider ... Connecting Riders!

Interested in talking motorbikes with a terrific community of riders?
Signup (it's quick and free) to join the discussions and access the full suite of tools and information that Netrider has to offer.

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?
  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:
  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?

  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.