the best mods are the simplest. needed; 1 file.

Discussion in 'Performance Mods' started by HeadSmess, Sep 8, 2011.

  1. HeadSmess

    HeadSmess Well-Known Member

    as i do more and more to a ht, im amazed they can run as stock.

    the first and simplest mod is to cut the piston skirt down.


    it was the ONLY thing that i did to my first engine. it screamed :)

    if you peer into the intake, and rotate the engine, you will note the piston never completely uncovers the port. so this does two things. reduces the amount of air that can flow through the port, and how long for.

    so. turn engine to TDC. scribe a line on the piston.
    disassemble. file down to scribed line.

    this has changed the induction timing from roughly 98 degrees (pathetic) to 120-130 depending on material removed.


    better to use a half round file than just grinding it all off :eek: dont overdo it! and get new circlips for the conpin! real ones that need pliers!

    i then had a bit of a chew at the port itself. mainly widened as i need it to be equivalent to 19-20 mm, without affecting the timing. flattened bottom of port a bit, hence the piston...
    this brings my induction timing to 148 degrees.

    um. then the exhaust. this needs a lot of metal removed!

    wider is better for volume, but at 138 deg stock, the top of the port can be raised somewhat.

    so here we are at 160 degrees exhaust duration, widened, the port is now basically parallel. much better :)


    dont smack the file into the opposite cylinder wall! :eek:

    last but not least was to clean up the casting flash in the transfers.


    they will need opening up as well, plus a little more duration, as they have about 110 degrees max now. this requires my mill to get set up again as die grinders can slip and ruin otherwise perfect jobs... :( the angles and depths are sort of important :(

    so just a clean for now.

    anyways. how does it perform?

    well, with a stock nt carb and a slightly restrictive tuned pipe... a scalded cat! and getting fatser daily :eek: so as soon as this mill gets set up its a cnc manifold for the mikuni and a new pipe :) as well as transfers...

    i guess i better put a vid up one day...this thing spools up like a proper 2 stroke now :D instantly, not when its thought about it for a while and decided that yes, that is WOT...
    Last edited: Sep 8, 2011
    Steve Best likes this.

  2. motorpsycho

    motorpsycho Active Member

    looks like i'm gonna try these mods on my 49 c.c. over the winter.
    at least the piston notching and cleaning up the ports....I can't do much with the transfer ports because i don't have a mill, just a dremel and some files.
  3. 2strokepoke

    2strokepoke Member

    I might also give this a try in the winter when i've got nothin better to do but wish it were warm out.
  4. HeadSmess

    HeadSmess Well-Known Member

    the mikuni manifold!
    first cuts...

    making the 18mm bore part...


    a woops :eek: this is why im not a cnc machinist by trade!


    forget about safe "Z" heights and things hit things... gotta learn somehow :D usually its when youre using the $150 tool...:rolleyes7: and ALWAYS on the last step!

    how the %$#^^$# do i upload pics and actually be able to see them? and why cant i do more than 4? everyone else can :(
    Last edited: Sep 17, 2011
  5. HeadSmess

    HeadSmess Well-Known Member

    continuing the "woops" theme...closely followed by an "ouch" :eek:

    linisher belt tore and copped me one... like being hit with sandpaper it was!


    and, of course, the finished item, with carb :) this, obviously, was the 2nd attempt :'( stoopid woopsies! could make millions now though :)

    i swear i had more pics! grrrrr!

    now just gotta tune it in :) pretty good now, sorta fixed the bottom end and really lets it open up at the top end :D uses more fuel, so hoping thats just richness...ha! with more air going in as well? keep kidding yaself!

    (lol at coming up behind some car and telling em to learn to drive...why brake just cause theres a car going the opposite direction in the lane its meant to be in? its not normal, and i wasnt expecting it...almost ran off the road! so im going faster than most fools can drive :D)

    regards piston skirt cutting...once again...DONT OVERDO IT! youll lose all the bottom end! im just bordering on it... :eek: but i was also mucking around with the port itself...

    get a degree wheel/ 360 degree protractor and measure things properly. :)

    the other way to change the transfer timing is to file the piston crown slightly. havent done this yet. but a piston at $13 is cheaper than a cylinder at $40 to stuff up :D

    can also use thicker base gaskets but then the inlet port and exhaust port times get affected, plus having to machine the head/cylinder to get compression back up again... :rolleyes7:

    oh well. two or three weeks of mistreatment, 40km a day, and still running :D

    ive also reverted back to a 20:1 oil ratio due to extreme loads and temperatures now experienced :D

    the pipe was chopped and stretched by 6" to bring the band down to somewhere useable. didnt touch the header cus the 60 extra mm it needs will cause it to hit the front wheel :(

    have to chop the HT muffler i stuck on, and change to a 14mm outlet instead of the 10 that it is...too restrictive. causes it to get too hot. the first 8" of pipe has taken on the oxide colours... starting from light blue and changing to the dark purple as it gets a bit cooler... so exhaust gas temps are up around 300 to 400 degrees C.... at the bend! way too hot!

    curious that the last pipe i made was super quiet with no muffler and this one is LOUDER even with a muffler... :thinking: different software...
    Last edited: Sep 17, 2011
  6. richirich1113

    richirich1113 Member

    Ive got a question.. is the skirting only on the intake side? also if it is only on the intake side does it throw out the piston balance? or is something esle involved to even it out?
    Thanks for your time.
  7. HeadSmess

    HeadSmess Well-Known Member

    yup. remove from the inlet side only.

    if you play safe and only file the amount required, i daresay it doesnt affect the balance doodlysquat. these things dont rev that high...

    the amount i removed might make a fair difference, but cant really say if its reduced or increased vibrations... the weight could be equal to the difference caused by different crown thicknesses between production runs of pistons... they are chinese and cheap remember! (or the vibration could be counteracted by the vibration caused by the badly machined clutch shaft that lets the clutchgear run out of true by about 7mm... just waiting for these gears to strip :D )

    i dont own any scales!

    i still have to brush up on balancing...all i recall is that a 2stroke can only be truly balanced for one speed and needs multiple cylinders to get it really good.... so why bother? :)
  8. ddesens

    ddesens Member

    I scribed a line on my piston and removed as much material, cleaned up my transfer ports this past weekend. It made a noticeable difference in power. It seems to rev slightly higher than before also. Thanks for the info headsmess!
  9. richirich1113

    richirich1113 Member

    Yup, thanks for clearing that up for me.
    RIDE ON>>>>