Carby Walbro Carb idling

Discussion in '2-Stroke Engines' started by Kit, Apr 18, 2014.

  1. Kit

    Kit New Member


    Had a few days off, so have had a good go at the bike.

    From a completely stock kit, I have added a Jaguar CDI, a voodoo pipe and a walbro carb and reed valve. I have also mildly skimmed the head, and gently polished and opened up the ports and outlet. (There were casting defects inside the ports, which I have tidied up, without altering the height or size of the ports.) I also brought a better (NGK)plug, but its the hotter type...I fear I should have the cooler one, but its only £3 if I need to change.

    I had to drill and tap the transfer port for the pulse pipe for the walbro.

    It started nicely, with a pleasing crackle though the new pipe. It starts well and so far I have let it warm up, and ridden a bit around my house, but not far. I can tell though the power has increased noticeably, just at low speeds.

    I found that the walbro carb would not idle down at it's factory idle, adjustable by the pointy screw. To get mine to idle, I have had to adjust the throttle cable, pulling it open, so that the cam is maybe 3mm off the screw, which is a lot of its adjustment. However, its idling nicely there.

    The carb is set at the walbro website recommendations of 1 and 1/2 turns back from seated at the mo, and is running, but predictably richly. Quite Smokey.

    Any Advice?

    Many Thanks

    Is this to do with the reed valve? Somewhere I have read that they will affect the mix?

    So far my carb is running at the
    Last edited by a moderator: Dec 18, 2015

  2. Fabian

    Fabian Well-Known Member

  3. darwin

    darwin Well-Known Member

    At 1-1/2 turns out and it's running rich turn it clockwise [tighten] 1/4 turn and see how it does.
    Kit likes this.
  4. Kit

    Kit New Member

    Hi Fabian,

    Yes, I have read several times your instructions on installing the walbro carb. Was very glad of them, as my carb came with no instructions or guidance at all, unlike it was advertised with, so thanks for the advice.

    Why is the carb supplied in the first place without this essential component?

    I wonder if RSE have changed the way they make the set. My Reed valve has a pipe on it, which passes though an alloy block, which in turn contains the mounting threads for the carb. The only way it all fits together is with an air space between the carb and block. The space is the thickness of the bolt heads. Obviously, the pipe from the Reed Valve mates air tightly via a gasket with the carb. So my set up is thermally isolated from the cylinder by an (approx) 4mm air gap. The only way for heat transmission is via the tube, bolt heads and carb mounting threads.

    During my test runs so far, the carb has remained cool to the touch, I have only run the engine for a few minutes at a time so far, so I think carb overheating is not an issue so far with my mods...

    But I'm puzzled as to why it needs to be set at say 1/4 throttle to happily stay ticking over?

    Any Thoughts?

    Kind Regards, Kit.
  5. Fabian

    Fabian Well-Known Member

    Please post hi resolution photos of your reed valve set up.
  6. dchevygod

    dchevygod Member

    Your low rpm circuit is too rich. Tune the carb, it really takes a bit of experience to get it right the first time. I bet if you first readjust the throttle cable to close all the way and then lean out the low adjustment till it idles and then start tuning for low rpm torque, and then tune the high RPM circuit for speed then turn back 1/8 turn rich for safety.

    Your setup could also use some reed port work. The piston needs windowed at least
  7. Fabian

    Fabian Well-Known Member

    Please post high resolution photos of your setup.

    The Walbro style diaphragm carburettor needs to be set very rich on the idle mixture setting to prevent a dead spot when rapidly snapping open the throttle; in fact i have had to cut down the idle screw spring to allow even greater idle throttle butterfly position. This is fairly consistent with just about every modern chainsaw that i've tuned. The older (1980's style) chainsaws don't seem to suffer from this condition.
    Maybe it's an EPA situation whereby carburettor manufacturers are forced to run their idle air/fuel circuit excessively lean.
  8. dchevygod

    dchevygod Member

    At fabian. Get an WT-813 and there won't be a bad flat spot when rapidly accelerating "they have an accelerator pump ;)" . They are setup as a primer carb with feed and return but there are parts to convert it to single feed from the wt-257.

    Also the size of your venturi may be a tad large for your engine size, there is a fine line between good torque and good HP on small 2strokes when it comes to carb size. I've messed with small bore motors all my life and have tried every possible carb on my goped/motors to see the differences. Yes they are a bit smaller but that just makes the power changes more noticeable.
    Fabian likes this.
  9. Fabian

    Fabian Well-Known Member

    Now that's exactly the kind of advise i am most grateful for.
    It needs to be said that i am using an overly rich low speed needle setting to compensate for the lack of an accelerator pump.
    The 12mm venturi Walbro style diaphragm carburettor (used with or without the reed valve conversion kit) is well suited to the 66cc engine when running sensible rpm.

    Many thanks for bringing that advise to my attention, because i will now have a WT-813 on order.
  10. dchevygod

    dchevygod Member

    Not a problem. I like making power, experience goes a long way. What model carb is on the reed now?
  11. Fabian

    Fabian Well-Known Member

    It appears to be a Walbro replica WT series, with a throat diameter of 14mm and a venturi diameter of around 12.5mm.
    The carburettor is fitted with a choke and does not use a primer bulb system.
    I have no idea what the model/serial number is, other than that it appears to be a WT replica.





    Last edited: May 11, 2014
  12. dchevygod

    dchevygod Member

    Knock off wt-257 looks like. I like the throttle linkage setup. Very different from the two styles I'm familiar with.

    You have the parts you need to convert the 813 to single feed. You will either need to tap a very small thread into the fuel outlet/ pulse hole on the carb body and use a small set screw, or a tiny bit of JB weld. Then use the diaphragm covers from your knock off to have a single feed. Or tap a return fitting into your tank.
    Fabian likes this.
  13. dchevygod

    dchevygod Member

    Your very welcome!

    And I need to know where to get that v stack with built in throttle cable holder, and the linkage arm on the throttle shaft. This is what I'm familiar with carb prep kit


    I just looked real close at your cylinder head...
    What the hell! There's two coils and three spark plugs!?!?!

    Please elaborate or point me to where you have before.
    Last edited: May 12, 2014
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