jumping time

i have an 86 k5 blazer 305, that keeps jumping time, we will set the timing, tighten the distributor down, it will run for about 50 miles, then will jump time again, the distributor doesnt move from where we set it, any help would be greatly appreciated. thanks in advance

guess i should of added, have replaced cap, rotor, wires, ignition coil in dist., cleaned egr valve

Well, I am assuming that the timing chain etc is okay..'cause if it was jumping that much, the cam and crank would be out of sync by now and your truck would not run at all.

I am also assuming this is a plain old HEI distributor...?

SO..... Check your distributor out. It is not uncommon to have either the roll pin that hold the gear on the shaft break and allow the gear to spin on the shaft putting it out of time.

The only other thing I have seen is the top of the shaft strip the splines out where the shaft goes into the mechanical advance weights holder. take off your distributor cap and grab the rotor and see if you can spin it or rock it on the shaft, it should only rotate just a few degrees in either direction, if it spins more than that something is wrong.

For the gear you will have to pull the distrbutor out of the engine and see if the gear is loose. Sometimes the gear will still seem tight even though it has spun..try taking a hammer and a small punch and try to drive the roll pin through (careful of the teeth on the gear, they are brittle...doesn't take much of a wack to break them), if the pin does'nt move, (drive through)..chances are the pin has snapped and the holes in the gear and shaft are no longer aligned. So try driving it part way out, if it drives about half way and isn't in pieces, that probably isn't the problem, drive it back in and check other things. If it is broken, you can buy new pins (and gears too) at most parts stores. (or off of a donor distributer..) Be sure to match your gear with the new one....

Good luck,

Zeeb