HF212 jet size

Discussion in '4-Stroke Engines' started by professor, Sep 26, 2011.

  1. professor

    professor Active Member

    The engine came stock with real lean jetting.
    I started opening the main jet untill it would stop bogging when cracking the throttle. This was with the stock exhaust and no aircleaner.

    I run a free flowing intake and exhaust now, but kept the jet size the same. Used a number drill to open the main jet to (I think) 33 thousandths.
    The idle jet stock is 15 thousandths, I opened it to 17.

    The gokart guys say 21 idle and 39 main jet size.
    Seemed too much to me, but because I still have to start with the choke on (unless the engine has run for a while), I opened the idle to 21.
    Big mistake- blubbering rich idle was what I got.

    Not being able to solider it up (with the jet incased in plastic).
    I used some seal-all glue with a guitar string stuck in to size it back to 17, until I can get a new jet.

    The HF 79 was similarily lean, I left the idle alone but opened the main jet about 3 thousandths. I assume all new motors are too lean.
    Just a heads- up to anyone with or getting one of these engines.

  2. bideronit13

    bideronit13 Member

    In your first paragraph you stated, "I started opening the jet". What exactly did you do?
    Im asking because I always do my own carb work and from what I interperate is you didnt have the jet screwed all the way in as described by you loosened it?
    Lets go piece by piece so we dont get confused.
    Last edited: Sep 27, 2011
  3. bideronit13

    bideronit13 Member

    All I can really do to help you is to give you future advice to insure the jets are clean and screwed in all the way as there is no adjustment on the jets.
    As far as drilling jets, I dont go there.
  4. professor

    professor Active Member

    Sorry for the confusion.
    When I said "open the main jet" - I mean increase the size of the fuel passage in the jet.
    Initally, I used a oxcy-acet. tip cleaner like a mini- file in and out of the jet- got a couple of thousandths out that way.
    Still too lean.
    Then I got a tiny drill (33 thousandths) to get a bit more fuel into the motor.
    By the way- if you clamp the drill in a vise and spin the jet in a drill motor, it is easier to drill jets.

    On the idle jet- I spun the jet by hand against a 17 thousandths guitar string and eventually it pushed thru. Thus, opening the jet to 17. I just ordered #66 & #77 drills from McMasterCarr so I would have the 33 and 17 thoousandths drills on hand.
    The SealAll diid not work, by the way.
  5. bideronit13

    bideronit13 Member

    One jets like aTornado and the other is like a sprinkler.
    They both need to work togethor. So by drilling you try to make a big tornado with a little sprinkle or a big sprinkler with not enough tornado.
    Its way risky I like to ride so I just leave or replace them and haveing extra Jet sets in nice too.
  6. bideronit13

    bideronit13 Member

    The only reason I'd have an extra set of jets is to enhance the performance and not to drill on. If I'd drill on them I might as well get my chainsaw out and chainsaw the carb.
    If I were to drill on them I would be designing and manufacturing them.
    If anyone else comes with the idea to drill the jets please provide pictures and video because the jets are to precise and I dont believe someone would attempt this. So I would enjoy watching the carb get chainsawed in the end after this is attempted.
    Heres a topic on the subject.
    Heres another topic on the subject showing the extent of the tools needed and lacks the science of the tornado and sprinkler.
    http://www.affordablegokarts.com/Drilling Main Jets.php
    Heres a topic that has a clue. The need for jewelers drill bits and a bunch of jets needed to conduct experiments.
    Last edited: Sep 28, 2011
  7. Silvaire

    Silvaire Member


    First, if you can find a source for HF212 jets, obtaining a small assortment and then tuning the carb to your taste is the most appropriate course of action.

    The best that one can hope for a simple carburetor to provide is a usable mixture for a decent idle, midrange, and full throttle. Many factors are at play in obtaining this - FAR beyond just what sizes the jets are. There are so many variables in getting a carburetor to work right.

    That being said though, if a carb is close to being correct for the engine, adjusting the idle mixture and the full throttle mixture will usually get you close to getting what you are after.

    Drilling/filling the jet(s) is certainly a functional solution (if you can't easily obtain new jets), and it seems that you are going about this in an appropriate fashion. Of course the diameter of a jet is usually the major factor in its flow volume, but the length of its bore, wall surface finish, entry and exit smoothness, etc. etc. are all factors of its flow characteristics. Of course all the rules of fluid dynamics apply, but so what? All YOU are looking for is for the jet to flow the appropriate amount of fuel, and just how you accomplish this isn't really all that relevant. You might find some useful info the this thread:

    I am NOT at all familiar with the carb on the HF212. Does it have an idle mixture adjustment? If so, I would work only with getting the main jet close (full throttle mixture) before I would do any fiddling with drilling/filling of the idle mixture jet though.
    Last edited: Sep 28, 2011
  8. bideronit13

    bideronit13 Member

    Thanks Silvaire
    I can help on fine tunning a carb also. This is textbook Honda 101.
    Its recomended to be done on a warm engine. So you'll need to find a spot to were it will start and run the engine for about 5 minutes. I also recomend using the 1/8 turn at a time method.
    To obtain optimal performance on a carb after it has been cleaned.
    1) Have the idle screw set a reasonable postition mayby 2 turns out from zero.
    2) Turn the mixture screw all the way in.
    3) make 1/4 or even 1/8 turns out and try to start the motor doing so till the motor starts. Write down this position on paper.
    4) Continue 1/4 turns or 1/8 turns out till the motor wont start. Write this position on the paper.
    5) The position between 3 and four is exactly were you want to be.
    6) Adjust the idle accordingly.
    Last edited: Sep 28, 2011
  9. scotto-

    scotto- Member

    WoW, you don't drill jets? It ain't rocket science and that's a great way to fine tune a carb that's somewhat compatable to the engine it's intended for. Open your mind and open your jets if your engine is starving for more fuel.....honestly.


    22mm Mukuni carb on a 212cc HF Predator. #95 main jet that I drilled out to .043" to make this race bike scream (and run properly). Any questions grasshopper?


    let's RACE!
  10. scotto-

    scotto- Member

    Umm, I've got 4hrs. on the motor now at like 40mph avg. is about 80 miles.....that's a long block lol.

    Yes you can, here's a short vid and there will be plenty more from the race track at Grange on the 22nd.....


    Last edited by a moderator: Oct 14, 2011
  11. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    Fantastic build scotto-
    That monsta motor is going to grind up the competition and spit
    them out! :D

    Hope you make it to the track on Friday so you have time to get
    the carb jetted right for 3400'.
    See you on the track at Grange on Oct 22nd....