engine sputtering and dies below 10mph- won't idle

Discussion in '2-Stroke Engines' started by Fletch, Oct 14, 2010.

  1. Fletch

    Fletch Member

    I just got my bike running and it has a speed carb with the blue filter, multiple jets and the gas/fuel and idle screws which I've been tweaking, but I can't seem to get it dialed in. I can't figure out if I have too much gas or air, or if it is something else?

    When I start it I need to immediately excelerate (give it gas) or it will die out. It runs relatively well above 10mph but it kind of 'sputters' a little as I slow down and then it starts to die as it drops below 10mph (sputters more then stops).

    I don't know where to begin trouble shooting. Should I adjust the gas needle, the float, keep toying with the air and idle screws?

    The carb sits at an angle because I have the extended 'Z intake' and I wonder if that has anything to do with it?

    What should be the order I troubleshoot this?


  2. HeadSmess

    HeadSmess Well-Known Member


    sounds like the idle circuit... um.

    ok. checklist.

    height of fuel in the bowl. if the carb has a drain screw and a nipple for a hose, helpful. hold hose on side of carby. open screw. fuel fills tube, shows level inside the bowl. you want it just below the bowl rim/gasket. if not, adjust the floats.

    having no hose/drain is a bit harder. you gotta remove bowl screws. hold bowl on by hand and let carb fill. turn off fuel. remove bowl and floats. then check height. HT carbs good in one regard, the float is loose in the bowl.

    idle circuits. remove bowl, and the smaller jet. blow through it. go crazy with the compressed air... but not tooooo crazy :p

    get a new plug. warm engine up on old plug. then fit new plug. just force pedal it at the speed its not running right...10mph?. holding the throttle steady at whatever position it has issues...(idle?) for at least a 30 seconds. then kill it with the killswitch, stop. check plug. if its black already, the idle circuits rich and needs leaning out. if looks new still, its too lean. so somethings blocked or needs richening up.

    air leaks. get aerostart, spray around joins/gaskets... any change? theres the leak...

    i hate it when a carb plays up n wont behave :(
    Last edited: Oct 14, 2010
  3. Fletch

    Fletch Member


    Thanks for great tips!

    This is the carb I have.

    It has multiple jets so I don't know how I'd figure out the small one. Someone else with this carb wanted to rejet it but couldn't figure out the sizes by eyeballing it.

    That bottom jet in the pic is the fuel drain which is always open. They have the tube hooked up wrong. It's not supposed to be attached to that upper one because it creates a vacuum. The one closest to the air filter is supposed to be open and the one to the right of it closed. The one he has the fuel drain tube attached to is also supposed to be open. Screw on left is air/fuel, and right is idle.

    Other than the fuel drain tube there is a screw to empty it I believe. Wouldn't the fuel drain on this carb (with the tube) just drip out any extra fuel as needed? If I unscrew it and empty the bowl, how would I know how much fuel is supposed to be in it?

    I am using this plug:


    It is single tip and I gapped it to the spookytooth directions for the stock 3 tip plug (.36 was it?) Could the gap have anything to do with it? I noticed this disclaimer on that page: Note: This plug may not be for all engines. It has not been evaluated in all engines. It may cause piston contact in 49cc engines and others. If you have removed head or cylinder sealing surface material or use no or a thin head gasket there may be piston contact. The purchaser is responsible for evaluation of this plug prior to purchase. I have no idea how to evaluate piston contact.

    Just wanna be sure I'm following you here...

    If it starts to 'choke' out below 10mph will pedaling keep the engine running? Or are you saying to keep it at the speed where I start to notice problems. Sorry I'm not exactly clear on this one.

    BTW, how do you post an active link? I tried the httml wrap- no luck.

    Thanks again, I appreciate it :grin5:

    Attached Files:

    • carb.jpg
      File size:
      30.7 KB
    Last edited: Oct 14, 2010
  4. GearNut

    GearNut Active Member

    I do not know the exact location of the drain hose on that carburetor, but with most motorcycle and moped carburetors, this picture should get you on the right track.

    Attached Files:

    • carb.jpg
      File size:
      27.4 KB
  5. Fletch

    Fletch Member

    Gear Nut,

    I gotcha... I knew that part... I guess maybe I'm slow or something because I'm not sure but I think I'm close. So he's saying if you hold the tube closed (to capture the fuel) and open the drain screw, it will flow into the tube? I would think it would just empty to the ground?

    I'm assuming that once you capture the fuel, you wanna poor it back into the bowl to see what level it's at. Does it matter if the carb is at the same angle it's mounted when you put the fuel back in? I guess maybe it doesn't matter. For example if you have a glass of water and tilt it...the water on one side is going to be higher than the other. So I'd guess that high side would push the float up first and it should be at the same angle when I measure it, but I'm not positive.

    Sorry that was such a long explanation/question...I'm still learning this stuff.

    Also, would it be a good idea to check for air leaks first?
  6. Fletch

    Fletch Member

    I just realized that he may be talking about holding the carb at an angle so that it would poor into the tube and not out the screw lol. Am I getting closer?
  7. GearNut

    GearNut Active Member

    You almost have it. You connect the hose to the overflow/ drain hose barb on the bottom of the fuel bowl. Do not close off the other end of the hose. It must be left open.
    Hold or tape the loose end of the hose to the side of the carburetor so the open end is reasonably higher ( an inch or two) than the gasket seam between the float bowl and carburetor body. Open the drain screw and the hose will fill up with fuel and stop level and even with the fuel level inside the float bowl. It is the most accurate way to measure the actual float setting/ working fuel level inside the float bowl.

    As for the carburetor's installed angle having an affect to the fuel level and the way the float reacts to the fuel, you are correct. These carburetors are designed to be mounted flat and level, not at an angle or rotated on the intake manifold.
    This is not to say that they will not work angled or rotated, it is just not how they were designed to work and any extremes in this will have a negative impact on how the carburetor functions.
  8. Fletch

    Fletch Member

    I got it running good now!! :bowdown:

    Here's what I did:

    * Tried to capture the fuel in tube by opening the fuel drain screw. Fuel just came out the screw every time. I think maybe because the overflow sits above the float in the carb, so it will only release extra fuel?

    * I opened up the carb to look at the float. I pushedd the float all the way up with the fuel on and it wasn't stopping the fuel completely. So I adjusted the float by bending up the little tab that the plug 'needle' (don't know correct word) sits on. Pushed it up again and fuel was stopped completely.

    * One of the nipples that is supposed to be open on the carb came with the tube pushed into a hole in the air filter. I inadvertently left it out of the filter sticking straight up, and noticed a night and day improvement.

    The engine is running great now and idling! What a relief. So it turns out it was getting to much fuel which explains why the choke helped and I had to have the air/fuel screw all the way out before.

    After the first successful run there was A LOT of smoke! I read that is normal to have some smoke when breaking it in, but it still concerned me. I was using this top-end spark plug: http://www.sickbikeparts.com/catalo...ucts_id=111&osCsid=ioeut49puqd2ckrsbogn0sqgt4

    I gaped it to .36 like the instructions for the stock plug, but I don't know if it should be the same?

    This part was bothering me: "Note: This plug may not be for all engines. It has not been evaluated in all engines. It may cause piston contact in 49cc engines and others. If you have removed head or cylinder sealing surface material or use no or a thin head gasket there may be piston contact. The purchaser is responsible for evaluation of this plug prior to purchase.

    So after the smokey run I decided to put the stock 3 tip plug back in. Ran it again- no smoke. Could be purely coincidental? I don't even know what the "head or cylinder sealing surface material" is? I definitely don't want to damage the piston! How can I tell if there is piston contact?

    Thanks to everyone for their tips... They really helped!
  9. GearNut

    GearNut Active Member

    Congratulations on your repair!
    If the spark plug was hitting the piston lightly you would see a mark on top of the piston where the contact is made and possibly see a mark on the spark plug itself.
    If it is hitting hard the piston would not be able to travel up the cylinder all the way and the engine would not be able to turn over.
    In some cases the piston can overpower the construction of the spark plug and ruin/ mangle the spark plug tip while still damaging the piston in the process. The engine will still turn over although you would feel significant resistance while contact is made.

    Based on your description, I am now assuming that this carburetor does not have a drain/ overflow barb. It only has an overflow barb. The drain screw port does not have a typical internal passage to the barb. Usually one only has to loosen the screw 1 or 2 turns to get such a device to drain the float bowl. It sounds like with this carburetor the hole that the drain screw threads into is the only drain port and one needs to remove the screw to drain it.
    As stated previously, I am still learning about this particular carburetor and the information I posted is typical of common motorcycle and moped carburetors.
  10. Fletch

    Fletch Member

    Thanks GearNut,

    Yeah, this carb only has one drain screw and an overflow barb. I'm still learning about this carb too. It seems not many people have it, and the ones who do have to experiment with it through trial and error because there isn't much info out there on it. Most of the info I found here: http://motorbicycling.com/f4/love-cns-carb-20155.html

    The engine is still smoking quite a bit (it seems to be on the first run of the day) and that has me concerned. Is this a problem?

    The plug I have is longer than the stock one by about a cm. I'd prefer not to have to take the head off and look at the piston, but I suppose I need to. There was no apparent damage to the plug when I took it out.
  11. GearNut

    GearNut Active Member

    You can inspect the piston for contact damage by looking down the spark plug hole with a flash light. From what you are sharing, I do not suspect that you are having any issues with contact.
    Excessive smoking can be caused by a rich air/fuel mixture or a heavy pre-mix to gas ratio.
    32:1 is good for most pre-mixes. Some folks have outstanding results with using Opti II oil which is designed to mix at 100:1

    If the business end of the spark plug is black you have a too rich fuel mixture.
    If it is white, it is too lean. Chocolate to leather brown is an ideal color range. My spark plug is actually a grey-brown color due to the additives in the pre-mix oil I run.
  12. Fletch

    Fletch Member

    My fuel is actually 20:1 according to the spookytooth instructions.

    When you check the spark plug each time do you clean it off for next time?

    I actually have a new problem...

    After the bike was running and idling well the other night I decided to cable-tie all the cables down and wire tape all the wires. I also had to open up the top of the carb to switch out the black rubber cap because I realized it came with a HD throttle I'm exchanging.

    After that the bike was no longer idling, and when coming to a stop now it revvs full rpm without going anywhere. I noticed the ground wire had broken because I had it tightly smashed down in the engine mount. I connected another wire to it to lengthen it and taped it to the frame. Next ride same thing with revving when coming to a stop, and the kill switch wasn't working.

    I also loosened the clutch cable a bit and then put it back to what I thought was the same tension.

    My neighbor said that he thought it was a clutch problem (although he knows nothing about these bikes. He said that whenever you starve the engine or pinch the fuel line for example, an engine will rev up like that?

    Could it be electrical? The clutch?

    I'm going to take off the air filter and make sure the throttle needle is seating all the way, and the choke plug is seated all the way down too. Just to be clear... I think it was you or Al that said the remote choke actually is like an additional fuel way? So turning up the choke on the handle bar would actually be giving it more gas?

    Any suggestion?

    Last edited: Oct 18, 2010
  13. GearNut

    GearNut Active Member

    Electrical issues will not cause a high idle speed. There is no way a clutch can cause a high idle speed either.
    A restriction in the fuel flow will cause an engine to rev higher due to the air/ fuel mixture getting too lean (too much air) just before the engine dies from lack of fuel. If this were the case you would not be able to ride the bike under engine power as the engine would not get enough fuel to run at higher engine speeds.
    A tight bend in the throttle cable routing can though. It will not allow the inner wire to move freely inside the housing. Also, multiple bends in the routing can consume the overall working length of the inner wire also causing a high idle due to not letting the inner wire extend back out properly thusly holding the carburetor slide up too high. Usually there is enough adjustment in the cable end adjusters to compensate for this *if* the inner wire can still move freely.
    The choke on your type of carburetor is an enrichment device. It opens up another port, sorta like an additional miniature carburetor, to allow a little more air and a whole lot more fuel into the intake charge.

    Double check all of your wiring to be sure that there are no connections that have come undone or wires that got pinched through the insulation when you cable-tied everything.

    It worked before, just carefully go backwards through all you have done until you find what has changed it's ability to operate. Assume nothing and check out every detail, even if you think the change had no affect. Something affected it!
  14. HeadSmess

    HeadSmess Well-Known Member

    aye. ze throttle cable be jamming... maybe :)

    um...black rubber cap at top of carb? ehh?

    make sure its sealing the cable entering into the top of the carb... make sure the top of carb is sealed full stop.

    yeah. air sneaks in through there n gains access through the slide... leans out the mix, and is only noticeable on idle.... ratio of metered air to leaking air...

    blocked choke jet thing... as geary says, an enrichment system. adds air and extra fuel. its why some bikes (motorcycles) idle high on choke. cus of the extra air.
    but! if the jet administering the fuel for the choke is blocked... your adding air with little or no extra fuel, equals lean, equals revs really hi at idle...(with choke ON. no effect when its off. unless the choke plunger is leaking...)
  15. Fletch

    Fletch Member

    So I worked backwards and took some bends out of the throttle cable. I noticed that I wasn't hearing it 'hit' when I released the throttle. I adjusted loosened it to where I could here it hitting (bottoming out) again.

    I did some reading and decided 20:1 was overkill and changed the fuel to 40:1.

    I re-grounded to the engine and not the frame.

    Noticed that there was a visual gap between the head and the 'body' of the engine. (where the head gasket is) Sorry for my ignorance when it comes to terminology. THey sent me an extra head gasket with my kit so I doubled up the gaskets and increased the torque from 13 to 17 with my less than accurate torque wrench.

    Next ride goes well but doesn't quite idle. I figure it needs more fuel so I tighten the air/fuel screw and it idles. It's idling high so I unscrew the idle a few turns until it sound right. Run it again...perfect.

    Then today I go out and run it and it's running pretty 'jerky'. I start to mess with the air/fuel and it smooths out the ride, but now it's not idling again. Turning in the air/fuel is getting it close but not quite. I figure I basically still have a low fuel/air ratio. Could be the float needs further adjusting but I decided to lower the needle clip one notch for the **** of it first. Run it again and it starts much smoother.

    Seems I just can't get this carb tuned properly! I've read other threads where people had similar issues and just gave up on it completely and switched carbs.

    Maybe the jerking today was due to the cold/rain/engine not warmed up yet, and I should have just left it alone?

    I bent the tab up on the float quite a bit, so maybe it's a little too low now?

    The carb didn't come with the threaded part that screws into the hole on top of the carb where the throttle cable goes in. It has one on the choke hole where the remote choke cable goes in, but not on the throttle hole. To remedy the extra slack in the cable I used that part from a caliper break where the cable goes in and you adjust the tension with the nut, but there are no threads in the throttle cable hole so it just basically sits in there and isn't tight. I'm wondering if air may be getting in through there and through the slide? If I take that piece out, I don't know if I can get enough slack out of the cable with the throttle adjustment alone. If I take it out, the end of the cable sits directly at the base of the hole. I hope that makes sense?
  16. HeadSmess

    HeadSmess Well-Known Member

    answered yourself right there.