212cc adjustable jet carb mod

Discussion in 'Performance Mods' started by Tony01, Sep 24, 2016.

  1. Tony01

    Tony01 Member

    A DIY on how to make an adjustable main jet for a 212cc predator carb, and pretty much any carb that has a single center screw holding the float bowl on.

    I used the adjustable jet off a Tecumseh carburetor.

    The first step was to drill out the main jet to allow fuel to flow freely through it. Ideal drill size would be around .055", I think I used a .062" drill which seemed to be too big but it still works fine. To prevent damage to the jet threads I made a small fixture out of some scrap delrin to hold it in the vise. The OD measured around .277" IIRC, so I drilled it five to ten thou under and screwed the jet into the fixture. This really should be done on a lathe, or at the very least in a drill press, but I just hand drilled. The hole wasn't dead nuts on the emulsion tube side but good enough.

    Installed the jet and found that the adjustable main jet off the Tec didn't reach the newly drilled jet. So I found a spare Tec bowl nut and cut a cross section of it to see how far I could cut it down. The needle will bottom out in the nut, so that had to be fixed. Also the nut must not block the holes in the 212 carb in the sides that permit fuel to flow in. So I cut off part of the bowl nut to prevent bottoming out. Well turns out it bottoms in other spots too, like at the oring seal, and on the threads. Even after drilling and tapping a few more threads it still bottomed. Too many threads. I drilled out the threads a size approx four thou bigger than the needle straight shaft, and pretty far down. So now there are only about .100" worth of threads, but it's enough.

    On the outside the head needed to be turned down so that the nut could thread farther into the hole.

    End result, the needle actually bottoms past the jet, so there needs to be some clearance in the emulsion tube. This is why the jet needs to be drilled to a max hole size of around .050".

    Adjusting this carb now is a piece of cake. I hit the expressway last night with my new $15 cateye speedometer and played with different settings. I found the best setting and the bike topped out at 53mph. Came home and checked the spark plug to find it was somewhat lean, richened it up in 1 second and done!

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