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• Top End

  Top End Related
  55 Topics

  668 Posts

  S

  My 3nc is fine as well. But nobody said they were bad. Just that the 3mb were a tadpole better 😜 …
  With the early bikes a fat strong spark can make a real difference. As soon as I put my lights on it zaps my tick over. I can see it going from blue to orange in my minds eye! …

• Bottom End

  Bottom End Related
  7 Topics

  42 Posts

  H

  Sorry for commenting on a 3 year old thread, but I have worked a little with balance shafts on humongous machinery and wanted to post a little for posterior knowledge.

  A single balance shaft on a single-cylinder doesn't really do much. It mostly just phase-shifts the vibrations so they go in another direction. Back and forth instead of up and down, like.

  Dual balance shafts is another thing. They cooperate in the right direction, and counteract each other in between - and if you put them slightly offset each other vertically they can even mitigate some rotational moment.

  But balance shafts also have another interesting effect - they can reduce the bearing load. There are actually two different optimums - one which gives the least vibrations but a bit higher bearing load, and another that is the opposite.

  So when I see that small engines have a single balance shaft, I usually assume it is to allow it to manage high revs by means of lowering bearing forces rather than reducing unpleasant vibrations.

  There was a bike back in the 90's(?) - some european single-cylinder 2-stroke 250 cc enduro (cross?) - that had a balance shaft and managed to output 70+ hp when all the balance-shaft-less big-brand competitors only gave 55 hp or so. Because it revved to like 12000 when the others quit at 9000 or whatever.

• Carburetor

  Carburetor Related
  93 Topics

  954 Posts

  H

  The cylinder sucks in air at intervals, leading to a pulsation in the inlet airflow. That bottle is a buffer that smooths out the pulsations.

  The piston goes up, sucking in air through the carburetor which is (and should be) the main restriction in the intake. This causes a low pressure on the engine side of the carburetor, including that bottle.

  When the piston goes down and closes the intake port, there would be a rather sudden pulse where air rush in through the carburetor and meets a dead end. Pressure rises, gas stops throughout the manifold, including the carburetor. Then when the port opens and the piston sucks in gas again, it would have to accelerate the gas mass from zero.

  With the bottle, the volume of that closed-off manifold is artificially increased, so it takes longer for the gas to fill the cavity and come to a stop. Hence it keeps moving, supposedly until the port opens again and the piston suck in gas again.

  The effect should be that there is a more constant flow of air through the carburetor, which likely improves the atomization or decrease condensation or whatever.

  4-stroke engines usually tune the intake manifold runner lengths such that the air pulsations should cause the "pillar of gas" that comes at the intake valve at high speed should "ram" through it when it opens. While the mass of the air is small and it is very elastic, the velocities are substantial so it counts. I wager this is sort of the same thing.

  In 4-stroke engines it is also not uncommon that the intake runner lengths vary with engine speed. I know my car has a valve that opens and shorten the length above some rpms. My Honda Fireblade also had something like it in the airbox.

• Derestricting

  Return the bike to how it was intended to be!
  22 Topics

  368 Posts

  C

  @Calum said in Restrictions:

  @tunertom Yeah that inlet manifold is a load of tosh, but don't discount Scrim, he knows his shit.

  @CYBER-NINJA haha yeah I did chop my exhaust up and removed the cat, but oh my seriously those pipes have some serious weight, so if not for the performance the weight alone is worth swapping out lmao!

  Hi I have his destruction manual and seems mostly since 🙂