I think I've got a vacuum leak!

Discussion in '2-Stroke Engines' started by Motorbike Wanabe, Dec 26, 2009.

  1. Just finished putting a Grubee GT5 on a friend's mountain bike. The engine runs like dog unless you switch on the choke, just a little bit. I'm thinking i might have a vacuum leak to locate. With the choke part way on, the engine accellerates much better, even when up to operating temperature. Turn the choke off and it instantly bogs down. It's a fresh engine so it might need a little more break in. I've got two good heat cycles in it right now. Will try another one tomorrow.

    Andy help is appreciated.

  2. professor

    professor Active Member

    The guys were talking about spraying something around the likely culprets and seeing if the engiine rpm and tone changes.
  3. AussieSteve

    AussieSteve Active Member

    Aerosol carb cleaner, brake cleaner etc. Start where the carb mounts on the inlet, then where the inlet bolts to the cylinder.
    Check that the nuts are tight and visibly check the inlet gasket for apparent damage.
    ... Steve
    Last edited: Dec 27, 2009
  4. arceeguy

    arceeguy Active Member

    Before you go chasing any leaks, raise the jet needle in the slide throttle to enrich the mixture and see if that helps. (lower the little c-clip a notch or two) You can also raise the float level a couple of mm to enrich the mixture too.

    Too many people start looking for vacuum leaks first, rather than last. If there was a vacuum leak that leaned out the mixture enough to cause bogging, it would be large enough to severely upset the idle mixture - enough to cause the engine to stall or to have a high idle even if the throttle stop screw is backed out all the way.
  5. AussieSteve

    AussieSteve Active Member

    Where I come from, I was taught to ensure that there were no leaks, the fuel filter was not clogged and that the air filter was clean before tuning, not after. This is especially important when there is a problem.
    There's no effort in, or harm done by, a quick squirt of aerosol to double-check the joints, before re-tuning.

    ... Steve
    Last edited: Dec 27, 2009
  6. Al.Fisherman

    Al.Fisherman Active Member

    I'm with you Steve, a spray or two is by far as the easiest check. Don't forget the carb cap where the throttle cable goes through. I'd be a little concerned as to using brake cleaner. That is what I used to remove ALL the paint from a "BLACK" painted engine, after the paint started to come off.
  7. arceeguy

    arceeguy Active Member

    Well, you guys do things your way and I'll do things my way. Like I said before, if there was evidence of a vacuum leak (stalling, high, unadjustable idle) then of course that would be the first place I would look. But the symptoms are not consistent with an air leak, but with a carb that is in need of adjustment. Where I come from, you troubleshoot based on symptoms and not waste time with things that are low probability cures. ;)
  8. AussieSteve

    AussieSteve Active Member

    Motorbike Wanabe said that he thinks he has a vacuum leak and asked for help. We merely told him how to check for it. And the choke thing is a good sign that there may be a leak.

    Ron, good point. I didn't think about brake cleaner as paint remover. Mine's an unpainted engine. I forgot that some are black.

    ... Steve
  9. arceeguy

    arceeguy Active Member

    Well, if the engine doesn't idle unless the choke is on, then yes, there may be a pretty large air leak. If the engine simply performs better with the choke slightly "on", then it is most likely a carb tuning issue.

    I've seen too many people in this forum chase their tails for "air leaks" or "vacuum leaks" to find out that their issue was something totally unrelated.
  10. Well, spent some time tinkering with it. I tried moving the needle, didn't benefit at all. I had some teflon tape wrapped like a rope around the outside of the threads but, as soon as we removed the top, the tape went south and we've not been able to seal it properly again. It runs better with the choke halfway closed. My understanding is that the additional fuel from choking it is compensating for the additional un-metered air that is coming through the various leaks.

    What have others used to seal up these little carbs? I used a little teflon tape on my engine and it runs great with the clip on the highest groove of the needle, just sipping the fuel.

    I got some O-rings and put one between the cap and the carb body and put one inside the carb, between the intake and the carb. I'm hesitant to keep one in there for fear it might get injested inside the engine. Maybe I worry too much. :confused:

    Anywho, with the 0-rings in place, I can get it to run somewhat okay. Mine is running much smoother than this one but, mine is broken in pretty well and that may make the difference. Still trying to work out the bugs on this one.
  12. arceeguy

    arceeguy Active Member

    If it idles with the choke off, then I doubt you have an air leak.

    An air leak is constant whether you are idle or full throttle. If you have a leak large enough to upset the mixture at full throttle, it will certainly be large enough to wreak havok with the idle mixture.

    Here are some random thoughts:

    If you have another carb that works well, you might want to switch them and see what happens.

    A quick check of float level would be to push the "primer" button while riding. The button doesn't actually pump fuel - it pushes the float down so that the fuel bowl can fill up more than normal. Raising the fuel level in the bowl will enrich the mixture. So if you push the primer for a few seconds, the engine may perk up for a little while until the fuel level drops to "normal". I've seen a few carbs that had misadjusted floats, causing flooding or starving.
  13. I'll have to try that with the primer. I hadn't thought of that. So, in this case, if it improves, I should assume that the float is set too low?
  14. Catfishjohn

    Catfishjohn New Member

    Are the air leaks at the cap and betreen the carb and intake pipe sealed now that you installed the o-rings?
  15. I think that the cap on the top is sealed much better because of the difference that it made. I want to take the carb back off of the intake and check to see if the end of the intake tube is cut on too much of an angle. There's just a little bit of area inside the carb that doesn't have a slot cut into the edge of the carb bore and I want to be sure that the intake tube goes all the way in, past those slots, for the entire diameter of the bore. If it doesn't, putting an o-ring in there is just going to make it worse, IMO. I'll be playing around with it a little this AM, before I "go to work".
  16. AussieSteve

    AussieSteve Active Member

    Hello Motorbike Wanabe, I had the same problems. I used a fibre washer intended for an auto sump plug inside the carb top cap and then bought an alloy inlet tube and cut a groove for an 'O' ring.
    See:- NT carby/inlet leaks - a reliable solution
    Also check the inlet gasket and double-check tightness of the inlet flange nuts.

    N.B. Before fitting the 'O' ring to the inlet tube, I used to use Permatex non-hardening 'Make A Gasket', smeared lightly on the sealing area of the carb.

    ... Steve
  17. I've been doing some playing around with this engine and I've about run out of ideas. I put some permatex on the inside of the carb and slid it on. I'm not sure if it helped or not but, it did fill in all the slots on the carb flange. I figured that should help some, if not all of the problems. I also put some teflon tape on the cap of the carb, along with the o-ring. I would think that would cure any leaks for sure.

    I've changed the needle setting numerous times, trying to get it to run better. I've got it on the 3rd from the top right now and if I go down one more, it starts to really dog out when accelerating. But, as it is right now, when it warms up, I hear, what I believe to be pre-ignition, or detonation when climbing a hill or accellerating. That's not good either. If I run with the clutch on just a little bit, the pinging goes away. That leads me to believe that it's still running lean at that setting. Idles fine without the choke on once it warms up. I even changed tanks to see if maybe the other one was creating a vacuum. Nope. I'm thinking of contacting Gas Bike, where we bought the kits and seeing what they have to say about it. Any other suggestions? I'm open.
  18. AussieSteve

    AussieSteve Active Member

    This sounds like a rich mix. Possibly you've gone from one extreme to the other. You say it dogs out when accelerating. Is that with the throttle wide open, or 1/2 to 3/4 open?
    Does the (four-stroking?) stop if you fully open the throttle, (WOT)?

    Have you tried going the other way with the slide needle? Mine used to bog down badly after I sealed all of the carb leaks and now runs best with the needle at it's lowest, (leanest) setting. (Clip in top notch)
    I also went down in main jet size, to 0.72mm. (My stock jet was 0.79mm but this varies from vendor to vendor.)

    This sounds like you might be describing 'four-stroking' rather than pre-ignition. Another sign of a rich air/fuel mix.
    There's more info and a link to a YouTube video of four-stroking in this thread:-
    NT Carb Tuning Basics

    ... Steve
    Last edited: Dec 28, 2009
  19. If I lower the needle (raise the clip) it starts to ping pretty bad. That would lead me to believe that leaning it out makes the condition worse. When I lowered the clip, it didn't ping as much and when I closed the choke a little bit, it went away. I thought that I could put the clip on the lowest setting and that should help but, it didn't. It just bogged down when I tried to go up a hill. Now, I'm thinking that I might be able to lower the clip and change the jet sizing to a smaller jet. That might help with the bogging when going up hill at full throttle. I would assume that the needle, while it mostly works from idle to 70% throttle, would still have some bearing on the overall A/F ratio, especially if the A/F was rich to begin with on the jet sizing. I just put some teflon tape on the carb/intake connection. I haven't had a chance to see how that does but I might tonight or tomorrow.
  20. AussieSteve

    AussieSteve Active Member

    Are you sure it's pinging? That's not likely.

    This indicates that it's too rich. If it bogs more when you lower the clip, (raise the needle), then it's probably not lean in the mid-throttle openings.

    Have you tried a spray of carb cleaner near suspected leaks yet?
    Have you done plug chops? Both at ½ and full throttle would be good.
    And did you check out the 4-stroking video, to be sure that what you think is pinging isn't 4-stroking?

    ... Steve