First build questions from a N00B -- 4 stroking, idling, etc.

Discussion in '2-Stroke Engines' started by jesse52, May 2, 2010.

  1. jesse52

    jesse52 New Member

    Hey everyone,

    Last weekend I finished building my bike, and man am I happy with it! However there are a few minor issues I can't seem to work out.

    My engine is a standard 49cc HT (PK50B). I replaced the stock plug which was slightly broken to begin with with a NGK B6HS, and just today I took it out after 10km, and it's already black with carbon. The problem I am guessing is a much too rich mixture, so rich that the bike is 4-stroking the entire time, with low rpm. I adjusted the screw, c clip, everything you could normally do. cleaned the air filter, pretty much everything but no change. It seems that when I got the bike it would stop 4-stroking when I accelerated, but it's not even doing that now.

    2nd problem... Idling. I'm new to this, but I'm not completely ignorant. Basically, should the bike idle when the throttle is completely down? Like... if the cable wasn't even there would it idle normally? Right now I have the throttle adjusted so that I adjust the idle by adjusting the throttle cable, the screw does pretty much nothing.

    Also, another note is that the kill switch that came with the bike did nothing so I removed it, and now I just kill the bike by pulling up the choke. Is this bad for the engine? Oh... and is 4-stroking bad for the engine also?

    Thanks. This is a great forum and without it I would be completely lost.


  2. AussieSteve

    AussieSteve Active Member

    You should be able to set the idle with the idle screw, if you turn it in far enough. It's not a good idea to do it with the throttle cable. It needs 2-3mm of slack, otherwise the engine revs will increase when you turn the handlebars to the sides.

    Sounds too rich to me. Mine was similar when I got it. I had to go down from an 0.79mm main jet to an 0.72mm before my engine would run properly.

    Four-stroking won't damage your engine, but as you've noticed, oil and carbon will build up quickly.
    It should be OK to stop the engine with the choke, but a kill-switch is preferable.
  3. jesse52

    jesse52 New Member


    So that means that It should Idle when the throttle cable is not pulling, right? OK.
    Yeah. Now where would you suggest getting a new jet?

    That's what I thought.

    Thanks for the reply.

  4. AussieSteve

    AussieSteve Active Member

    Jess, if you're in the US, try DellortoDirect

    If in Australia, either OzMiniMoto or Rock Solid Engines, (Email Tony).

    For an NT carb, the right main jet is the one for a Dellorto 'PHBG' carburettor. It has a 5mm thread.
    I bought a range, 0.72mm, 0.73, 0.74, 0.76, then worked down through the sizes until I got it just right.

    You might find this thread useful, too:- NT Carb Tuning Basics
  5. jesse52

    jesse52 New Member

    I'm in Canada actually :p and based on what i've researched, I'm surprised you would recommend DellortoDirect.

    I'm not even sure what size my main jet I'm using right now is, and I've read that NT Carb tuning post already (many times. :p) It says the stock would be about .7mm, so I'm not sure that going higher is a good idea... but again, I don't know the actual size of the one I'm using right now.

    Thanks anyway.

  6. AussieSteve

    AussieSteve Active Member

    No, I don't recommend DellortoDirect, just letting you know they exist. They're the only source of jets that I know of in the US.
    My problem stemmed from the fact that I'm in Australia. Apparently it's normal for funds to be frozen during an international transaction. I'll personally never deal with them again.

    Regarding your jet size, you could try asking the vendor that sold you the engine kit.
    It's most likely an 0.7 or 0.79. Possibly you could buy one or the other and compare the new one to the current one, then work from there.
  7. jesse52

    jesse52 New Member


    So far I've fixed the idling bit, just had to put the idle adj. screw in more.
    I drilled 2 more holes in the air intake casing for extra flow, and cleaned out the filter which was surprisingly soaked with gas (is this a problem?)

    It seems to run a bit better, but also seems to be misfiring. That just might be the fact that my plug is already pretty much fouled by the extremely rich mix that I can't fix. I really don't want to have to buy jets or anything, if there's any other way I'd like to know.

    Another question... is it normal to have to try at least 4 times to get it started, and to have it fire very infrequently until it does?

    I'll post a video in a while for anyone who wants to see what I'm experiencing.

  8. AussieSteve

    AussieSteve Active Member

    It's normal for these 'port scavenged' 2-strokes to spit some fuel back through the carb. Because your's is running rich, it'll be spitting a bit more than usual.

    The only alternative to buying jets is to solder and re-drill the existing one, but I don't recommend it. Jet drills need to go up/down in 0.01-0.02mm increments, so you can't use normal drill bits. By the time you find jet drills, you might as well buy jets.

    It should start fairly easily. In my case, 2 attempts are needed - one with the choke fully on, then one with the choke off and it fires right up. (I kick-start on the centre-stand.)
  9. jesse52

    jesse52 New Member

    Yeah that's what I thought (to first and 2nd comments)

    Why do you think it's so hard to start? Could it be too much fuel in the carb to start? Because I can never get it to start with the choke fully on, I put it 3/4 or full the entire time.

    Tomorrow I'll post a video of my problems if stops raining over here long enough for me to make one.

    The one thing that confuses me, though, is that the only reason I know I'm 4-stroking is because it used to occasionally go 2-stroke and it would be much faster and smoother running without extreme vibration. If I hadn't experienced this, I would say it was running ok. Wierd.
  10. AussieSteve

    AussieSteve Active Member

    As you mentioned, the hard starting is probably the result of a fouled plug. Try cleaning it with a wire brush and fuel, then set the gap to 0.024", (0.6mm).

    Regarding the rich mixture, check the float level, (21mm as shown in 'NT Carb Tuning Basics') and also ensure that there is no fuel in the float, (punctured float).

    Mine doesn't start with the choke fully on either. It fires once, then I turn the choke right off and it starts instantly.
  11. jesse52

    jesse52 New Member

    It was the same when the plug was brand new. The plug is already conveniently gapped at 0.024", I checked ;)

    I will do that tomorrow before I make the video. Sounds easy enough.


  12. jesse52

    jesse52 New Member

    Alright, I think I have it running pretty decent now. Still quite rich, but it will do until i can get the new jets.

    I went to Dellorto's website, and I can see here that the jet sizes are NOT in mm, but in a # system that I have no idea how to understand.