Carb Rebuild, Expecting A Transformation.....

Discussion in 'Builds & Projects' started by edb, May 13, 2019.

  1. Another carb related issue, sorry I have looked but cannot find any similar threads….

    750ss 1998


    The bike starts and runs on the choke with some backfiring (hopefully the right term, a loud pop from the exhaust) but generally quite smooth.

    When I take it off the choke idles at 2000rpm, smoothly with little or no backfiring.

    As soon as twist the throttle revs rise, drop quite slowly and then very quickly the engine dies.


    Recently cleaned carbs, dynojet kit stage 1, pipercross air filter and Laser exhausts.

    Uprated coils etc from Exige.

    I was hoping to put everything back together and hey presto, right first time………

    I should mention the throttle cable has been adjusted as it was sticky, now the mechanical action is spot on.

    My (inexperienced) thoughts.

    I thought the throttle issue may be fuel starving but as it runs on the choke, fuel feed should be ok?

    The carbs were set up by an experienced Ducati mechanic (off the bike, not on a dyno obviously).

    Any help or ideas would be really appreciated.

  2. Have you talked to the experienced Ducati mechanic to get his thoughts yet? Could be many things but possibly a mixture issue, i’m not sure how free-flowing the Pipercross filter is, but have you tried it with the stock air filter yet? Do you know how the idle mixture screws are adjusted/ how many turns?
    #2 Chris, May 13, 2019
    Last edited: May 13, 2019
  3. Air leak between carburettor, rubber and manifold would be my first port of call if the carbs have been off
    • Agree Agree x 1
  4. Thanks for the initial thoughts guys.
    I haven’t tried the original filter, I will have to order one as the old was pretty manky.
    I am waiting for a response from the mechanic.
    When I get home tonight I will double check all connections for the air leak and also the idle screw. If there was an air leak would that not effect running on the choke too? It runs fine on the choke.
    Many thanks
  5. The idle mixture needs to be set up on the bike as every one is different (exhaust, air filter, air box, valve clearances, etc.) I would suggest that the idle mixture is a on the weak side which will lead to the engine stalling when the throttles are opened.
  6. Sorry I retract the comment about running on choke... it further restricts air right so if there was a leak it would have less of an effect. I think...
  7. Thank you Derek,
    I will double check and try a bit richer.
  8. Sorry edb, I was a bit cryptic and Derek has spelt it out better re: possibly running too weak. If this is the case then if you can run it and let it idle until fully warmed the throttle response could improve but idle mixture screw settings still need carefully checking. Have you got a drastically cut-down air filter top cover or is it stock? Did it run ok before carbs overhauled?
  9. Thanks Chris,
    The air box lid is standard.
    It ran ‘ok’ before the overhaul. It ran better with standard cans, the open Lasers did not improve matters (unsurprisingly without a setup). I had hoped that the overhaul, new filter and dynojet would have improved the situation with the open cans. The dynojet needles were set for open cans (you get the option).
    When hot it idles well, then literally falls off a cliff with any throttle.
  10. Firstly, I reckon you should have a Stage 2 Dynojet kit, because the Lasers are straight through.

    If you have the airbox trumpets fitted, take them off when you have fitted the Stage 2 kit.

    Are you sure you have a proper Dynojet kit in the carbs.......for example, how do you know the needles are 'set for open cans'? DJ needles aren't marked with any sizes. Are the springs definitely DJ?. Are the jets DJ? (They will only have a size on them, no other mark)

    In other words, check the photo below.

    Needles & Springzah.jpg

    When you are satisfied you have the right parts / kit; check the float measurement - if you have set them at 14mm, it will run weak - they need to be about 12mm (or even 9.5mm).

    Also check the diaphragms are torn/split and causing the slides not to lift - up to 2000rpm - 2500rpm, the carbs use the pilot circuit only.

    It would be a good idea to take the pilot screws out to check the springs; O-rings and tiny washers are there (or actually exist).

    The file below is the Dynojet Stage 2 kit instructions and specification.

    Attached Files:

  11. thanks Chris , i will check out Brad's notes!

    thanks Arquebus. i supplied the DJ kit and have the old parts returned so it is all good in that sense. the carbs are generally very clean, the dealer told me they were quite new and the mechanic who refurbed them backed this up saying they were very clean. the air mixture screws were 3.5 turns out as instructed.
    i was told you have two options in the set up, one for standard and one for open cans - this may be float measurement you refer to?
    after just locking the garage for the night it occurred to me that the idle
    was a bit low 1250-1500rpm, would increasing this has an effect on the throttle response?
    (i cant see how but as you have all probably realised, my experience is very limited).
    thanks for all the advice
  12. My 750 would tickover at 1000rpm with Stage 2 DJ, Laser cans, OEM filter and no trumpets on the airbox.

    Brads notes are remarkably similar to what I posted in 2014.

    If you look on the SS forum you will see I have commented on Desmoquattro's question.

    Have you looked at the components of your DJ kit compared to the photo I posted yet?
  13. thank you Arquebus.
    Good to hear your setup. I will read the other threads and try a few combinations over the weekend, will report back success (hopefully)!
  14. Arquebus, yes the needles and springs are dynojet, exactly like your picture shows.
  15. Iirc, a Porsche 924 filter from a motor factors will fit your airbox perfectly and save you a few quid over oem.
  16. Has the bike ever had a scottoiler on it then had it removed do you know?
  17. Update.

    • I removed the carbs and refit, there is no obvious air leak from anywhere. Is there a ‘usual’ culprit for a leak?
    • Again it starts on the choke second or third time, running on one pot for a few seconds before settling.
    • As soon as you come off the choke and touch the throttle it dies.

    I had a small breakthrough (I think)

    • Set the air mix screw to 2 out on the horizontal feed and 3 out on the vertical. I presume this is the right approach.
    • I then blocked the trumpets on the airbox loosely with some paper towel.
    The immediate response was a higher idle on the choke that surged.

    No change when on the throttle.

    I think this gives more confirmation around the lean running when off the choke?

    I will order an OEM filter and see what happens…..
  18. Never had a scottoiler Noobie
  19. I wouldn't buy a std filter just yet - if it runs on one cylinder then there's more to it than just the air/fuel ratio. Has anyone balanced the carbs on the bike yet? Are you happy that this is not ignition related?