Any solution suggesting fuel problems doesn't address the problem of the jumpy
tach which will be getting it's signal from the ignition system. This
car has " electronic
I just went through a similar problem with almost the same symptoms on my
Mother's 77 Datsun 810 with electronic ignition. This distributor has a
magnetic pickup and a 6 point reluctor. The car would idle fine (650 RPM) but
as soon as the engine speed went much over 1000 RPM the tach would read
double the RPM, and the engine would miss. At first it was a cold weather
phenominum. The colder the more likely. As the engine warmed up, it would
run normally. I tried replacing the fuel injection black box, the ignition
black box, coil, alternator, ballast resister, tach, a resistor in the
tach feed wire all from a parts car, and new magnetic pickup, cap rotor,
ignition wires, and plugs. I finally took the car in to the shop. The
mechanic noticed the rotor had some wobble to it and rebuilt the distributor
using the one from the parts car. Two weeks ago when returning from a
1500 mile trip it started doing the same thing about 150 miles from home.
After I got it home I pulled the distributor cap, and sure enough the rotor
would wiggle side to side. I did a jury rig repair to tighten up the upper
distributor bushing and now it's just fine again.
The wobbling reluctor sends false and miss timed signals to the coil. This
causes erratic timing and the extra signals cause the tach to read higher
RPMS than normal.