Engine Misfire under load. 74 2.7
 

My 911 developed a misfire after hitting a large unmarked pot hole on a BC classic car tour.

I discovered that the distributor cap was unclipped on one side and on a slight slant. Re-securing the cap did not cure the problem. I limped with miss back to civilization for about 100kms. I replaced the cap and rotor with the only jobber parts that I could get on a Sunday and it ran fine for about 500kms including a climb over a large mountain pass.

The next morning the miss had returned and despite much fiddling around on the side of the road, I ended up driving the last 400kms back to Calgary with the engine miss.

Since my return, I have done the following:

-Replaced the cap and rotor with the correct bosch items
-Replaced all spark plugs, gapped slightly wider than stock
-checked that all plugs are firing
-checked timing
-checked firing order.
-checked mixture via "field test" method.

Now the car idles fine and runs okay under light throttle but misses under load.

Other notable items
- Pertonix ignitor II installed last year
- Pertronix flame thrower coil, last year
- New Behr ignition wires last year
- original factory CDI box
- the battery is marginal and should be replaced. It starts the car fine if left on a battery minder but will need a boost if the car sits with out one after a couple of weeks.

Is it possible that the marginal battery is causing the miss under load? The Alternator is bosch unit in good shape. I seem to remember reading that the Petronix unit will not work properly under certain voltages.

I am open to suggestions.

Cheers,

Mike Bennett

Its possible the CD unit was damaged.