red67

hi guys, had my shop strip and clean my carbs, had everything tuned up, 6 new plugs and it ran like a dream for a few weeks. i waited with getting CD for now (to save on my cash output). then it happened again. i found that it was the same cylinder that kept doing it, just replacing that cylinder's plug would cure the problem. the plug was completely sooted but looking close the contacts where it fires was clear. also the garage had put in top notch plugs that really dont like to run cold. sticking a less quality regular plug in helped. after testing resistance on the connector from the ht lead found that that rod was shortcircuiting. replacing the rod has meant trouble free motoring (so far) with good plugs all round. curious to see how long it goes without trouble. done just under 800 miles so far and it's working pretty good. doesnt like warm starts though. very reluctant to give revs immediately on accelarator depression for minutes after warm starts, then all is fine.

LaughaC

Adding an MSD box and replacing my points with a Pertronix kit greatly improved my car's response and performance.

These are easy upgrades to add yourself in an afternoon. I would start with the Pertronix upgrade. The hardest part about adding the MSD box is finding a suitable place to mount it.

randomhick35

I also have a pertronix ignition, pertronix coil, and an msd ignition box [6 al].
works like a charm, it gives you one heck of a spark.
I'am also running REALLY hot plugs: ngk bp4es.
I've had a similar problem to yours except more extreme [did you know that it'll run and drive on 3 cylinders?]

if you still need a cd box, i think I still have my old one, presumeably functional. won't run with the new ignition, so if you'd like it, i could see if i can dig it up
Bob

Mike Murphy

Plug heat ranges are very important on all engines. I learned this lesson when I bought 8 splitfire plugs for my hot-rod Chevy Nova. I ended up fouling all 8 plugs and the car ran terrible after awhile. New plugs will always make the engine run great, but the wrong style, brand, and heat range will be noticeable only after a few miles...

sounds typical on older, carburetted engines.

red67

it happened again today.
i replaced all my plugs again with hotter ones as a quick fix for fun driving today.
thats it - i'm going into my mechanic tomorrow with this string printed out and say 'fix!' (i'm too busy)
thanks very much guys for the advice.
bob - thanks for your real kind offer, but its summer here and i wanna enjoy the car every day i can so i can't wait. i want them to do it real fast. lifes too short for not driving our 911's kinda thing. stuff from the US sits in customs for a while here. muchos gratias.
P

Graufuchs

Just rebadge the car to a 912 and keep driving on four cylinders...

wdonovan

You go, PZ. Nice diagnosis.

red67

spoke to my mechanic. he refuses to fit cd or petronix ignition kit even when i explain to him all of this post and beg for it!!!!!
he's a stickler for keeping things as original as possible and wont let me **** it up and says the cars too original which i do respect but i just wanna enjoy my car also. hot plugs can burn up cylinders on the old cars so he wont do that either. he's gonna take a real good look at it monday.
last time i changed all the plugs and tested them one by one, and it was a different cylinder gone down to previous i noted. last time he fitted a new coil and new spark plug socket on one cylinder (the spark plug socket on the end of the ignition wire was short circuiting with weird ohm resistance readings) so he's guessing its the others needing to be replaced - but lets see - i dont reckon it'll work judging by what you guys say - hmmmm.
i'm bored of this thread i want it to work.

Mike Murphy

You have one of 3 potential problems, so let's try to elimate some variables.

1.) Too rich on that cylinder. Solution: why do you try swapping the carburettor with another cylinder? Can you do that? That will elimate the carb.
2.) Too much oil blow by on that one cylinder. Action: make sure the "sooting is gas and not oil. Oil will damage a plug pretty bad.
3.) Faulty spark on that cylinder. Action: swap the spark wire with another cylinder.

By the way, have you run a compression tester on those cylinders? If you're getting blow by, chances are, you have low compression on that cylinder...