No offense to everyone that's trying to help, but shouldn't we start at the beginning?
First of all, what has Alan told us?
- It's backfiring through the exhaust system
- He's changed fuel, wires and dist cap
- He's checked all his alignments, including flywheel, dist, valves, cam, etc.
- He's running an external resistor coil with no ballast resistor
- The motor starts and runs
The cause of backfiring is timing. But timing means a lot of things, both mechanical and electrical. Alan's checked all the mechanical stuff, but only part of the electrical.
The motor starts and runs. What does that tell us? The mechanicals have to be fairly close to right. If you get the distributor off 180 degrees, the motor won't run. If the cam slips one gear, the motor won't run well. A couple of gears and it won't even start. If a valve breaks, the motor will run rough.
His runs, it just backfires.
Most likely cause? Points and/or condensor. The points are probably badly burned, Alan. The purpose of a ballast resistor is to drop the voltage from 12 volts to about 6 volts. This makes the points last much longer.
Don't forget, the amount of time the points remain open determines when the spark plug fires and how strong the spark is.
You don't need a ballast resistor if you buy a coil with a built-in resistor. NAPA sells them. They're much more reliable than the external resistors ever were. Get one. And while you're there, buy points and condensor. (I'm assuming you replaced the rotor with the cap.)
Now, let's clear up some misconceptions. The timing can't "slip" on driven gears unless one of the gears has plastic teeth (which his 235 does)
and teeth are chipped off. I've seen this on the Pontiac 327, where the timing chain slipped because the teeth became brittle and broke off. But Alan said the teeth were
worn, not broken. I doubt seriously that the timing gears are improperly located. Besides, the engine runs. If the cam slipped one tooth, that would throw the entire timing mechanism off by several degrees, which should be more than enough to at least make it
very hard to start, and it would run very rough.
Second, jbennett is right. The distributor will only go in two ways. Either in the right place or 180 degrees off. If that was the problem, it wouldn't run at all.
Third, on the 235, the intake valve opens at 1 degree ATDC and closes at 39 degrees ABDC, and the exhaust valve opens at 42 degrees BBDC and closes at 9 degrees ATDC. So, if Alan had the motor at TDC, the exhaust valve would be open, not closed, if he was 180 degrees out. If he was 1 degree past TDC, both valves would be open, the intake on the intake stroke and the exhaust on the exhaust stroke. So what he is reporting is normal for the 235, if he's off just a hair or he's 180 out. Only Alan can answer that.
Finally, no one seems to have asked: Alan, has the motor ever run right? Has it always backfired since you've had it? Was it just rebuilt?
