clackity clack goes my 268

258... What is this noise? It happens under load. I was up in the mountains and the noise stopped wtf? '73 cj5 258.

 

Pre Ignition maybe? Is it sort of a ping, or a rattle? Did it just start out of nowhere, or has it been doing it for a while?

 

Try adjusting your timing or changing to high octane.

 

If it has not done it previously.....and you have not messed with the timing....it "may" be bad fuel. But....it also may be carbon build up on top of the pistons and in the combustion chamber. There are a few ways to clean this off. The cheapest way is just plain water, slowly pouring in the carb with the engine running at slightly higher revs to keep it from stalling.
The other is a product called "sea-foam"...You pour it in like you would the water. I normally never would endorse snakeoil fix everything products...but seafoam actually works pretty good as a top engine cleaner.

 

There's also a product called "Top Engine Cleaner" available from GM dealerships that is outstanding. BG 44K will also clean the cylinders and valves in one tank, just make sure you run the entire tank out and refill before leaving the vehicle sit for any length of time. The stuff will gum up the fuel system pretty bad if you indadvertently leave it in a half full tank over the winter.

Since the problem lessens or goes away with altitude, I'd also check timing first, then chase carbon buildup. I would expect bad fuel problems to get worse with altitude.

 

Na...weak fuel that would cause ping would get better with high altitude. Your engine has less compression the higher you go above sea level, less heat, less chance of pre-ign.
And I'm not sure of what his laws are...but I have heard that you cannot buy GM top cleaner in CA anymore (or so is the rumor). Would not shock me to find out it's true.

 

me neither if it gums up your fuel lines... kinda a bad product. it has a good cause just bad effects. like sleeping pills =D