I don't post here often (okay, ever) but I've almost run out of ideas. I have a 68 el camino with a 307 in it that gets me around (don't worry there is a 383 stroker in the garage that I haven't finished yet). I took the car off the road when I bent a rear axle and decided that would be a good time to upgrade to a 12 bolt with 3.73 gears and an Eaton posi from my open 3.08, 10 bolt. When I started driving it again I noticed its started missfiring at about half throttle in the upper rpm range. I have a fairly new edelbrock 650 carb with an electric choke that I haven't modified at all yet. Here are some of the things I've replaced trying to get this to work and not misfire: plugs, spark plug wires, the coil, I checked the fuel filter, installed a new electronic fuel pump (don't know how old the last one was), points, condenser, cap, and rotor. The only thing left I have really are the mechanical advance springs (either not moving out at all to advance or not enough) though I lubed them and they move when I move them by hand. It could be that my distributor bushings are shot and its messing up my timing at higher RPMs or perhaps I need to rejet my carb to get some more fuel in there at higher RPMs. My uncle has a chasis dyno that he uses for smogs so I'm going to talk real nicely to him to see if I can use it to figure this out but I want any input you guys can offer or if you've ever dealt with something similar. Any suggestions? I'm really looking forward to getting to drive it with this 12 bolt but haven't really got the chance yet. Thanks a lot.
Brandon

Actually, you'd be better off if you could find an independant shop somewhere that still has one of the older style oscilloscopes. This can help determine if the problem is ignition related or not. From your description of the parts you've replaced, it would seem you've eliminated the ignition system as the cause of your problem. The scope would help by eliminating any doubt.

Did it run good with the Edlebrock before you took it off the road for the axle problem? When exactly did you switch carbs? At this point, I'm leaning a bit towards the valve train. There might be a weak or broken valve spring that's causing a valve to float at higher RPMs.

As for the distributor, with it installed on the engine and cap removed, you should be able to move the rotor about an 1/8th of a turn clockwise, and it should snap back quickly when released. That would indicate that the advance mechanism is functioning properly.

I was trying to get in a few test runs with an oscilliscope hooked up to it before I spent my time on the dyno. The problem was deffinately ignition related somewhere in the primary circuit. Turns out that the ignition wire that runs from the coil to the starter was disconnected and managed to short itself to my header on that side. So when I put a big load on the engine the coil couldn't make enough power to get the spark to jump every 4th cycle or so. When I took the wire off everything went back to normal again. So $150 later the problem (no thanks to any of that money) is solved and my buddy and me just went out and spanked a 69 mustang on the street. Thanks for all your help Randy.
Brandon

Sounds like another happy ending!! http://www.chevelles.com/forum/biggrin.gif