More Tuning Questions

Hi guys

During a reasonably long drive recently I noticed that my car seemed to be running very lean (69T webers). Light ash grey deposits within and around the tail pipe. I haven’t changed the jetting set up on the carbs and my mechanic who took care of the car for the PO mentioned that it has already had a larger main jet (and corresponding air correction jet) installed. The only other matter worth noting was that it was reasonably warm day (<>30C) and I did traverse a mountain range, whilst the car has only ever been tuned at close to sea level (don’t know if this is relevant). Average engine speed would have been at around 4000rpm. I have started running the car back on leaded fuel after a short stint on hi-octane unleaded, which would have been long flushed out of the fuel system.

Normal driving around the city the car seems to run with a normal mixture

With previous experience with English sports cars this tail pipe sign is usually the first indication of a pending burnt out valve, which is something I really don’t want to deal with at the moment (ever?).

My questions are:
Do these cars burn out valves readily and given the limited information provided, should I be concerned?
Are there any obvious indicators of a burnt out valve that I should look out for, in case I have already done damage?
Would an octane boosted additive to the fuel provide a short term solution to a lean mixture problem should I experience this again?

Thanks for any comments or advice.

Regards,

Paul, It does not sound to me like you are in any danger of burning any valves, in fact, light gray is the 'perfect' condition for a tailpipe after a highway trip. If there is no pinging or knocking going up those mountains, there is no need for an octane booster. If you want a good indicator for your mixture 'correctnes', pull your sparkplugs immediately after a one-hour highway run and check for signs of too lean a mixture. Light gray-brown is normal, whitish, or cracked center insulators would indicate a mixture problem. And, if you do have a too-lean mixture, an octane booster will not help or alleviate any potential damage to valves, it might eliminate the pinging up the hills! If there is a problem with the spark plugs, you may want to discuss the matter with your mechanic and see if he has a portable air-fuel mixture gauge that he can take along with a test drive, with a 'sniffer' probe in the tail pipe.

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Warren Hall
1973 911S Targa

Damn, wish I knew this earlier. Backed off on the throttle for most of the return trip home and missed out on some great driving through some of our state forests. Anyway, it gives this paranoid early Porsche owner great comfort to know that he is not about to do great damage to his car. Many thanks once again Warren.