TheSamba.com :: View topic - Desirable jetting for Porsche 356c at high altitudes

Thu Dec 28, 2023 10:19 am

I currently have a 1964 Porsche 356C, with Zenith 32 NDIX carbs, stock main jets (130) and idle jets (55). Altitude at my location (Kerrville, TX) is about 1,600 ft. I am considering driving the car to Colorado Springs, CO, maybe in June of next year. CS altitude is about 6,000 ft. Question...I am sure that the car will run much better at the higher altitude with different main/idle jets. Can anyone tell me what size jets you are using at the higher altitudes in your Zenith carbs?? Also, if you have the name of a vendor who can supply those size jets. Thanks

Fri Dec 29, 2023 7:44 am

Several yrs back, had a 356 tour of about 15 cars, started in Colorado Springs, down the east side of the Rockies, over them & then up the west side to Denver. This was a 10 day wonderful trip.
In preparation we had lots of discussion on carb jetting. I live in N Wi, likely about the same elevation as you. One guy from Mpls shipped his car to CarQuip in CS I believe & had them change jets for the elevation. Big mistake. Lasted about a day, he changed back in the hotel parking lot not fun!
I spoke with my trusted mechanic (who rebuilt my engine) he said unless you’re moving there permanently, don’t touch the jets. Learn to live somewhat erratic idle & loss of about 10% power. He was right. That’s what the other 13 cars did. My 62 cab still has the throttle cable for warm up so I just left it slightly pulled to compensate for rough idle.
All worked fine. Don’t fool with the jets unless you’re an expert and like fiddling with carbs.

Fri Dec 29, 2023 11:47 am

One thing to be mindful of if you leave the jetting alone (I would): when releasing the accelerator to go downhill at altitude, expect the engine to backfire. My '64 C coupe with the 75HP 32NDIX carbs isn't so bad; my son's now 1960 1600S Roadster with the same carbs sounded like a war zone. No big deal--once we got/get out of the SoCal mountains the cars ran/run fine. Have fun!! 8)

Sat Dec 30, 2023 3:49 pm

Don't mess with the jets, it won't be that bad, if anything bring a spare set of plugs incase you foul one.

Tue Jan 02, 2024 8:24 am

For a trip, it's not worth changing the jets. You may want to consider advancing the timing if you're here for a few days. Most air-cooled cars add about 5 degrees for our 6000+ feet. Most definitely add some timing if you're going up Pikes Peak.

Feel free to contact me if you need some tourist tips or want to stop by my shop. I restore 356's, among other things.

Thu Feb 08, 2024 4:39 pm

No, this wont be just a trip to Colorado Springs....my son will take possession of the car and I don't know how long he will choose to live there, He may be moving back to Texas some day, but who knows? Considering the vehicle will likely be operated at the higher altitudes where he lives for an unknown period of time, I'd rather set him up with a jetting combination more likely to succeed in those circumstances. I've only driven the car in Texas for the 15 years of my ownership and have no experience or knowledge of how the higher altitude will (if any) affect the engine (i.e. performance, maintenance, etc.). Thanks for your input.

Fri Feb 09, 2024 7:56 am

If you plan to drive the car there don't changes the jets before you go or you might be too lean and burn a valve or two, wait until you get there to change the jets. The main jets go down to 115 and cost about $15 each, idle jets cost about the same. www.dellortoshop.com has them in stock. As far as what size you need to know if you are lean or rich at your altitude to know how many steps to go down. You can take a plug wrench with you and take a 10 min. drive at 50-60 mph down the hwy. then put the clutch in and switch off the motor (don't let it idle) safely pull off the road and pull a plug and read it, White=lean, tan=Good, and Black=rich.

Mon Feb 12, 2024 8:45 am

mmorrison wrote: No, this wont be just a trip to Colorado Springs....my son will take possession of the car and I don't know how long he will choose to live there, He may be moving back to Texas some day, but who knows? Considering the vehicle will likely be operated at the higher altitudes where he lives for an unknown period of time, I'd rather set him up with a jetting combination more likely to succeed in those circumstances. I've only driven the car in Texas for the 15 years of my ownership and have no experience or knowledge of how the higher altitude will (if any) affect the engine (i.e. performance, maintenance, etc.). Thanks for your input.

If he needs assistance in setting the car up for altitude, I'd recommend Sportcar Service, here in CS. He has a lifetime of experience in tuning/jetting.

I mainly stick to metal restoration.

Thu Feb 15, 2024 5:19 pm

Thanks to all for the info...I spoke with Carb Rescue and got a rough idea of the size jets (main/idle) that might work for the higher altitude of C.S. Ordered from Alfa 1750 in Italy and requested 125 main and 52 idle jets. I'm thinking that I will probably trailer the car to C.S. this summer and change out the jets once I get there and see how it performs. Got good recommendations on Alfa 1750....he appears to have cornered the Zenith carb parts world market...will post the final results when we get to Colorado.

Mon Feb 19, 2024 12:03 pm

FWIW....I ordered main/idle jets on Feb. 15th and received them today (Feb. 19th)...brand new and sized as requested. Kudos to Alfa 1750

Sun Jun 29, 2025 5:11 pm

The Porsche 356C has been successfully transported to Colorado Springs. The stock idle jets (55) were replaced with 52's; the stock main jets (130) were replaced with 125's. I read on the Porsche 356 registry website that the general rule for high altitude jet sizes was a 6% reduction in jet size for every 1000 meters increase in altitude from sea level. The altitude at CS being about 4400 meters, per the above general rule, the idle jets should have been adjusted to about 51 and the main jets to about 120. Will try the readjusted jets for a few days of driving, check the plugs for any evidence of lean/rich and go from there.
It seems to start, idle, accelerate and wide open throttle as expected with this combination of jet sizes.

Spoke with Randy at Sport Cars shop in CS and yes, he seems to be very knowledgeable about the 356 animal and will likely get acquainted with him in the future.

Sun Jul 27, 2025 5:32 am

I live in New Mexico at 6200'. I run 125 mains in my stock nearly 1600S (Mahle P/C's, very mild elgin cam) Seems to be best with the stock 50 idles, although its a little rich. I have tried 45 idles but its just a bit too lean then when cold. Fine when hot.

Sun Jul 27, 2025 5:46 am

Thx for the New Mexico input. The engine seems to be running fine for the month it has been in CS; will be visiting Randy this week for some other minor issues and will ask for his thoughts on the jetting as well.