’70 911T engine performance vs ambient temperature

[911T70]

With the warmer than usual fall we’ve been having and the lack of snow in central-eastern Canada, I’ve been enjoying my 1970 911T much longer than I expected to this year, which is a real bonus! Driving it in cool temperatures, I’ve noticed that the engine performs much better. Since I’ve had it, the engine hesites and sometimes backfires, belching a black cloud of smoke at around 3000 rpm but pulls strongly elsewhere. It hesitates and backfires much less when the temperature is lower, being almost un-driveable on very hot days. I’m assuming it needs a good tune-up, which I am planning on undertaking starting my first valve adjustment over the winter.

The last tune-up was done by a ‘Porsche expert’, according to the PO, in Cedar Rapids, Iowa, which is at a different elevation than its new home, thus at a different atmospheric pressure. I was assuming this was the reason.

Does this behaviour relative to temperature hint at some other problems or does it confirm that my Zeniths need to be tuned to their new environment? Any advice would be appreciated; now that I’ve experienced how much fun this car can be, I want it to run this way all the time!

[stownsen914]

Belching smoke and running better when it's cool are both consistent with the engine running rich. Cool air is denser (i.e. more oxygen), so other things being equal, more fuel is required to make the ideal air/fuel mixture. Elevation can definitely play a role in the running of your car, but I'd be surprised if at is last tuneup the mechanic changed jetting in your carbs. Assuming your carbs are jetted correctly, they may need a good cleaning or a rebuild. It's not uncommon for carbs to get gummed up, especially if the car has been sitting. If you haven't already, check a couple sparkplugs. A rich mixture will yield black sparkplugs. This would be good information to know for starters.

Scott

[911T70]

Thanks a lot, Scott! I suspected it's running rich - I'm only getting about 13 mpg and if I hold a rag near the exhaust for a second I can smell fuel. Checking the spark plugs will be the first thing I do after I park it for the winter.

I also suspect the car wasn't driven much after the carbs were rebuilt and tuned so they might be gummed up. The PO is a collector who has a few 911s so it probably sat in his garage.

I'm new to carburetors and find them intimidating but I'm looking forward to start playing around with them. I will probably make them worse before I make them better but that's how we learn. I've found a lot of good information here on this forum that I will try to digest before I dive in! Thanks again!

Michel

[Reiver]

I always 'tune' my CIS for the season leaning it in the summer a tad and a bit richer in the cooler weather. (on board AFR)
Cold air is 'denser' so changes the AFR...the cooler air is doing you some tune up help,

[stownsen914]

Michel - it sounds like you're in for some fun learning to tinker with your 911! I'm not familiar with Zeniths, but if you search the forum there's probably some good info. on the steps to take to check/clean the carbs. I'd guess between doing that and a tuneup, you'll probably address the rough running condition you've been experiencing. Have fun!

[Trackrash]

Sounds like its time to rebuild those Zeniths. I'm guessing the needle and seat assemblies are shot.
I used to get 25mpg on the highway with my Zeniths.

[Norm01]

My only recommendation is to do only one thing at a time then start and run the car before the next little project. Doing the carbs, plugs, cap, rotor, valves, fuel filter and others all at once may lead you to chase a lot of small problems when you start it again. Setting up the carbs is not an easy task at first and what you think are carb issues could be something else.

[911T70]

Thanks for the advice, Reiver, Scott, Trackrash and Norm01. The carbs were rebuilt a couple of years ago and the car then probably sat until I got my hands on it so I’m hoping that they only need to be adjusted, worst case maybe a cleaning. Doing a tune-up one step at a time is great advice. I’ll start by doing a valve adjustment during the winter and then the rest when I can drive the car in the spring.

You’re right, Scott, it’s a lot of fun tinkering with these cars because they are simple and so well engineered and built. I’ve already fixed quite a few problems with it, thanks to a lot of good advice and information I received or found on this amazing forum. I found great info on Zeniths from two members, A Quiet Boom and 1QuickS, that will be useful. I can share links to those threads if anyone needs Zenith information.

[67R69S]

Hi Michel, I'll take those Zenith links please! Yeah, 13mpg is not normal; I was usually around 23-27 under normal driving, less under "other" driving.

Cheers,

Gunther

[911T70]

Hi Gunther,

13 mpg is a rough estimate - I kept track of my fuel consumption and mileage this summer after having fixed my speedometer last spring. I'll calculate my real mpg soon. I usually go out for short rides where there is a lot of acceleration/deceleration so my mileage suffers. The noise it makes is addictive! It's another good reason to get those carbs tuned though; fuel economy and less emissions.

Here are some links; there are many more I'm sure:

Scroll down this one and look for A Quiet Boom's comments. He describes full disassembly/assembly/set-up procedures:

Zeniths on Stock 2.7L

Thread by 1QuickS:

Zenith 40TIN service and design review

Hers'a another good one:

Zenith Rebuild - My First Carby Project

And finally the ultimate reference (I printed this one and put it in its own binder):

Zenith 40 TIN Carburettor Manual

You're in Ottawa and have Zeniths? We should get together next summer and compare notes. Porsche guys regularly get together on Sunday mornings for coffee. Here's the only time I could attend, the meeting being held at Landsdowne:

Ottawa Cars & Coffee

Michel

[67R69S]

Hi Michel, thanks for the great links! I'm tearing into my Zeniths in the next few weeks, just waiting on some new fluid for the ultrasonic cleaner. I would definitely like to compare notes on these carbs at some point.

Great looking picture of your car at Landsdowne; I have a bit of an obsession for 911's in slate grey. To me it's one of the best colours. Mine is run of the mill white.

Gunther

[911T70]

You're welcome, Gunther. Good luck with your Zeniths. It's good to know that we have a local expert; I'll be consulting you next spring!

911's look great in white also. Which one do you have; 67R? 69S? Both?! There's no info in your 'Garage'!

[67R69S]

Feel free to consult so you don't make the same mistakes I will likely make

I'll have to figure out the garage info thing; I have a 1971 911T. The 67R69s refers to one of my motorbikes, a 1967 BMW R69s.

[911T70]

In preparation for my first attempt at adjusting my Zeniths when spring finally arrives, I removed the AC compressor to give me better access along with the condenser. Although it blew cold air when I first got the car, I never used the AC and wanted to remove it eventually. I was planning on waiting until the spring to do this so that I could drive it to a garage equipped with a freon removal system. Wanting to confirm that there was any freon in the system, I loosened one of the joints and nothing happened; the gas had already leaked out. Getting rid of that stuff really opens up the engine bay - looks so much better!




You can see in the first photo that the breather hoses to the oil tank were reversed. I will re-install them the proper way, which will hopefully reduce the amount of oil that ends up in the air box. I will install a catch can to scrub the oil out of the re-circulated air, as suggested by wayner here on Pelican.

I also removed the cold air distributer from under the dash. Love the minimalist look in the cabin.


The spark plugs were removed to see if they can give any hints on the engine health and the carb settings. The plugs in cylinders 1,2 and 3 were as expected, suspecting that the carbs were set rich. I was happy to se they weren't oily, which hopefully means the engine is not too tired!



I was pleasantly surprised to see the plugs in cylinders 4,5 and 6.



Any comments on what the plugs indicate and what I need to adjust on the Zeniths would be very appreciated!