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    Re: Re-engine Buggy with a (modified) Predator 670
    #81
    Keeper of the Asylum K-fab's Avatar
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    I'll second on not straying away from either Mikuni or Keihin and messing with the Chinese crap.
    They're cheap and that's exactly what you get.
    I've seen a couple really cheesy castings on them - to the point of the mounting ear snapping off as you tighten it down to the intake cheesy...
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    Re: Re-engine Buggy with a (modified) Predator 670
    #82
    Senior Member darwinpayne2000's Avatar
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    No end to the fun when it comes to dialing-in the carburetor.

    I took the buggy out to the sand dunes this weekend. With the old cam, the motor tended to run rich from 1/4 to 1/2 throttle and then run lean on WOT. I kept going bigger and bigger on the main jet, but I never quite got it down. With the new cam, it tends to run lean on 1/4 to 1/2 throttle and run great with a smaller main jet.

    However, the difference may not lie totally with the new camshaft. One change I made was adding a gasket between my intake manifold and the carburetor boot. The rubber on the carburetor boot would, theoretically, seal up against the intake manifold, but the boot is a little old and the rubber is fairly hard. That would possibly cause an air leak that would get progressively worse as the carburetor opened up.

    So, with the new camshaft, the Keihin PWK 39 carburetor and a boot gasket . . .

    The carb ships with a 160 main jet. During my testing, it seems to run best at WOT with a 150 jet. I'm sitting at 4000 feet above sea level, so that made a certain amount of sense. It actually felt like it would run even better with a smaller jet, but the smallest I have is a 150. Mid-range, however, was a different story. I was running an EEG jet needle, which is considered fairly rich and I raised the needle all the way up to make it super-rich, but the plugs were bone-white after testing it at mid-throttle. When I put in a bigger main jet, such as a 170 - 190, the performance at WOT was noticeably worse. The bigger the jet, the bigger the bog.

    Maybe the Keihin isn't the best carb for this application. I dunno.

    The old Vanguard engine had a Mikuni VM34 that was dialed-in really well. I'm thinking that I want to try the VM34 on this engine and see how it performs. I have multiple slides, jet needles and main jets for this carb, so that gives me more tuning options.

    Stay tuned . . .
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    Re: Re-engine Buggy with a (modified) Predator 670
    #83
    Keeper of the Asylum K-fab's Avatar
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    Quote Originally Posted by darwinpayne2000 View Post
    Stay tuned . . .
    LOL!! Sounds like that should be what we say to you.

    The super white plug - I remember an issue where the new plug I was using along with such a rich setup, that the fuel was actually washing the plug and it wouldn't show that it was blubberingly rich.

    Is it a bog or is it starving? Could it be that you're too rich in the mid range? Just throwing out thoughts.

    Two-strokes are so much easier to jet...
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    Re: Re-engine Buggy with a (modified) Predator 670
    #84
    Senior Member darwinpayne2000's Avatar
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    During my testing, everything seemed great with a 150 main jet. Mid-range response was pretty good. As I increased the main jet to 170 - 180, I could tell that the mid-range was getting richer, but it wasn't running real blubbery.

    Yeah, if tuning a carburetor was easy, I would have my kids doing it, like mowing the lawn.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #85
    Senior Member darwinpayne2000's Avatar
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    I dug out the VM34 carburetor from the Vanguard engine and gave it a quick cleaning. I put it away with all of the fuel flushed out of it, so it didn't take much to get it ready. The main jet in the carb was a 240, which seemed larger than I remembered, so I dropped it down to a 220 for the first test.

    After cranking on it for a bit and getting the carb bowl filled up with fuel, it took right off. A quick drive around the neighborhood confirmed that it was running smoothly.

    A couple of blocks from my house is a street with a short but steep hill. I gunned it and the power died out slightly at full throttle up the hill. That confirmed to me that it probably had a 240 main jet for a reason. I swapped it out real quick and tried it again. This time, it pulled hard and steady at full throttle all the way up the hill. Success!!!

    I pulled one of the plugs and it was soot black. I was driving around mostly at 1/4 throttle, so I can probably adjust the jet needle to get the air/fuel mixture a little leaner. At least I'm close with this carb!
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    Re: Re-engine Buggy with a (modified) Predator 670
    #86
    Keeper of the Asylum K-fab's Avatar
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    Cool - good to hear you're headed up the right track now.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #87
    Senior Member jimmyg's Avatar
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    Anyone noticed fuel doesn't like to sit long without use in carbs like it used to?

    Bit of an OT question for your thread. Just curious.
    All dressed up and waiting for the beer to arrive.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #88
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    Quote Originally Posted by jimmyg View Post
    Anyone noticed fuel doesn't like to sit long without use in carbs like it used to?

    Bit of an OT question for your thread. Just curious.
    Ethanol. Use Sta-Bil.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #89
    Senior Member darwinpayne2000's Avatar
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    The ethanol blend in gasoline is probably the biggest cause, although I don't know if they blend ethanol into gasoline in Australia. Modern gasoline is also more highly refined and probably has more volatile components, compared to gasoline of yore. Sta-bil is a good option, but I know that some of my friends always buy ethanol-free gasoline for their ATV's to avoid carburetor issues when they let it sit. From past experience, it seems that once a carburetor gets gummed up, it has a tendency to gum up much quicker.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #90
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    From personal experience, ethanol tends to plug up carburetors on smaller engines like lawnmowers and ATVs because they have smaller orifices in the carbs. We always use ethanol free fuel in all small engine applications.
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