Dreaded exhaust leaks..


  • Global Moderator

    After owning a few 2t mx / enduro bikes and having never purchased a new pipe for any of them, I've become accustomed to the traditional 2t cylinder / exhaust leaks that always seem to occur on a well used bike!

    I bought a second hand, (but not too badly dented) FMF pipe for my KTM 125 sx and it now seals against the cylinder perfectl!

    So next is my KTM 250 exc, I can visibly see the smoke coming out of the leak and as I'm having trouble finding a second hand pipe for an 02 model, I've temporarily bought some general exhaust sealant http://pages.ebay.com/link/?nav=item.view&id=301682878989&alt=web

    Just wondering if this is good enough to not bother with buying another pipe? Is a leak really that important on these bikes?!



  • It's a bodge. What you want to do is fix the problem.

    So on those isn't it tightened down by springs?

    There is usually an exhaust gasket that sits inside the flange. So renewing that is also a good idea.

    I don't like using exhaust paste as it wasn't the case when it left the factory. Best thing to do is to completely clean and dry the mating faces to a aid a better seal.


  • Global Moderator

    Yes from factory they mostly all use bloody springs... Ridiculous idea imo. Every time you drop a 2t bike the pipe is gonna get dented, or worse in these cases dented and bent out of shape. When they are bent out of shape, the mating surfaces no longer sit flush. The only thing would be to physically try and bend them all back into shape, but the risk is that they will split.



  • A good article about dents in two stroke pipes. Quite interesting really lol

    http://motocrossactionmag.com/news/this-much-we-know-the-truth-about-speed-dents-rock-dings-and-pipe-catastrophies


  • Global Moderator

    Seen that somewhere on old forum lol.. But I still don't know whether the exhaust not sealing against the cylinder fully is causing any sort of unusual behaviour from the engine. I know if it doesn't seal very much, my kx 100 will not idle, so that's something. I read that exhaust pipes allow each puff of exhaust fume to kind of use each others momentum like F1 cars do. Slip streaming kinda..



  • So two strokes are supercharged.

    They employ forced induction.

    The sound wave creating in the exhaust pipe causes a pressure wave that forces lost fuel and air charge that is lost off of the exhaust port back into the combustion chamber.

    No representation describes this better.

    Having a decent seal between the exhaust header and cylinder is crucial.

    Putty isn't the solution here. What is is a quick skim of the exhaust port and proper fastening of the header. So from experience these sorts of bikes employ different methods of mounting the expansion chamber to the bike than the DT's.

    From memory they have an exhaust header, this fastens to the cylinder, then the expansion chamber slides over the top and springs pull it close.

    I won't lie, I don't know why this is the case, but I haven't seen a motorcross bike that doesn't leak oil from the header. I will research this and find out though.


  • Global Moderator

    My 125's exhaust goes into the cylinder's manifold, where as my 250's exhaust covers the cylinder's manifold. The 250 is definitely worse at sealing, however it is an 02 compared to my 125 being at 06.



  • I think I know what you mean. But wouldn't mind seeing some photos. I wonder why they've done that.