At 20000 to 2400 rpms I get a spark knock when I accelerate quickly. This happens more when it is hot...
I'm not sure where to start to look for the problem. I changed plugs and that did not change it.

 

Low octane fuel ? Try some Rochet fuel additive sold by Cal Products 312-927-4888

 

Use a lower gear also.

 

We could use a little more info, carb, jets, ignition, compression ratio, octane of gas used, gearing, plugs, and any other mods. that would help with the cyber diagnosis.

 

I agree with this answer. tell us what rpm range you use. Start fueling at Chevron for a while & see if that helps!
Hatch.

 

I agree with Mike, sounds like you may be lugging the engine.

 

not infrequently through laziness to shift maybe and I never get any pinging and use all different brands ( always the Hi-test) of gas. I would check your timing advance setting and also the VOES ( vacuum operated electric switch) that tells ignition module when to advance or retard the timing. If it's not working properly the timing is staying advanced when it should be retarding for exactly that reason; to prevent pre-ignition/pinging. It has a vacuum hose going from the carb backing plate to it and is probably up under the gas tank. If memory serves me it's a "normally closed" switch that advances the timing when the intake pressure is high and pulls the spring open to send "advance" signal to ignition module. When the intake pressure drops ( i.e. lugging) the spring closes and the switch tells the module to "retard" the timing to prevent pinging. Confused? Me too. There are a few very informative threads on here about the exotic and mysterious VOES.

 

It only happens when I am at 2000-2400 rpms cruising. Should I keep the rpms higher?

 

What gear are you in when it does it? If you are wound out in say 3rd your vacuum pressure will be high and VOES should be advancing the spark ( signal to ECM) . If you are in 5th gear lugging it the vacuum pressure will be low and the VOES should be calling for retarded timing. If you are in 5th gear lugging and it does it then I would look at the VOES switch which may be sticking at that exact vacuum pressure. Also double check the vacuum hose to the switch for tightness, leaks, cracks, etc.

 

Usually in 5th gear....I'm no mechanic but I'll check it out.