This one totally has me stumped. It's my 68 bird, 400, 4-speed. I purchased it this summer & finally got to start driving it after doing the interior work & some engine compartment stuff. It starts fine, but when your driving at cruising speed the car hesitates every now & then, not where the car shakes & buckles, but just enough that you can "feel" it. Here's the set up. cam: unknown, the guy I purchased it off of had no clue what the cam is but it does have 4.11 gears at the rear. I replaced the oil pump, water pump, new MSD 6AL box, new MSD distributor, new Holley fuel pump, new Holley 600 cfm double pumper, new fuel filter, new spark plugs, new MSD coil, & new spark plug wires. It has an Edelbrock Torker intake & headers. It did have a 670 Holley Street Avenger carb on it & it did the same thing. I thought it may be the carb & got the new 600 & it didn't cure it. I've even tried another 6AL box & that didn't work. In order to make the engine start/idle/run I have to have it about 18 degree's advanced, so it must be a serious street/strip cam. Starts up fine, idle's perfectly fine (around 800 rpm), & if you punch it there's no miss at all. The miss is only present at cruising speed, which is around 3000 rpm for 55-60 mph with the 4.11 gears. So...any idea's on this one. I'm completely out of ideas. I've talked to the MSD & Holley tech guys & they all said "something's not right"....no duh. I've heard everything from fuel to rich, fuel to lean, the cam jumped a tooth, a cam lobe is worn down, & a few others. I may over the winter just pull the 400 & tear it down & rebuild it but before I do that I figured I'd run it past you guys while theres still a couple weeks left before I put her in storage. One other thing, it runs hot in stop & go traffic....it'll soar up to 215 degrees. I do have a 160 thermostat in it with an aluminum radiator.
Sounds like its not the carb since you changed that out, that would have been my first guess too. I have never ran the msd stuff but there are enough that do that have had problems would focus on that.
I would see if you could install a stock points or HEI system to bypass the existing MSD to see if that helps. Ohm out your spark plug wires. Check that you are getting a full 12Volts. Maybe reset the preload on the lifters if they are adjustible. Pull you spark plugs and see if they give any clues if it only on one cylinder.
I know some people have used their timing light to find what cylinder was missing but that might not be an option if it only does this while cruising.
It sounds like you might have to much advance at cruise. Do you have vacuum advance? If so try disconnecting it and plugging it off. What is your mechanical advance?
Was the original 1968 pos coil supply wire used? If so its a resistance wire that only runs at 8 volts and can cause headaches. If you want to try a quick fix to see if it works you can run a 12 ga. wire directly from the batt to the MSD coil. Just remember you won't be able to shut the engine off with the key and will have to pull the jumper to stop it. Don't leave it connected to the coil for very long without running the engine as it can damage the coil.
Obviously not the problem but your original 400 came with a 750 cfm quadrajet carb. I think you'll find more power with another 750 cfm carb (edelbrock, holley etc.) save the 600 for the 350 guys LOL !!!
