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I've torn down Nissan-Datsun Z car engines that had a knock, only to find it was carbon. The years were 81-83 inline 6 cylinders. There was about 40% of the top of the flat top piston (earlier models dished) that was just a little more than 1 millimeter from the cylinder head. Add enough carbon and you get a knock that sounds TERMINAL.
The worst trip I ever had was 180 miles to DC to pick up a car for a customer who made that drive to my shop for his service. Hauling a 2900 pound Z car behind a Toyota truck (4cyl) with no trailer brakes at 3 PM in the afternoon, at the intersection of the DC beltway and I95 south, after a flat with no jack is a migraine for sure.
I would take my old Binks Number 7 paint gun and fill it with water, remove the air cleaner and set the gun in front of the intake, use vise grips to lock the throttle at 2k RPM and set the gun to provide a good supply of atomized water. Leave it until the paint gun cup was empty and test drive, rinse and repeat if necessary.
That would clean the carbon up nicely, visual inspection as well as test drive verified the results and I never had any problem with the cat or emissions systems since the water delivery was very well atomized and not excessive to the point of liquid becasue of the paint gun.
I did that to the car I towed for almost 200 miles (DC nightmare trip) and told the customer I fixed it in 1 hours labor. I guess I could have told him I took the head off but he paid me well for the nightmare tow and I did not believe in telling people tales for profit, even when I could without them ever figuring it out.
regards
Mech
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