Dellorto flaw

Discussion in 'Performance Mods' started by jaguar, Jan 27, 2013.

  1. jaguar

    jaguar Well-Known Member

    Trying to perfect my race engine I found it almost impossible to get the jetting right. It seemed like there was a small air leak. I pressure tested the crankcase and there were no seal or base gasket leaks. My intake and reed valve were tested to be leak free. I messed with how the Dellorto carb mounts on the reed valve and finally got that perfect. But still the problem persisted. Then I took an investigative look at the Dellorto carb and found an air leak path designed into it! Taking the carb off and putting the slide in place and shining a light into the carb I could see light between the front of the slide and the carb. Air comes in over the slide from the air filter (and also from where the throttle cable connects to the top of the carb), goes past the front of the slide, and then gets into the air/fuel passageway through that small space between the slide and the carb. I can't believe they designed it that way. The design should be that the slide slightly scrapes the carb to seal and prevent any major leak. So now I have a Mikuni 18mm carb on order and until then I will put JBWeld on the slide and sandpaper it down till it just barely touches the carb. That should allow me to finish testing on a new pipe design.
    I will probably have to let the machine shop lathe a new aluminum piece to replace the manifold piece of the Rock Solid reed valve to match the carb. Once I get the carb I can tell you all the inner diameter of its clamp so you'll know what size to tell Rock Solid to make the manifold on the reed valve you're ordering.
     
    roughrider likes this.

  2. Bob K

    Bob K New Member

    A Mikuni 18mm ? Why not a Kehien ?
    Just asking. I mean I like Mikuni but I know you tend to think these things through.
    Your thoughts?

    Bob K
     
  3. jaguar

    jaguar Well-Known Member

    near impossible to find a 16mm-18mm clamp-on Keihin. But they are good carbs. I had one on my CR250 but it took using a computer program to get it jetted perfectly. Stupid Honda couldn't get it right from the factory.
     
  4. jaguar

    jaguar Well-Known Member

    The inner diameter of the clamp-on part of the Mikuni is 23mm. So either I need a plastic sleeve of 2mm thickness or I need to have the reed valve part made from scratch to match.
     
  5. roughrider

    roughrider Member

    Well, this is a bummer. I just ordered the performance carb from JNMotors. Not sure if I'm visualizing the problem though. I don't have the carb in my hands to see what you are talking about. But, anyway, thanks for sharing your discovery, Jaguar.

    PS: Just had a thought. Have you/Did you test the carb when it was hot?
     
    Last edited: Jan 29, 2013
  6. jaguar

    jaguar Well-Known Member

    The two sections of metal that should be mating perfectly to disallow air leaks are thick enough to not change with heat. Maybe the space between them varies from one carb to the next due to manufacturing tolerances. Like I said, it can be mostly fixed with JBWeld. But to get jetting really perfect you need a carb that allows adjustment of all three ranges (low, mid, high RPM) which is what I'll have with the Mikuni carb. After correcting the air leak the Dellorto should be fine for most people though. I am aiming at absolute maximum performance from mine though.
     
  7. Bob K

    Bob K New Member

    I just had a flash back from my past . When putting too big of a carburetor on an engine the airflow was too low through the carb to draw fuel reliable.
    On trick to enrich the idle mixture was to take .020" off of the back side of the slide. This caused the emulsion tube to get a stronger signal and it helped
    get a proper idle and more importantly, a good off-idle throttle response. I do not recall if I had to fuss with the needle height ( hey 30 years ago...)
    this was a way of enriching the idle with out affecting the main or mid range. This was with Mukuni's.
     
  8. Bob K

    Bob K New Member

  9. jaguar

    jaguar Well-Known Member

    the ideal carb size for my 60cc engine with reed valve reving to 8300 rpm is 17.5mm, so 18mm can't be considered "oversize".
    The carb I have on it now is a Dellorto 12mm bored out to 14mm.
     
  10. Bob K

    Bob K New Member

    That's what I like about you, you run the numbers! My point was/is, by altering the effective slide cutout height, you can independently alter
    the idle/off idle mixture without affecting the other jetting circuits. I just shared an (old) experience with a hopped up motor and a slightly oversized
    carb. This does not alter the fact that slide cutout geometery is an effective tool for tuning.(even with a "correctly sized carb)
    Possibly, ( imagine that) the chinese did not get the
    carb slide cutout correct. Or at least not in the 'sweet spot". I'm at sea level and you are at high altitude. Our results may/will vary to some degree.
    I look forward to reports from your experiments with your new Mukuni. If you have not yet received your new carb, You might entertain the idea of
    taking a bit off of the back side of your exisiting slide to richen up the idle/off idle and then backing off the main jet and see if it works.
     
  11. jaguar

    jaguar Well-Known Member

    you're absolutely right but the Dellorto doesn't have a traditional slide. It is just a flat wall in front of the jet column. I thought that raising the intake side of a traditional slide leans the mixture though. I have used that trick for fine tuning the idle circuit but I don't remember the details or which motorcycle I did it to.
     
  12. Fabian

    Fabian Well-Known Member

    All the more reason to make the switch to a diaphragm carburettor.

    I haven't looked back after fitting mine !!!
     
  13. Ludwig II

    Ludwig II Member

    I remember those carbs, they were used on a lot of Italian moped engines in the 70s and 80s. They're 2 position devices, idling and flat out.
     
  14. Fabian

    Fabian Well-Known Member

    You must be thinking of a different carburettor.

    Do a Google search for "Walbro Style Carburettor for Rock Solid Engines Reed Valve Intake"
     
    roughrider likes this.
  15. roughrider

    roughrider Member

    Thanks for that. I'm gonna run the stock NT carb for a while. Then I'll swap the Dellorto clone and mess with the tuning. If I get an improvement, it's all good. I'm not looking for max power, just smooth, predictable power curve.
     
  16. roughrider

    roughrider Member

    I do not doubt you my friend, but those carbs cost as much as a whole new engine! Yow! if you know of a deal, well, do tell!
     
  17. Fabian

    Fabian Well-Known Member

    You get what you pay for, and in this case it's billet components and CNC work from start to finish, and the components work as designed, i.e. they work to perfection !!!
     
  18. Fabian

    Fabian Well-Known Member

    Been there and done that and after testing the genuine Dellorto, the clone Delorto and the standard NT carburettor, i went back to the standard NT carburettor, before installing a diaphragm carburettor and everything became rainbows and sunny fields after that !!!

    The laws of physics determine how much air you can move through a 14mm hole and the NT works just as good as any of the other carburettors,once it is set up and jetted properly; in fact the NT works just as good as a diaphragm carburettor with the same size 14mm venturi, with the exception that the diaphragm carburettor is easily adjusted for air/fuel ratio with a simple twist of a screw, not to mention that it does not leak "at all", be it air or fuel.
    Once the mixtures have been set, they stay set, unless atmospheric conditions change, and even so, it's a 2 second adjustment to bring the air/fuel ratio back in line with changed atmospheric conditions.

    Next to the SBP shift kit and the Jaguar CDI and the Rock Solid Engines reed valve intake, the Rock Solid Engines diaphragm carburettor kit is the best thing since sliced bread.
     
  19. lazylightning@mail.r

    lazylightning@mail.r Active Member

    BUMMER! %-D I got the Dellorto =-(((
     
  20. jaguar

    jaguar Well-Known Member

    Let me know how bad it is. Can you see much light coming thru that slit?
     
Loading...