Engine Trouble Runs rough at low RPM?

Discussion in '2-Stroke Engines' started by Tressie, May 22, 2013.

  1. Tressie

    Tressie Member


    Just got my 48cc Happy Times running and have a quickie question about how poorly it runs.

    Regardless of throttle position, while at low RPM my engine seems to run very rough and might even be firing only on every 2nd revolution. As speed increases it reaches a point where power increases markedly, and it starts to sound like it's firing more consistently.

    My guess is that at low RPM I'm only getting enough fresh charge in the combustion chamber to ignite on every 2nd or so rev. It's maybe taking a couple of turns to exhaust enough spent charge and replace it with fresh mixture.

    So how do I address this? Enriching the fuel air mixture by changing needle valve setting might help, but I'm worried this might mess-up my fuel consumption. Lengthening the induction column length sound a better means to tackle the problem, so I would be very interested to hear others experiences with this mod.

    Thanks for any help or suggestions.

    T :ack2:

  2. crassius

    crassius Well-Known Member

    I'd first wait till the engine gets thru the run-in period.
  3. Purple Haze

    Purple Haze Active Member

    Sounds like the needle is set too rich. Raising the clip (lowering the needle) should fix it, and improve your fuel mileage.
  4. Tressie

    Tressie Member

    Thanks Purple Haze and crassius,

    Yeah, there are a couple of factors I hadn't considered then. The 'tightness' of a new motor may have some impact, as may the 16:1 run-in fuel mixture. Changing the plug to an NGK B6HS gave it a better idle, but it would only keep running at idle with the idle mixture screw all the way in.

    Well I guess the best place to start then is with the needle valve setting. Okay, I'll give leaning-it-out a shot this afternoon, and get back with the result.

    Thanks for the tips, Guy's. Muchly appreciated.

    T. :)
  5. Big Red

    Big Red Active Member

    Hey T man, First, If you have a stock carb, It doesn't have a fuel mixture screw. All it has is an idle ajustment screw. This has NOTHING to do with the fuel mixture, it only raises or lowers the idle. On a stock carb you will need to adjust the needle setting or change jet size to affect the fuel getting to the cylinder.
    Using a NGK B6HS is only the first step. The next thing to do is replace the plug wire and boot. Just use a good auto grade wire for this.
    Then, Make sure all wires are secure, not touching bare wire to metal, and soldered if possible. And never use the white wire for ANYTHING. Cut it off, cap it off or whatever, just don't use it. not even for the kill switch. For that simply take one kill switch wire to black, the other to blue. BUT, BEFORE you hook up the kill switch, hook up the ENGINE ONLY. Sometimes the kill switch itself is defective and will cause problems. So, Unhook the switch and try it.
    And crassius is right about letting an engine break in some. They always seem to smooth out a little after being broke in.
    Big Red.
    And I also meant to say, Purple Haze is probably right about the needle setting. It's something to try anyway.
  6. Tressie

    Tressie Member

    Yep, leaning-it-out improved the situation quite a bit. The idle speed went right up, but settled back to a nice smooth 'putter' when the idle mixture was wound out from it's fully-in position. I didn't really get a chance to warm the motor up much this afternoon to see, but low RPM running appears now much smoother, though has become a little sensitive to throttle position in achieving this.

    The spark I got with the standard CDI gear seemed pretty strong. I believe there are some issues with the stock ignition at higher RPM, but I'm aiming for a lower speed, de-tuned configuration of about 200W.

    Standard 20mm (19.1mm OD) copper water pipe and fittings appear to be available quite cheaply in oz, and promise to make an attempt at the extended induction column length mod fairly manageable. I wonder how well soldered copper fittings would take an electro-deposited coating of tin? Guess we'll find out.

    Thanks for the great input, fellas. Much appreciated.

    T. :)
  7. Tressie

    Tressie Member

    Okay, I'm slowly running-in this engine and it's running better with each slow 15-minute ride.

    What's the deal with chain and idler noise, though? I've got the chain nicely lubricated, and the idler is positioned straight, level and central, yet the rattle is driving me nuts. What's the cure for that? Do I have the chain on inside-out, or something?

    T. :confused:
  8. crassius

    crassius Well-Known Member

    the standard chain will stretch about 1/2 inch over the first 100 miles or so - adjust it as it gets looser and it will get quieter unless it's rubbing on something - I also like to grind the sprocket teeth narrower at the tips to help the teeth engage the center space of the chain more easily & quietly
  9. Big Red

    Big Red Active Member

    Pain in the @&& chain

    First, Start with making sure the drive sprocket is PERFECTLY straight and in line with the rear sprocket. You can use a straight edge, But I have a very simple way of doing this. Unhook the chain, make sure the tensioner is out of the way, and pull the chain straight back. You can move the chain left and right and will be able to see, on the drive sprocket, when the chain is coming STRAIGHT off the sprocket. The teeth of the drive sprocket will be centered in the chain. Then, very carefully, bring the chain STRAIGHT BACK from there. The chain should be hitting the rear sprocket dead center. If not, adjust the rear sprocket to do so. You may have to use thinner, (or thicker,) spacing to accomplish this.
    After you're very sure the sprockets are lined up, next comes the tensioner. Install the tensioner and lay the chain across it from sprocket to sprocket. I think you'll notice that the chain is not running straight in the roller. It goes from one side to the other in the roller. This is because the tensioner is straight, but the frame always goes to the rear at an angle. So simply twist the tensioner arm to fit the angle of the chain, and that the chain rides in the center of the roller.
    Then, (What!!! thought you were through?) Install the chain, raise the bike, pull in the clutch and spin the wheel. Because if you're using a rag joint the rear sprocket IS NOT perfect. I've been doing this for YEARS and hardly ever get this PERFECT. There is probably a little back/forth in the sprocket. This will leave you with a spot thats just a little tighter when the chain goes around. This tight spot is where you adjust the chain. If you adjust on a loose part, then when it hits the tight spot it will be TOO tight.
    Besides getting the engine straight and tight, This is the most critical part of any build. A chain that's not in alignment, or very, very close, will give you nothing but headaches. Yeah, A PITA, but it's gotta be done.
    Big Red.
  10. Big Red

    Big Red Active Member

    Hey crassius, I agree a chain settles down with some break in, But, If the chain is installed properly there should be no need to grind on the teeth. If yer having a hard time getting the chain aligned then, yeah, grinding the tips will help. I just gotta get the alignment right if it kills me though. Then again, Like I've always said.
    Big Red.
  11. crassius

    crassius Well-Known Member

    since the force of the chain is applied to the valleys rather than to the teeth, it won't hurt to grind the teeth and makes things quieter & easier

    costs the customer about $30 to install the sprocket, so $30 more to reinstall to push it over a 1/4 inch left or right isn't reasonable ($7 tooth grind is included in $30 install)
  12. Big Red

    Big Red Active Member

    Are you saying you charge $30 to install a rear sprocket, And $7 to grind the teeth. If you install the sprocket yourself it's free, except for the time involved. And I'm sure the guy can find a grinder or a file to knock the edges off the teeth. The whole idea, for me anyway, is to build MY bike MY way. I might ask for advice, But come he!! or high water, I'm building MY OWN bike.
    I've only got around 100 builds behind me. I'm sure there's lots of people have more. And like I said to the guy, The sprocket/chain alignment is one of the most important parts of the build. If you get that right there's really no need to file on teeth. If your chain alignment is 1/4 inch off then you did something wrong and need to do it again anyway. 1/4 inch is A LOT for a chain to be off, And if you don't think so, you need to talk to some of the other guys on the forum that have some experience. 1/4 inch might as well be a mile when you're talking chain alignment. If you are going to tell someone how to do something, PLEASE at least tell them how to do it the right way.
    Big Red.
    P.S. I charge $20 to install a rag joint, Including truing it at my station. And no grinding.
  13. crassius

    crassius Well-Known Member

    I get lots of college kids - building in their dorm room with 99cent store tools : (
  14. Big Red

    Big Red Active Member

    Now thats cool for a few reasons. Ya get a new batch of customers every semester, And when they leave I'm sure a lot leave their bikes behind, so you probably get some good used bikes and a lot of good used parts.
    I live in a college town too, except I'm in silicon valley, San Jose State. Most of the kids here drive VERY NICE cars. Most of my customers are a little older and looking for a cheaper way to get around. If they bring me a bike thats engine doable, I can get them "out the door" for around $300, A LOT cheaper than a car. I only charge $100-$120 labor for a stock build. anything special is more.
    So I guess I can't blame you for charging $30 for a rag joint install. If I had to deal with a bunch of kids screwing up EVERYTHING, I would have to charge more too, If just to stay sane.
    And OK, grinding the teeth wont hurt a thing, I just don't see the need if properly installed. But THAT'S nearly impossible with tools from the 99cent tool bin. So CHARGE them up the wazoo, I would.
    Big Red.
  15. crassius

    crassius Well-Known Member

    what started me on grinding was a youtube vid showing bike spokes flexing in a turn at only 25mph

    they move a LOT and with the sprocket mounted to them, I became nervous about how the chain was moving at that time when these things go closer to 30-35mph
  16. Big Red

    Big Red Active Member

    Yeah, Spoke size has always been a problem with me. A standard spoke size is around a 14 guage. I recommend at least a 12 guage, and even better a 10 guage. Also, if you've ever read many of my posts you know I recommend using a sprocket adapter. I just don't think driving a wheel from the spokes is a very good idea. I've installed a lot of rag joints and most of the time they work fine. Then I also get a lot of broken spokes, always on the rag joint side. So I try to talk my customers into a sprocket adapter, but if they can't or wont I warn them about the possible problems with the rag joint. Then I charge them accordingly for replacing broken spokes.
    I don't really have many chain problems with any of my builds. The problems arise when the customer lets the chain run too loose and neglects to do maintenence such as lube it once in a while. I run my personal bike at around 35mph almost every day. The chain has never come off yet, (knock on wood.) But now that I've made this claim I'm sure it will come off today.
    Big Red.
    P.S. Just noticed I suddenly became a "Senior Member". Guess it's 1000 posts to get there.
    Last edited: May 27, 2013
  17. crassius

    crassius Well-Known Member

    if the adapter came with the kit I'd try it, but then if like most I see, it will only fit coaster brakes I wouldn't try it since I refuse to build a coaster brake bike

    got a pile of dead coaster hubs out back that all cracked or completely sheared off the end of the hub from excess heat
  18. Big Red

    Big Red Active Member

    Never seen a sprocket adapter come with the engine kit. And they aren't cheap, $60 - $80. But you can get one for almost any hub, not just coaster hubs. I just put a Clamshell Adapter on a tadpole recumbent multi-geared bike. My Cranny has a coaster hub with the Howard Adapter, No problems after 2 years of riding, but I don't rely on the coaster brake. In fact I hardly ever even touch it. Sometimes just to stablize the bike when applying the front brake hard.
    Big Red.
  19. Big Red

    Big Red Active Member

    Sprocket Adapter.

    So, just so ya know. Heres a couple pics of the livefastmotors clamshell adapter on a multi gear bike. I just took it to 7-11 and back to get smokes. The thing runs great.
    Big Red.

    Attached Files: