too jumpy in first ~3% of throttle openi

Discussion in '2-Stroke Machines' started by mattology, Mar 4, 2018.

  1. mattology

    mattology Well-Known Member

    yeah, i dont plan on giving up on this carburetor just yet. depending on the tolerances and what not i may try and 3d print a new slide, and then have nickel metal vapor deposition on top of the print to make the part metallic. why throw money at a problem when you can throw time, money, and complication at it?
     
  2. Algonquin

    Algonquin VRRA and DOCC no.92

    Well said, awards should be presented for such category-leading examples...

    Impressive, just wait until you do the part surface-grinding or other fettling. Minor issues, good stuff.
     
  3. mattology

    mattology Well-Known Member

    I went with a factory TPS equipped TMX38 from a 2007 Honda CR250R to replace my non-tps TMX38. Also I went to a solenoid powerjet instead of my thunder powerjet, which kept vibrating itself out of adjustment on me.

    The tps will allow me to retard the timing in the lower throttle percentages to make the bike less jumpy not just as a result of extra fuel, but also less ignition timing.

    I also added a wideband o2 sensor so i can see what kind of air fuel ratios i am actually running on the dang thing while on the pipe, and not just reading my egt and cylinder head temp.
     
  4. Kurlon

    Kurlon Well-Known Member

    Your wideband isn't going to last very long with premix hitting it. :D
     
  5. mattology

    mattology Well-Known Member

    replacement sensors are like 70 bucks, i've been using them with leaded gas and what not for years. i bet they last as long as a piston ring does. i think the big one is warm the bike up without the wideband in, and then once its warmed up, install the wideband into the pipe.
     
  6. dave3593

    dave3593 What I know about opera I learned from Bugs Bunny

    On my TZ I battle that transition by not closing the throttle all the way and keeping very light pressure on the rear brake.

    Maybe someone already mentioned this but raising the idle speed will decrease this transition providing your carbs even have an idle setting.
     
  7. mattology

    mattology Well-Known Member

    thank you for the insight, dave
     
  8. dave3593

    dave3593 What I know about opera I learned from Bugs Bunny

    Two strlkes are cool but persnickety. ;)
     
  9. mattology

    mattology Well-Known Member

    hey there, updates

    the TPS carburetor is almost done being fit for the solenoid power jet, and the wideband is installed

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    i also designed a new combustion chamber with modern squish design and a compression ratio of about 15:1 down from the >19:1 that the bike previously had

    all of these should help the bike not only be easier to tune and see what's going on, but also make it a little bit easier to open the throttle while leaned over....
     
  10. rob linders

    rob linders Well-Known Member

    What cc's are you using in the combustion chamber. I didn't read your entire thread, but I read the first post and the last post. When we ran a Honda RS125 we ran from 9.8 to 10.2 cc's. It seems you are running a sweedtech gokart engine, I'm guessing a Honda CR125? Karts don't worry about throttle control nearly as much as bikes.

    I could be totally wrong but I seem to remember we were under 10.1 on compression ratio, but I honestly didn't ever calculate it or even try to figure out how.
     
  11. mattology

    mattology Well-Known Member

    Yes, it's basically a swedetech stock moto go kart engine., cr125.

    8.8CC's with .8mm squish. 100 avgas. with 54mm bore, 0.8mm squish, 54.5mm stroke, and 8.8cc combustion chamber volume the compression ratio ( excluding the exhaust port yadda yadda) is 15.1:1
     
  12. rob linders

    rob linders Well-Known Member

    Well if that is anything like a RS125, there is your problem. Try increasing the CC's. I really can't remember the squish we ran, I gave away my notes but we rarely ran 9.8cc because it was too snappy, usually 10.0, or 10.2, even did 10.4cc at times. For some reason .032 is what I'm remembering on squish, if I calculated correctly you are at .031...so that is the same.
     
  13. mattology

    mattology Well-Known Member

    well i just designed this cylinder head insert based off what the aprilia race team recommended.. had it cnc'ed and it was about a 16 week lead time for the price i paid, so i am gonna give this one a go. That's interesting that you guys ran even lower compression. It makes sense with the snappiness though.

    Keep in mind, i am coming from like a 6cc combustion chamber previously,, so i hope this makes some good improvement as is... if i like where it's going, i may go more in that direction, but i'm gonna at least give these a shot
     
  14. rob linders

    rob linders Well-Known Member

    Understood. If you want to have another head insert shaped, I would suggest getting a hold of Roland Cushway he's up in the Bay Area and he turned all our stock heads, to different cc's.

    He can probably turn them within a week or two. Amazing guy with a lot of knowledge, tuned with Erv Kanemoto on the MotoGP circuit.
     

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