1968 911 - Altitude Adjustment

NordschleifeFIA · 08-26-2007, 05:15 AM

We've just moved from southern Arizona (5000 feet above sea level and dry) to Woodbridge, Virginia (near sea level and humid). I took my 1968 911 2.0L with weber carbs out for a first spin yesterday and was underwhelmed with the performance. Time to adjust the carbs!

I've never had a 911 before I bought this 1968 coupe this summer, so I'm learning.

As a baseline, Dave Radmacher (Tucson) did an excellent rebuild/adjustment job on the Weber 40s just 3 months ago and greatly improved the initial performance.

Observations:

- car ran very well before we left AZ.
- car now lacks power above 5000PM.
- some spitting through carbs on start-up.
- car has brand new Taylor ignition wires and NGK BP8ES spark plugs.
- float levels were synchronized by Mr. Radmacher.

I've read some excellent posts on this Pelican board by T-Bird and others but have a basic question: should I just pull the spark plugs, assess and adjust the idle mixture accordingly? Or is there a better starting place?

Also, as a newbie to the greater Washington DC area, I'd appreciate any advice as to a good mechanic or shop that knows these early cars.

Thanks to all in advance for your advice.

Special thanks to Grady Clay who gave me such good advice when I first bought this car.

Best Regards,
Andy
1968 Porsche 911 base coupe "Hot Rod"

Joe Bob · Joe Bob's Garage

That's a good start.....Moving to higher altitude creates a richer mixture because you now have less air. You will need to re-jet the carbs for higher altitude. You MIGHT try turning the idle mixture screws out one full turn. Make sure you do each screw the same, document WHAT you did so that you can return it it the original settings when you go back "down below".

I like to set carbs when having the use of a sniffer in the tail pipe. Having a unisync also helps. Is there a local wrench that has air cooled experience? He might be your new best friend....

911s55 · 08-26-2007, 07:04 AM

Andy, I'm at sea level and have similar problem in reverse going over 5,000 ft mountain passes. I think Mike saw your post backwards, what you have now is too much air mixture, and you'll want to try to richen it up a bit.

Start by turning in the individual idle mixture screws (large ones with springs) in until motor "begins" to drop RPMs, and back off. Do all these adjustments with motor at operating temp, and go slowly allowing motor to respond to changes made before moving on. Luckily you shouldn't have to worry about fouling during set up since you are already lean.

What plugs are you using?

A synchrometer is a good investment $50 or so and let's you monitor your air draw and balance carbs, very important.
David

Joe Bob · Joe Bob's Garage

yeah...I do that....

2.7RACER · 08-26-2007, 11:31 AM

Andy,
No doubt Dave Radmacher did a great job of optimizing your '68 911 2.0L with Webers for the 5,000 ft. altitude.
Probably running smaller main jets, and larger air correction jets as well as possibly smaller idle jets than what is typical for sea level.
The ignition timing is likely set 4 or 5 degrees more advance.
The problem going from 5,000 ft to sea level is the carbs will run the engine significantly leaner because you have 15% more oxygen at sea level.
This, combined with advanced ignition timing, will run your engine hotter, in some cases hot enough to burn valves.
Find a good local independent Porsche shop and explain the car has been optimized for high altitude and now it's time to change things back to sea level settings.
Re-set for sea level, you'll like the new found horsepower and performance.
Most books that spec out jet sizes and timing are for sea level. Use that info for a great starting place.

NordschleifeFIA · 08-26-2007, 04:00 PM

Thanks to all who responded! I found Dave Radmacher's notes regarding my Weber carb settings. Currently I have:

Venturi: 30mm
Air Correction Jet: 185
Main Jet: 120
Idle Jet: 52

What do you think? Should I increase the Main and Idle jet sizes to Mains = 125 and Idles = 55?

Best Regards,
Andy

NordschleifeFIA · 08-26-2007, 04:14 PM

Another note: I removed and assessed all the spark plugs today. They were all light brown to white, so lean condition is likely.

Next, I noted all six idle mixture screw settings. I then slowly screwed in the cylinder #1 idle screw to see how many turns it took to completely seat the screw; it was 1 and 1/4 turns out from full in. Then I carefully returned that idle mixture screw back to where I'd started. From a quick glance, all 6 idle screws seemed to be more-or-less the same tension (judging from how much binding each idle mixture screw spring's coil binding displayed).

Finally, I then screwed out all 6 of the mixture screws 1/4 turn to enrich the mixture. Then I drove it again at normal engine temp (190-200F).

Clear improvement. Throttle response between 2800 - 4000 RPM is greatly improved. BUT above 5000 RPM at wide open throttle (WOT) I started to get the same mis-fire and resistance - doesn't want to rev. So I didn't push it, brought it back home.

Is it likely the Main Jets are simply to small and can't flow enough fuel at WOT and higher revs?

Thanks to All,
Andy

2.7RACER · 08-26-2007, 05:49 PM

Hi Andy,
The info I have indicates a 30mm venturi, 125 main, 180 air correction, 55 idle jet and a F26 emulsion tube. These are the stock spec's.
A 2.0S with a hotter cam runs a 32mm venturi, 130 main, 180 air correction, 55 idle and a F3 emulsion tube.
I would guess Dave R also advanced the ignition timing from stock.
The smart play would be to call Dave and discuss this with him.
He may have the parts you need.