I have 350 in my '60 F100 that started running a little rough about a month ago. Right around 1100 rpms, if I'm cruising in town in 3rd gear there is an obnoxious vibration- the shifter chatters back and forth, keys rattle, etc. Above that RPM it seems fine. I originally thought it felt like a bad plug, so I changed those and when I found the #8 plug fouled, I was sure I had solved it. It didn't, so that was when I started making note of the RPM when it happens. It runs normal at higher RPMs. It also idles kind of rough...It seems like I have to keep messing with the idle speed screw...once warmed up, in Neutral it was idling at around 1200, so I cranked down a smidge. 5-600 and it's threatening to stall, back up a bit...1200. It seemed somewhat evened out, but odd nonetheless. Lastly, and most confusing, I thought I would be able to diagnose a misfire by pulling plug wires and listening for a change in idle or not. I tried it on 4 different plugs and there was no change in idle on any of them. I can't figure that. Not even the slightest change in pitch or idle speed. Is it possible that a vacuum leak is causing all of these issues? Any ideas?
I should add that I put urethane engine mounts a few years ago and that made for some vibrations, but this is different and new within the last month. I tightened up the shifter and transmission bolts when this first began.
HEI. I knew I forgot something. Maybe a new cap and rotor are in order? I would think if the HEI module was messed up, it would either work or it wouldn't work, but I don't know for sure.
Well here is a tid bit that most don't know or want to admit. A bad battery like in one with a loose cell or weak cell will screw with electronics. I would not throw money at it until I was sure what the problem was. get yourself a can of WD-40 or starter fluid and fire it up and start spraying it around first and see of you haven't developed a vacuum leak. A car with a substantial vac leak will become impossible to adjust the idle speed.
Sounds good...I was thinking rotor and cap, vac leak...I just didn't want to waste what little time I have barking up the wrong tree. Sounds like I have validation from you guys. The battery is relatively new, but I won't rule that out...I may try it last, but it's not off the table until this thing starts running right again. Like Porknbeaner says, finding a vacuum leak is free... The way this truck has treated me, if it IS a vac leak, it's going to be in the most unlikely place, like the stereo or drivers seat (I know that defies any intelligence, but with the way this truck is, I wouldn't doubt it)!
Carb clean works well for finding vacuum leaks. The idle will pick up as it is*****ed in. Make sure it is flammable. Some of the "green" formulations are not. In my experience an HEI will give warnings before it gives up. The symptoms you describe can be caused by erratic spark from your module starting to die.
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