I have a 69 XR7 with a 351w 4v. The engine was recently rebuilt and during my daily test runs, I have noticed it runs fine when cold, but after about 10 minutes it starts to backfire and miss under load, at all RPMs. It idles fine at all times. Here are the details:
Fresh mostly stock rebuild, .020 over bore, 10:1 compression, mild cam, rebuilt Autolite 4100, rebuilt distributor, Pertronix ignition, new plugs and wires, Flamethrower coil, C4 automatic.
New gas tank, lines, fuel pump, fresh gas.
Plugs gapped to .040", removed and inspected, all greyish-brown, dry
Firing order verified.
Vacuum 18 Hg, steady at idle
Timing - I have experimented with settings between 6 and 20 degrees, 15 works best, but backfiring occurs at any setting. Ported vs manifold vacuum advance also had no effect on this problem.
A couple of possibilities. First, a coil can go bad when the engine warms up. If you have another coil handy, that’s a quick and easy test. Secondly, it’s possible that your distributor is worn, and is causing your timing to jump around. When it sputters, any chance you can get a timing light on it? Another possibility is that your condensor is going bad.
Thanks Midlife - I will try swapping coils from another car - good suggestion. The distributor was rebuilt by a trusted shop, and felt pretty solid when I installed it. Also no condenser since its a Pertronix setup. I will check out the timing light idea, although the symptoms are most noticeable under load, and not sure if I could recreate that in the garage.
This is the wiring diagram for the Pertronix, which came from them. Red wire goes between the tach and ignition switch, bypassing the pink resistance wire.
Here is an update on my recent tests: I swapped out the coil for a known good one, and there was no change. Also did the “dark test” to see if spark was jumping from coil wires, that was OK also. On my long test drive today, I noticed that if I was stopped at a light for a long time, the idle started to get rough and sputtered/backfired on acceleration. If the stops were short, the symptoms were barely noticeable. So I think the carb is loading up and possibly flooding, so I will check the fuel pump pressure, floats and seats to see if that is the cause. Let me know if you have additional ideas.
Today I checked the fuel pump pressure and measured 7.5 PSI. Seems a bit high for a 4100, as the spec is 4-6.5 and Mikes Carburetor recommends 4.0. Could this be enough pressure to unseat the needle valve and cause flooding? Has anyone installed a fuel regulator? If so, what type and where did you install it?
PROBLEM SOLVED: While diagnosing a weak spark, I found that the Pertronix was wired per Pertronix instructions (see diagram posted previously in this thread), but that was WRONG! This resulted in less than 12v at the coil, causing the weak spark. I have since found the correct wiring diagram (attached) and now the missfire is gone. I have sent the correct diagram to Pertronix.
Then you must have three wire tach. The factory tach works wen current is pulled through the tach. Power follows the path of least resistance so you have a parallel lower resistance path going straight to the coil. Now if you ran the red jumper directly to the Ignitor, not connecting to the coil, that should work.
The 3 ohm coil really needs the full 12 volts. Glad you got the 1.5 ohm version.
