I think I have a timing problem?

Discussion in '2-Stroke Engines' started by Fletch, Feb 1, 2011.

  1. Fletch

    Fletch Member

    My engine is running great, except when under heavy load going uphill for an extended time.

    *expansion chamber

    *no base or head gaskets

    *no porting

    After going uphill for a while at full throttle the engine starts to bog and then die. I don't have an air leak.

    After it quits, I can hear the engine continue to rev in a few short bursts like, "rvvvvvv.....rvvvvvvv.....rvvvvvw." Once it does that I can restart the engine and go again. It's like it is clearing itself.

    I'm not sure if this indicated too little timing or too much? I just read that under load the timing increases (or needs to).

    I pulled my plug but it has been used at different settings (ie. it's already colored). the tan ring around the center electrode insulator only goes about half way around on the side of the side electrode. There's some carbon build up on one side of the center electrode. If someone is really good at reading the plugs in detail I can post some pics.

    I'm wondering if I need to put a base gasket back in, or if I just need to tune the pipe? Maybe lengthen the copper adjustment tube?

    By having no bottom gasket that should be good for intake but bad for exhaust right? Instead of raising the exhaust it is = to lowering it a little. I'm stuck on this one.

  2. wackey101

    wackey101 Member


    simple check your ignition wires some times they come loose when you rididng i use to have the same problem figured out it was my ignition coming looser and looser and if not check your spark plug if its looose
    thnx and put gaskets how they were
  3. Fletch

    Fletch Member

    Thanks for the tip. My wires are soldered so I don't think they're loose. My plug isn't loose, although maybe this is a good time to ask what the torque should be (in ft. lbs)?

    If it was loose wiring/plug... why would it only happen under the conditions I described, and not randomly?

    I'd really like to know if this could just be a tuning issue with my expansion chamber. Maybe I took too much length off? This would be the easiest fix.
  4. a/c man

    a/c man Member

    Sounds to me like a fuel flow problem.
    I would bet you're running your float bowl nearly empty
    with sustained uphill wide open throttle.
    After it sits for a moment it has a chance for the fuel level to recover
    in the float bowl.
    Here are some things to check:

    Try running your bike under the same conditions you are having trouble with, but without the fuel cap on the tank. A clogged vent is like holding your thumb on the end of a soda straw, doesn't drain

    Clogged inlet screen on "in tank fuel petcock"
    Restriction in fuel petcock itself... Pull off your fuel line at the carb and see how well the gas flows out with valve open, a small stream is enough, it doesn't gush but dripping in not enough.
    The fuel valves have a very tiny crappy opening for fuel to flow thru.
    SBP has a nice one.

    If it's see thru, it should be nearly fuel under all running conditions

    I'm willing to bet it's one of these, or a combination of a few.
    Don't be afraid to experiment by using the process of elimination, stating at the tank..
    Good luck and check back in when you find the culprit..
    Last edited: Feb 1, 2011
  5. Fletch

    Fletch Member


    Thanks man... My first thought was that it was fuel starvation too, but the revving after it dies threw me off that. I did wax the tank and cap so maybe it's blocked. I wasn't using the screen until recently, but the engine was doing this before I put it in.

    THere's no chance that it has to do with my float level setting right?

    Ok...No gas cap made no difference. Normal hills are fine, but the one I'm riding up is a monster! My filter is always full, and I really don't think it is a gas flow issue. I did lower the float a bit because I had some leaking out of the carb. Maybe I lowered it too much? That shouldn't matter though because going up a large hill is going to drain the bowl eventually right?

    Maybe because the expansion chamber uses up more fuel, I need a larger carb (new Speed NT, or CNS) if I want to full throttle monster hills. I did try going up the hill half throttle and that was fine.
    Last edited: Feb 1, 2011
  6. wackey101

    wackey101 Member


    honestly if it were me i would buy a new carb if you have the stock one get a speed carb thats all i can say
  7. Fletch

    Fletch Member


    I bought a speed carb but the threads broke out where the air filter screws in using an aftermarket filter. I found out I had a broken rear mount stud so the vibration probably caused it.

    I tried to recreate the Speed carb using a dremel on a standard carb. I haven't given it a go yet because I put all the jets and needles in the same bag and don't know which ones came from the speed carb. Do you know what the needle looks like? Is it shorter and thicker?

    I think I just set my float too low, but I want a better carb anyway. I don't have good luck with them though... 3 broken CNS, 1 broken Speed.
  8. wackey101

    wackey101 Member


    the pin for the speed carb is shorter than the one on the stranded nt at least that like mine
  9. wbuttry

    wbuttry Member

    Well i got similar problem on flat land if im running full throttle for over a minute it starts bogging down .And eventually dies and half to restart it. 1/4 mile is all i get at full throttle before it bogs down. And i dont see nothing on that carb to adjust beside the idle screw and that doesnt do much just idles it down or up what yah think is ours similiar or what?
  10. Fletch

    Fletch Member

  11. Fletch

    Fletch Member

    I don't think so. On flat land or even slight grade I have no problem. It sounds like you have an air leak. Do you have an expansion chamber?

    I have concluded by process of elimination that I am running out of fuel on decline/incline because the float is topping out sooner, so the bowl is not filling all the way. This is normal of course, but my expansion pipe must be revving higher and burning more fuel than the stock carb can replace with the small bowl.

    Someone please correct me if you think I'm wrong. I don't have an air leak...I checked with butane everywhere and my carb is siliconed closed and to the manifold.

    If I lengthen my header (lower revs)... will that fix it?

    This can't be the result of having no base gasket right? I mean it can't thrown the timing off that much?

    *** Update ***

    OK so I want to fix this dying and revving a few times after on steep hills problem so I have a list of options here. Please let me know which ones I should try and in what order, or which would be more relevant...I think I have blow back into the intake possibly?

    A) Get some copper pipe and lengthen the header some on my expansion chamber
    B) Wait for a new carb upgrade (Speed or CNS)
    C) Try using the jet from my broken Speed carb if I can figure out which one it is (ie. increase jet size)
    D) Use a base gasket
    E) Use a head gasket
    F) Take the muffler off the pipe and see how it runs
    Last edited: Feb 3, 2011
  12. Dave C

    Dave C Member

    I'm thinking that without the gaskets when you pull a hill the temps go up and you have more expansion of parts and your seizeing up and when it's cooled down the clearances go back up and there's no problem. Look for scuffing or hammer marks on your parts when you put the gaskets back on:cool:
  13. Fletch

    Fletch Member


    You're right!

    I rode 80 miles on it 2 days ago up some monster hills! What I did was add a head gasket and gap my plug to .40. I know that seems huge, but Al fisherman recommended it in a thread and said it helped with hills. I did both at the same time, but I'm sure the head gasket made most of the difference.

    On the 40 miles back home, by that time it had dropped from like 67 deg. to the mid 40's I'd guess, and there was zero vibration on the bike. Maybe I was just numb from freezing my *** off for so long, but I noticed a huge difference. This could also support your theory (which I now believe is correct). Doesn't the fuel lean out when it is cold, and that's why you need the choke to start in really cold weather?

    I might even add one more head gasket to be sure there is no piston contact. I always assumed the piston would hit the plug 1st, but after looking closer I saw that the outer edge of the piston would hit the head before the plug would.
  14. jaguar

    jaguar Well-Known Member

    Sounds like you solved your problem. I was thinking that if at full throttle it dies but not at half throttle then your main jet is too small.
  15. Fletch

    Fletch Member

    That's kind of what I was thinking. I switched over to the speed NT carb which supposedly has a bigger main jet, only I had taken the 3 nt's I've had apart and don't know for sure if I put the correct jet back in it. I had the same problem going up that hill again only I was able to pedal assist it without it dying. I ordered a Mikuni 18mm carb which will be here Mon. I'm going to see if that makes a difference. I ordered 2 jet sizes below what it comes with (90) and 3 pilot sizes to play with.