We used X-66 in my shop in the 80's, mostly on GM V8s w/carbs (feedback or not) that had verified carbon build up issues. GM tech training suggested it as a last ditch fix before engine disassembly. The quart used a kit that included a plastic tube (that yellowed & became brittle w/use) and probe/tip that was poked down into the venturi section of the carb. When used w/DualJets & Quadrajets you moved it from one venturi to the other or you used two kits. The exhaust was wicked.

In a shop that worked on 50-75 cars a week, I witnessed very few cars that improved. It was VERY easy to damage an engine if you poured it in the carb while blipping the throttle.

We saw a lot of carbon'd up EGR ports & intake manifold exhaust gas crossovers. We suspected a lot of the "it's carbonned up" performance complaints were really poorly functioning EGR systems. X-66 seemed to clear EGR's of the time if you blocked open the EGR valve while running the juice into the carb at 50% throttle.

D.C. Klasse: I would not use it on a fuel injected car that was frequently run hard. No need.

Good luck!

What a fun read. I think it's been 10yrs since I last looked at this thread. This place has been so very helpful to me over the past 15 yrs :)