Thread: Newby 429-460 question

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Originally Posted by OregonHighboy

it does not want to shut off right away and there is some minor valve chatter when I get on at certain RPMs.

Running on after shutting off the ignition is called "dieseling". Here's one of the best write-ups I've seen explaining it.....
http://mgaguru.com/mgtech/engine/ro101.htm
I used to give my motors a "water tune-up" once in a while. Take one gallon of water and the truck out of town where there are few other people to complain. Take a buddy with you. Remove the air filter. Fire the motor and have your buddy modulate the throttle in Park or neutral with foot on brake while you pour a small stream of water down the carburetor, alternating side to side into the primaries. The water will try to kill the motor, so your buddy will have to keep up by giving more throttle to keep the motor from dieing. After a couple of tries, you'll get the knack of how much water to pour. You won't believe the crud that comes out of the tailpipe(s) during this operation. That's why you don't want to do it near any populated areas. The water is steam-cleaning the combustion chambers and getting rid of the carbon buildup that is the probable cause of the dieseling.

Dieseling was quite a problem with the early to mid 70's motors as Detroit was forced to lower static compression ratios, retard camshafts and raise engine operating temps. I remember an anti-dieseling solenoid mounted on quite a few of the GM cars I drove in that period. It was a solenoid that when activated, allowed the primary throttle blades to open to the idle position. When the key was shut off, the dashpot relaxed and pulled the throttle blades completely closed so that no mixture could pass into the motor, thus, no run-on.

The valve chatter you hear is more than likely muffled detonation. It's the same thing as hitting the crown of the piston with a sledge hammer. Here's a pretty good explanation....
http://www.misterfixit.com/deton.htm
Not mentioned in the article is water injection. It provides a fix for motors that were built with too much static compression ratio for the available fuel without having to tear into the motor to change components. I have made a simple and effective water injection unit from an old Ford windshield washer bag with the little motor/pump in it. Run a line to the carb and terminate the line with an old carb jet (drill a blank one) drilled to about 0.030" and using a microswitch to operate the pump at about 1/2 throttle or more. Of course, there are commercial water injection units available at the cost of hundreds of dollars. You can add up to 50% methanol, ethanol or isopropanol to the water to gain back a little energy and aid cooling the mixture further.

If I had to guess what is going on in your motor, it is probably either or both of these....
1. Static compression ratio too high with not enough cam to bleed off excessive cylinder pressure. Using what you perceive as the proper ignition timing probably isn't helping any either.
2. Piston used in rebuild was short compression height, leading to excessive piston deck height and lack of squish. Squish "jets" a part of the fuel/air mixture from the coldest side of the chamber over into the spark plug area when the piston is coming up to TDC on compression. This homogenization and turbulence of the mixture allows a higher static compression ratio engine to operate on lesser fuels which might be the best available at the pump. (Think 11.0:1 on 91 octane fuel as a for-instance)