I have a C900 (I am a new owner of this truck) with a 477 SD motor and Allison Transmatic single axle dual with two speed rear. The truck runs great at idle and while shifting into first and second. When it shifts into third the motor has a tendency to die then catch and then backfire. Without a load I can throttle the motor up and it runs fine so I decided to check the vacuum (responds as it should) and the timing (does not match).
Assuming I am doing the timing correctly then the timing is was off. I found buy reading this site that #1 is on the passenger side front of motor. I connected an inductive timing light. Found the marks on the timing wheel (goes from 0 and up) and marked the wheel. Fired the truck up and the timing is forward (that is mark is 1/3 of the way toward the bottom of the flywheel) of the marks on the flywheel. With the vacuum disconnected the mark comes back about ½ the distance to the mark. Ok so I figured all I have to do is adjust the distributor and here is where I ran into difficulty.
I cannot turn the distributor. I loosen the restraining bolt and cannot budge (it is cramped in there) the distributor. I have WD40 (general spray because the water pump, water hoses, and carb block general access) and waited several days with no change. Any trick to this distributor? General consensus, am I on the right track for misfiring? Truck is a 73 and carb has been rebuilt recently.
I had similar backfiring/quitting difficulties on my 534. Timing got rid of most of the quirks. There was only one bolt holding the plate that held the distributor in place, and my distributor easy to turn after the bolt was loosened. Good luck!
Back in July I decided to trust my judgment and work on the distributor. I went off the assumption that the distributor was stuck in the intake. So, with a friends help, two fair size hammer and long bars, we hammered on the distributor to knock it back a forth until it came loose. Everything went fine until the last hammer stroke broke the oiler inlet. An application of bonding compound, a through cleaning around the distributor and intake opening and I was successful in adjusting the timing. The truck now runs well except the muffler was shot.
PaMnd - Glad you got it going. In another thread, I was asking for opinions on muffler sizing for a big block engine. The person I bought the truck from, suggested that I get a muffler with as little restriction as possible, in order to not overheat the valves, exhaust manifold, and so on. Not sure how correct this information was, but it seems like a good idea to play it safe. Greg