Hi again,

Ive got an interesting problem. 1987 Montero 2.6l. Just rebuilt (for reference)

Every morning that I get on the highway, once I hit about 55-60, it feels like I lose a cylinder and it starts to make a pinging noise (or clicking/rattling/etc). At first I thought maybe the advance mechanisms aren't working properly, but I went through and cleaned/greased the mechanical advance, and also checked the vacuum advance. Both seem to be working normally... The timing is set to 6 degrees BTDC.

When it does this, it's only when under load AND at speed (getting on highway, going up a hill at 50mph+). And when it happens it's very sudden, and feels JUST like one cylinder is just gone.

To remedy it, I simply pull off the side of the road, let it idle for a minute, and it drives normally.

Plugs are new, wires are new-ish, cap and rotor are new. I'm wondering if maybe this could have something to do with a jet valve or something.

How is your secondary diaphragm on the carb? Could be a lean condition...air/fuel rather than fire?

When was the fuel filter last replaced? A partially clogged fuel filter could be limiting the amount of fuel to the carb and causing fuel starvation when the float bowl empties. A bad fuel pump could cause the same issue.

I'm not sure regarding the secondary diaphram. I'm assuming you're referring to the "Depression Chamber"?

I did replace the filter last Thursday just to cover my bases. It actually seemed to help SOME. But going up hills at speed (throttle mostly wide open) it begins to make the pinging/rattling sound. If I pull off to the side, and let it idle for a minute or two, then jump back on, it seems to recover.

I'm in agreement with you both, as I believe this to be a lean condition, more so than a timing issue. It's time to take a closer look at my pump and carb. I was needing a good excuse to get a weber anyway..