Flat spot??Carb prob.

Discussion in '2-Stroke Engines' started by grinningremlin, Nov 5, 2013.

  1. grinningremlin

    grinningremlin Active Member

    I've only heard the term in VW circles, I'm thinking there's a flat-spot in my carb (Tanaka PF4000, Walbro WYK28B, plug nice chocolate brown).When I get to about 3/4 throttle the engine bogs a bit, push the throttle past the point the engine takes off fine, go a little back from the spot and it accelerates fine.Is this a rebuild/replace or am I missing something?

  2. HeadSmess

    HeadSmess Well-Known Member

    if its always done it, since brand new, while under load... its more related to the piston port. i know some of my engines get super grumpy in the mids, depending on what pipe and manifold im using...

    if its just started doing it....yup, tweak the carb a bit. rebuild kit is a bit pointless unless theres actually something busted. should get away with the strip, blow out and reassemble.

    WYK is the rotary barrel type, ja? some are completely unadjustable. some have a hi speed screw. some have a low speed, thats a tiny lil screw in the centre of the barrell. some look like they adjust but when you pull that screw (actually its the needle) you find its bottomed out as standard, unadjustable! if it has a spring, its an adjustable type.

    and then some have a nifty choke that lifts the barrel.

    (ha ha, i couldnt be bothered googling your carb)

    reminding me. youll notice as the barrel revolves, it also lifts up on a ramp. this obviously lifts the needle from the jet, increasing fuel flow as air flow increases. regards the choke, lifting it without rotating will obviously give you a rich mixture, or choked :) easy, huh?

    the ramp might be worn down? would make it lean at times. something , ie the cable, might be lifting the barrel off the ramp? that would make it rich at times.

    of course, if it is a butterfly type, forget all that nonsense, just strip and clean and get a rebuild kit if required.

    just remember that normally, most of these engines are intended for idling, then full throttle operation, and are usually accelerated with no load. think of how you would use a chainsaw! the engineers design for these two conditions with these modern unadjustable carbs. mid range operation is a secondary concern. smooth transitions are seemingly pointless, as it isnt a car or a motorbike that needs precise throttle control to remain safe...
  3. grinningremlin

    grinningremlin Active Member

    Much thanks.It's one of the adjustable HP models, with the velocity stack.Got it VERY used, so it may just be getting old, your last paragraph eased my concerns, until it don't work I'll just work around the flat.