Strange ignition problem

This is a strange one. Occasionally after hard acceleration - gotta love that v8 in a cj5 - and letting off I get lots of ping. It almost sounds like a diesel. If I feather the accelerator it goes away. The only things I can think of are the vacuum advance is sticking or a spring from the mechanical advance is busted. I popped the cap and pushed on the vacuum advance and it appears to operate smoothly. Obviously, other than popping the cap, I have not opened anything up yet, so I am just speculating right now. The application is a '74 amc 304 with stock duraspark ignition from an '83 waggy. Any ideas?

Karl

 

could it be a partial blocked fuel filter? The hard run accell drains the bowls and then you have to feather the throttle until the fuel pump can fill them back up?

Not having any experience with a carb this is my only WAG.

 

Hmmm, your comment made me think. A partially blocked fuel filter seems plausable given that I hae never changed it and that I was recently into the carb there might even be a chunk of dirt in there, but I cant imagine how that would contribute to pinging? Tune me cause I am missing somethin here.

Karl

 

Insufficent fuel causing a lean condidtion and premature ignition during the compression stroke? That is what I was thinking.

 

although anything is possible, it also could be carbon knock. Hard accel loosens up carbon build-up on the valves or pistons; sticks on top of piston(s) until loosens up and exits through exhaust.
Hot carbon can cause pre-ignition
just a thought
pinging on de-acel is unusual

 

Hmm, I was not clear. I dont get pinging on decel. I get it in light accel after a hard accel. I think I might have to open up the dist. To observe the mechanical advance bits do I have to yank it, or can it be done installed?

Karl

 

that makes more sense.....
I'm going to say if the carb is stock Motocraft 2bbl, probably the power valve. Holley power valve should be a direct replacement.
Remove cap rotor and dust cover to visual centrif advance mechanism. look for broken spring(s), rusted/stuck weights, and even the sensor hitting the trigger wheel.

 

Might be just good old fashioned low octain fuel. You need to suck on the vacuum advance hose and use your tounge as a plunger and see if the vacumm advance holds vacumm as well as checking the weights. And of course, check the timing.

 

Well I was reluctant to go to work and since I have to walk by the Jeep to get to my bike I took the dist apart. I found no broken springs and the vacuum advance works ok accept that the grease on the pivot for the advance assembly was gummy. So I cleaned it all up and used - I couldn't think of anyhting better - dielectric grease on the pivot. I works smooth now. Perhaps this was the problem. I dont know yet cause I hoped on my bike and came to work.

Karl