Carby What Jet Size Do I Need?

Discussion in '2-Stroke Engines' started by Waxxumus, Jul 14, 2015.

  1. Waxxumus

    Waxxumus Member

    I live 500 ft above sea level and the average temp here is 80-98 F, I have a 14mm RT Delorto clone and a BPR7HIX spark plug
    .
    I havent done one thing detailed in this report on SHA Delorto carbs. (Probably should lol)

    http://www.dragonfly75.com/motorbike/SHA.html

    But my bike has a .70 and it runs too lean @ 32:1 when going more than half throttle for extended periods of time. When running slowly 15 mph or so for awhile, it comes out too rich.

    So im having the exact OPPOSITE problem that you were when you got your Delorto and moved down to the coast. (if that was you lol)

    Im having trouble telling because the plug was fowled when it originally had a .75 in it and ran too rich, and keeps coming out with a real light tinge of white over the black fowlness. Does this indicate detonation? and What does a detonation sound like?

    When I ride, she has great acceleration up hills, and has no problem accelerating down them, but after long periods of high rpm, it seems to pop once the rpm drops, as if its having backfires or detonations. (im still not sure if it is or not, im not entirely sure how to tell what a detonation is or sounds like)

    I could go up to a .75, but it will run way to rich and bog out.
    I could go to a .65, but the lowest recommended jet for where i am is about a .68~.70 @ 500-800 ft / 85F using the table provided in that article.


    B = Base Jet Size (says size that came with engine, but you dont put a jet in an engine? and the NT carb is meant for less than 66cc, because it always bogs out at WOT)
    P = Number From Chart (Estimated As roughly half of 1640 ft)
    820 ft.
    So somewhere inbetween the two on the chart. .98

    B x P = N

    .70 x .98 = .686

    Jet Calculator.jpg

    (Not sure If my calculations are right. Guessing that the stock RT dax performance carb 14mm comes with a .70 jet, but im not sure what the engine originally had)

    If anyone could help me calculate this I would greatly appreciate it.

    I could go out and get another spark plug, or play around with the fuel mix, but the fact that the carb sometimes runs leaner than other times makes me not want to get a different plug.
     
    Last edited: Jul 14, 2015

  2. Waxxumus

    Waxxumus Member

    Turrrrrns out.
    There is an air leak on the engine intake, so the gasket is not doing its job.
    Ran the engine for a tad, brought it into the driveway and sprayed abit of carb cleaner on the intake where the valve meets the motor, and it died almost instantly.
    Choked her to death. :(
    I think I may go the brass gasket route, sand the intake valve down a tiny bit to ensure flatness, and jb weld that thing to death.

    And roughen up my intake a tad just for kicks.

    Or just buy a reed valve with a high performance intake valve?
     
  3. Waxxumus

    Waxxumus Member

    Wonder if I have had that the whole break in....
    Hasnt been too obvious, or I would have noticed a dramatic loss of power.
    But it sure runs lean man. Leaner than it should.
    No wonder it survived 16:1
    Perhaps this will fix it.
     
  4. Waxxumus

    Waxxumus Member

    Well, it got richer after I tightened my intake down.
    Gapped the spark plug to .031 and she runs just fine.
    Tad bit rich, but ill eventually get my bpr6hix

    Hey look at that I made a rhyme.
     
  5. Fabian

    Fabian Well-Known Member

    There is no such thing as a "correct jet size", because every engine is different, and they can vary wildly in their fueling requirements from one engine to another.
    I have every Delorto jet size in single number increments from No 65 to No 90.
    There will be a jet size where the engine runs optimally, and it could be anywhere in this range, but in my location of lower south eastern Australia, most of my engines seem to run best with a jet size somewhere close to a No 76
     
    Legwon likes this.
  6. 45u

    45u Active Member

    Jetting is trial and error but from what you are saying I would start around a #65 and work my way up.
     
    Legwon likes this.
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