NT Carb.... No air adjustment

Discussion in '2-Stroke Engines' started by danlandberg, Mar 20, 2010.

  1. danlandberg

    danlandberg Member

    I want to know what a technician thinks, or any body who knows 2 stroke engines. I had a thought about adding an air mixture screw into the intake manifold to create a slight intake leak. With my past experience on a 40mi. ride (one way) 1/2 the way back, the bike was running GREAT for about 5mi. (37 mph) on flat ground,:D Then I could not get it over 18 mph.:( I stopped at the next town, (I was in B.F.U.) and found I had developed an intake gasket failure. So when the leak begain it was small & got bigger to cause a drop in power. I used a buisnes card to make a gasket.(It worked) I made it home! It's just a question I've been pondering:whistling: I have rebuilt & tuned many 2 stroke weed eaters, but not to the point they could cut down a forest with 1 role of string!
     

  2. motorpsycho

    motorpsycho Active Member

    an intake leak would cause the engine to run lean, even if it was made by using an adjustable screw of some sort. A mixture screw does just what it's called...it allows you to adjust the air/FUEL mixture at the carb. an adjustable screw in the intake would only allow you to adjust the air and it will always run too lean if it is sucking extra air anywhere after the carb.
    you will always get more power / shorter piston life out of an engine that is running too lean.
     
  3. danlandberg

    danlandberg Member

    Lean is lean no mater where the extra air comes from. Carb or aft. the fuel/air mixture goes into the crank case via intake manifold (where fuel & air can be mixed together) then the combustion chamber. I think that the NT carb needs improvement! But I guess you get what you pay for. I think that an adjustable jetable style manifold would work (if you are using a stock NT carb). Also a thumb screw adjustment, so you can tune as needed (on the go) The only problem I see in this, is it will be unfiltered air! I am talking about a minute bit of air! Oh by the way, at 5500 F.A.S.L. wee need to run leaner!!! :ack2:
     
  4. danlandberg

    danlandberg Member

    And how does a boost bottle work? Intake leak?
     
  5. AussieSteve

    AussieSteve Active Member

    Good to see you thinking outside the box, Dan, but motorpsycho is right.
    It would only allow adjustment in one direction, (leaner and leaner) and could only work if the mixture was too rich to start with, at all throttle openings.
    That's where the problem lies - where we need more specific tuning of mixture to throttle opening, this method would reduce it and lean out the mix at all throttle openings.
    That's where a carb with an idle circuit with pilot jet and idle mixture screw is good. You can then set the mixture independently for idle, idle to 3/4 throttle, then 3/4 to full throttle by altering pilot jet and idle mix screw, needle position and main jet size.

    Sorry, forgot to add. I think that a boost bottle works by capturing the fuel that would be blown back out of the carb, to be sucked in on the next cycle. Someone else can add more or clarify, hopefully. Otherwise, my usual advice - search 'boost bottle'.
     
    Last edited: Mar 21, 2010
  6. motorpsycho

    motorpsycho Active Member

    yep, a boost bottle collects fuel vapor that is spit back out of the intake. the vacuum from the intake sucks the vapor back into the engine. If you fully understand how these 2 strokes work with valve ports, some exhaust actually comes out of the carb on the very beginning of the exhaust stroke. this slight exhaust backfeed is what pushes unburned fuel vapor into a boost bottle. on the intake stroke, this fuel vapor is sucked from the boost bottle and back into the engine. It's a very simple but effective way of cramming more fuel vapor into the cylinder. sort of the same effect as supercharging but in a simplistic way. it is not an intake leak because the fitting on the intake and the fitting on the boost bottle are sealed. there is no extra air involved, it's all fuel vapor and whatever tiny amount of air is backfed from the intake manifold into the bottle.

    by the way, a hole with a screw in the intake will not work!!!
    why do you think intakes are sealed at each end with a gasket? the air/fuel mixture takes place in the carb venturi...not in the intake manifold. my 50 c.c. 2 stroke doesn't even have an intake manifold...the carb is bolted right to the cylinder. if you want to slightly alter the way an engine respnds, long striaght intake runners (or tubes) are better for high rpms, shorter intake runners are good for low end torque. look at a tunnel ram on a drag engine. they have long, striaght intake runners that go straight into the cylinders...better for high rpm power. Look at a stock v-8 carb. intake manifold. the runners are short and curved...better for low-mid range power.
    I have built many engines (cars), played with nitrous, intake manifolds, multiple carb set ups, and all that. The intake must be sealed..ANY hole will cause loss a of manifold pressure, (a vacuum leak) and will ALWAYS cause a lean condition and make the motor run terrible.
    True, more air and fuel will make more power, but it's all how that air and fuel are able to get into the cylinders. adding more air will make more power, but you will lean the motor out so much, you will lose a piston sooner than you should.
     
    Last edited: Mar 21, 2010
  7. danlandberg

    danlandberg Member

    Thanks Steve, I Hopped you would reply to this thought! And I think I had mentioned before, I am at over 5000 F.A.S.L. And had to drop the needle clip a notch to lean the mixture so it would run. I wish the NT carb did have a circuit separate for air/fuel mixture. I now have a ''speed'' carb, I can't see a difference, they are supposed to have a larger throttle and slide than stock. I know these are scavenging engines, but I think this mod would work when you need it!
     
  8. motorpsycho

    motorpsycho Active Member

    If you think it'll work, then why did you ask the question in the first place?
    You ask the question "I want to know what a technician or anyone who knows 2 stroke engines thinks..." you get some replies and thoughts that say it will not work.
    then you say "i think this could work".
    so why don't you just do it and see what happens?
    it's like when you have a flat tire and someone tells you that it's flat, and you respond "it's only flat on the bottom".
    sometimes i get tired of wasting my energy offering suggestions /thoughts when people do not listen to what i'm saying.
     
  9. danlandberg

    danlandberg Member

    Thanks MP; it was a thought I had after 2 experiences of intake gasket failures, the thing ran it's best! (at this altitude) I want to create the same results when I need it!
     
  10. motorpsycho

    motorpsycho Active Member

    That is what i'm telling you!!! even at your altitude, it will run too lean with an air leak at the intake manifold.
    It ran so good with a blown gasket because there was a ton of air being sucked in...IT WILL KILL YOUR PISTON!you can only run an engine lean for so long before the piston gets a hole burned in it or, it gets so hot that the piston will seize up.
    but you do what you want to do.
     
  11. Gh0stRider

    Gh0stRider Banned

    Do the Gh0stRider aquarium pump mod!

    There are no air/fuel adjustment screws on carburetors.

    Either it is an air adjustment screw, or a fuel adjustment screw. Air adjustment screws are generally on the air cleaner side of the carburetor, and fuel adjustment screws are generally on the engine side of the carburetor.

    I have personally done the modification to my generic chinese engine as it was running too rich at idle and loading up the engine with fuel. Moving the needle lower would fix the idle but cause an overly lean condition at mid throttle. Adding the air bleed worked well, but you have to go back and forth on the idle speed and air bleed screws to get it balanced perfectly.

    Go to a pet store and buy an air regulator valve and an inline air filter and hook it up to the intake tube with a brass barb obtained at a hardware store. Drill and tap a hole in the intake tube and screw in the brass barb. (I sealed mine with high temp PC7 epoxy)
     
  12. danlandberg

    danlandberg Member

    SORRY MP; Did not mean to waist any of your time! But I'm thought it MIGHT work. I just wanted an opinion!
     
  13. Gh0stRider

    Gh0stRider Banned

    Read one post above - it does work, I've done it.
     
  14. motorpsycho

    motorpsycho Active Member

    sorry but you are wrong.
    There are air /fuel mixture screws on carbs (maybe not these little 2 stroke carbs).
    opening the screws allows more air in, closing the screws allows less air in.
    actually what you are adjusting the the air that mixes with the amount of fuel that your jet is flowing, so technically yes...they are air/fuel mixture screws even tho you are not actually adjusting the amount of fuel.
    so you aree saying that a leak at the intake manifold is good with your pet store filter idea? wow!!!!
    if it's runnning too rich after adjusting the needle jet, the only RIGHT way to lean it out is by changing the jet to a smaller size.
    go drill a hole in the intake manifold of your car and tell me how well it runs.
     
  15. Gh0stRider

    Gh0stRider Banned

    So I'm wrong, but then you prove that I am right?

    What I did was to install an external air bleed screw on my carb. It does the exact same thing an internal air bleed screw does. The amount of air introduced into the intake is very small and will only affect idle mixture. This is how an idle air bleed works on cabs with internal air bleeds. Opening the throttle does not close off the idle air bleed adjustment, it still continues to introduce air at all throttle positions - it is just so little air that it has a negligible effect on the mid and high speed circuits.

    Go ahead and make fun of the setup, but I know that a good aquarium pump air regulator valve has O-ring seals and can precisely adjust the amount of air introduced. Adding the inline filter solves the problem of drawing unfiltered air too.

    Go for it Dan, you won't be disappointed.
     
  16. danlandberg

    danlandberg Member

    There was not a TON of air getting in, It was only a 1/4 oz or so. LOL.
     
  17. motorpsycho

    motorpsycho Active Member

    there's nothing wrong with a good old fashioned debate....i do not look at this as an argument.
    each of us has our opinion on the subject...neither of us is right or wrong i guess.
    If you say it works, then it works.
    In my opinion, i wouldnt do it...i can not prove to you that you are right or wrong.
    and i can not prove that i am right or wrong.
    i'm just going by experiences that i've had, and my mechanical knowledge.
    No, i was not mnaking fun of your pet store idea. actually, i love it when people think of alternative ways of making parts.
     
  18. Gh0stRider

    Gh0stRider Banned

    Give it a try then! You won't be disappointed.

    Every carburetor that has an air bleed screw for idle mixture has an "introduced vacuum leak" - but the key is that it is a controlled vacuum leak, not one introduced by a bad gasket or a hole drilled in an intake. These air pump valves can be adjusted so the air introduced is a infinitesimal amount. Just enough needed to get things running crisply.

    These ultra simple generic Chinese engine carbs do not have any adjustment for idle mixture, they don't have a dedicated mid range circuit other than the jet needle in the needle jet and a main jet that seems to be lean for EPA considerations.

    My fix was to keep the jet needle high enough to allow for a good mid range (no lean bogging) but the idle mixture was too rich - so I corrected that by introducing a controlled amount of air through the aquarium valve and balanced the throttle stop and air bleed to provide a nice smooth idle. The setup is like a motorcycle carb where you must go back and forth to balance idle speed and a/f mixture with the two adjustments.

    These simple carbs do a good job out of the box, but can be made much better with some simple, common sense modifications.
     
  19. danlandberg

    danlandberg Member

    I understand MP; No argument, it's just a discussion and learning curve! I also have over 20 years technical experience (all hands on). I just had a thought, and wanted some other input! I did not get mad about anything. But I did think about sending you a link that is funny. Just don't know how to yet.
     
  20. AussieSteve

    AussieSteve Active Member

    Not good advice. Besides other considerations, GhostRider, you set out to cure a lean mid-range condition, not a rich one, which is Dan's problem.
    Also, as you say, you only affected idle mixture with your mod - that's not what Dan wants, although in reality it will slightly affect the mixture at all throttle openings, of course.
    Dan wants heaps of air, so that it noticeably affects the mid throttle range.
    Your trick worked for you at idle, but with enough of a leak to do what Dan wants, there's the potential to cook the top end.


    In reality, the majority of people find that the jetting is too rich standard and have to go down in main jet size. In my case, an 0.79mm jet was stock but I needed to go down to 0.72mm before the WOT mixture was right.
     
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