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1990 Tercel running excessively rich

I’m “shade-treeing” a poorly-running 1990 Tercel for a friend. The '90 model was the last one to have a carb, and it’s a fairly oddball one: with a constant-velocity piston, it’s more like a motorcycle carb than an auto carb.

I conclusively determined that the main problem is excessively rich operations. I concluded this from: rough running, sooty plugs, exhaust with a fuel odor with grey smoke upon abrupt throttle application. (Also, when the CV piston was manually opened fully at idle, the idle smoothed out and RPMs increased markedly.)

So, what could be causing the problem? I’m considering a float that doesn’t shut off, or possibly a way out of whack adjustment. I need to diagnose this (if possible) without tearing down the carb, because friend in question does not have much money for diagnostics, and most certainly can’t afford a profesionally-rebuilt carb.

(Also, a series of shade-tree-ers before me…with zero carb knowledge…have fiddled under the hood, so I have to allow for the possibility that something could have been adjusted way out of whack.)

Most CV piston carbs have an adjustable needle within the piston. Pull the top, ahd there should be a screw holding the needle in place. You can normally remove that screw, and pull the main jet needle. There should be an adjustment there so you can lean the mixture at high rev’s. At low rev’s, there’s the mixture screw.

Good luck,