Something happened suddenly it seems. You were driving, things in high gear were ok, then something happened and now you can wind out in low gears but everything goes flat in high gears. Rusted parts inside your muffler could have fallen off while you were driving and formed a dam in there. I've seen that happen. Sudden vacuum loss is a possibility. Failed coil is a possibility, but if you're ok in lower gears it isn't a very large probability. I suppose a failed/failing gas cap is a possibility (must be vented correctly or at a point the pump can't suck enough gas)...but sort of a stretch since that would not happen suddenly.
I cut off the muffler last evening and am in the process of gutting it to steal the inside pipe diameter to build a quick sleeve from so I can reattach the turn-down to the pipe from the manifolds. As is it points onto the hydrovac which I dont want hot exhaust gasses hitting.Ill fire up and go for a test run today.
I replaced the gas cap during the rebuild (and filler neck). I checked it last night and it appears to vent properly (pulls air in, dopesnt let it out). Unrelated, gas makes your lips numb for a few minutes if you use your mouth...
I am with you on the distributor. The seemingly perfect operation at the low end makes me think its not, but I guess you never know.
Now you need to leave the carburetor alone and find the other problem. This requires a vacuum gauge, a compression tester, and a multi-meter.
Since you didn't mention that the engine dies, I don't believe that you have a fuel delivery issue.
This leaves ignition, compression and vacuum leaks.
Test all components involved in each of these areas.
Once I am done doing heads on the other truck today ill break out the compression gauge and check the mighty 6 and report back. As far as testing the ignition, what is the best way to do so? I am young and new to distributors as a whole, so please forgive any ignorance I may have. Ultimately want to go with HEI, but I was adamant about running with old style first to force myself to learn it in depth before ever upgrading.
Thanks for the feedback all. Ill keep digging and then report back
*EDIT*
I know you mention it above Jon, but after poking around again this morning I realized that while I replaced the distributor, I never replaced the coil pack, and lord knows how old it is. Given the seemingly fine low end, could it be breaking down at the high end with heavier loading on the engine?
