79 K5 400 Ignition Problem
 

I have a '79 K5 with a 400 and am having ignition timing issues. Recently it started spark knocking quite badly during accelleration. If I'm going (lets say) 40 mph and come to an incline it will begin to knock and I have to ease up. It knocks on take off as well. Sometimes the knock on take off is worse than others. Either way, I'm having to take it very easy.

I have replaced the plugs, cap, and rotor and have been trying to time it with no luck. I've had to retard the timing so much to reduce the knocking that power and drivability is now very poor.

The only time my truck had spark knocked before was after getting off the highway after having driven 65+ MPH in the summer heat for a moderate length of time. When this would happen I would just baby and it would go away. The knocking I'm getting now, however, begins as soon as it reaches operating temperature. This issue came up suddenly one day - it was not a gradual thing.

I'm wondering if the mechanical advance in the distributor could be the problem. If anyone has any ideas of what I can check, it would be greatly appreciated.

I would take the distributor apart and make sure that the mechanical and vacuum advance are working properly. If the vacuum advance is stuck in the advance position and/or the mechanical advance is stuck out, it could cause the problems you are experiencing.