Help with an old mini bike issue.

Discussion in 'The Gastank' started by cornishrooster, Aug 12, 2017 at 8:09 PM.

  1. cornishrooster

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    I spruced this late 60s sears roper up that I scored for next to nothing on CL. After messing around with lots of different engines and carbs I landed on the predator 212 and lots off upgrades. Billet flywheel, upgraded valve springs, Billet crank arm, autolite spark plug and a real mikuni vm22 carb, I messed around with the knock off that all the popular go.kart sites sell but couldn't get it dialed in, not even close. Anyhow.. Long story long I put the new real mikuni on yesterday and it fired right up much to my surprise. Took it for a ride, the first time in almost five years of having this thing, and got home with a grin on my face. Loads of power and torque. Turned it off and later tried to start it, plug was foul, and I mean soaking wet.

    Aired the plug out and checked the numbers on the Jets, pilot was 30(!) and main 100. Obviously 30 was too high, lowest I had laying around was 17.5 pilot so I popped that in and it fired right up. Turned it off and same issue, wouldn't start again and the plug was wet though not as bad. I have pilots size 15, 12.5 and 10 on the way but I'm curious is there anything else I should be thinking of? Haven't touched this engine in four years so I changed oil and plug. Couldn't tell you what the valve lash is set at but I do remember setting it.

    Also I know the carb float isn't too high cause the carb isn't pissing fuel.

    Appreciate any help here.
     
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  3. racerx

    racerx Administrator
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    That doesn't look like any VM22 I've seen...the float bowl is way different. The ones I've run have a shorter float bowl, with a main jet holder along the rear edge, and NO drain valve.

    The #30 pilot/#100 main jet combo is what the real VM22s come with. You're right, that pilot is friggen huge and main tiny. I'd like to know how those jets were selected by the manufacturer. With a known, genuine, VM22, 110cc Honda singles usually want either a #20, or #22.5 pilot and #150-170 main. 125cc needs a #22.2 pilot, main size can range as big as #210, depending upon how much power a given tune makes.

    With this engine, you're pretty much on your own. Dialing-in the pilot is straightforward enough. Find the lean limit, then work back toward the rich side, until you have no off-idle stumble. If you need to set the pilot airbleed screw less than 1/2 turn out, from seated, go one size bigger on the pilot jet...more than 2-1/2 turns out, go one size smaller. Sizing the main is more work, you'll need to keep testing main jets until you find the biggest that allows the engine to rev-out cleanly, then dial the mixture back 1-2 steps in the lean direction, as needed.
     
  4. cornishrooster

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    Thanks. I figured as much just wanted to see if there's anything I hadn't thought of. Posted a picture of the wrong carb, that's the knock off carb before I replaced with a real one, which is damn near twice the size.
     

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