'77 gt, knock on stop

MY '77 GT 400 is knocking when i turn it off? Any ideas?

I have a 72 GT with a 77 motor in it and it does the same thing. I run premium in it and it helps. Checked the timing several times. Nothing. Near as I can figure is the carbon in the combustion chamber gets a hot spot and keeps detonating.

What, like dieseling or run-on? If so, you could have carbon causing hot spots in the combustion chamber. Another possibility is if you adjust the idle using the screw on the back of the carb, or the throttle kicker. It's a 400, so I'm just guessing it still has the factory 2bbl carb on it. If you adjust the idle with the screw on the back, it's holding the throttle plate open allowing gas to still be drawn into the intake, and self igniting in the combustion chamber via the hot spots. Adjust the idle with the throttle kicker on the drivers side-front of the carb, and not by the screw on the back. The throttle kicker is only energized when the key is on. Thus when you shut the key off, you completely close the throttle plates keeping the gas from being drawn in.

As Montego mentioned, this was a biggie problem all through the 1970's. Was a product available at (initially) speed shops that you used to spray down carb. while engine was running at idle. It then poofed out black out the tailpipe. This was fine for those cars equipped without a catalitic muffler. Hot Rod mentioned this in one page review, but months later got lambasted for they did the project on some 1960's era auto. Not a then current one.

I don't remember the title for the product, but have heard of folks using a common atomizer and water or alcohol in spraying a mist over inlet at idle. My worry still is plugging up the exhaust system during it. Hot Rod got a heap of complaints when they introduced a water injection unit on to the market, for then too they omitted mention of using a open exhaust system.



Wm.