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The jet won't stay put when inserted. It prevents the flat side of the plastic part from seating on the carb body. Anybody? Thanks

You need replace that tiny o-ring that holds the jet in place or the complete jet..

The oring, jet and plastic piece are new.

Are you trying to install the jet in the correct direction. The end with the O rings goes down the tube first with the mushroom head part facing the carb venturi when installed.

I find it easier to sit the jet on the end of a punch then lower the carrier plate over it
The jet is stepped and unless dead square will not sit properly in the tube
Once the jet is in position the carrier plate is popped into the bowl & the bowl offered to the carb upright then when in position flipped over to put the screws in .
There are a lot of rebuild kits for these carbs on evilpay/ ammozone that are defective , probably more than for all of the other carbs added together.
Also the Chinese unbranded Nikki knock off's are slightly smaller so their rebuild kits will not fit a genuine Nikki & visa versa .

Usually on these single barrel Nikki I never buy a kit, Just the bowl gasket and maybe an o-ring for the jet; unless, I lost the darn jet on the shop floor. But I have learn to use a large pan with a cloth or paper towel in it when taking these carbs apart. The biggest cost of these repair kits is the needle valve which seldom ever needs replacing.