1971 911T dies at idle when warm

karlekroth · 04-07-2014, 10:53 PM

1971 911T dies at idle when warm. Starts up if gas pumped, once running can pump it until it finally maintains idle. I drive it 2 miles, get it a little warm, pull to a stop, and then it wants to die.
Very little back firing at idle before warm. No back firing when accelerating ever.
I thought idle jets, but why would it even start and run to an idle when cold?
When I mean idle, I mean under 2-3K RPM.
Idle screws or what? Carbs are CLEAN.

gamin · 04-08-2014, 02:42 AM

Assume engine restarts after it dies. Check your fuel pressure. Check inline fuel filter and mesh filter at fuel tank (tank empty). Linkage. Idle setting at carbs. My 70T starts cold with lots of pumping then rough idle with spitting and then smooths out. Normal with no choke. Idle is low when cold then normal when warm. I'm guessing fuel problem.

gamin · 04-08-2014, 04:04 AM

Doesn't seem to be a lot of early owner posts here. You might want to try early 911s registry. Great bunch with a lot of knowledge. If you are not a member you should join.

karlekroth · 04-18-2015, 12:21 PM

All electrical replaced. Fuel pressure is 4-8psi constantly. I'm back to thinking either the jetting is off or the coil is bad when hot. I ordered another coil to put in after the car heats up, that will tell me if the coil was the issue.
Note, I just found out that OIL coils should always be mounted vertically! On our porsches they are mounted at an angle. So I ordered an EPOXY coil to solve this issue since they do not require a certain position.

karlekroth · 04-18-2015, 12:33 PM

Idle jet is 47.5. Oddly the number on the jet says 947.5P. But the 9 is a symbol, not a numeral. Terrible. I live at sea level, so I might have to increase the idle jet from what I'm reading. But waiting for the coil first to make sure that wasn't the problem.

I have Marelli rebuilt with pertronix ignitor and flame-thrower oil - currently getting the epoxy coil to avoid vertical placement issues. Zenith 40 tin. Aftermarket deadhead fuel pump 4-8 psi with no return. valve clearances ok. brand new plugs, wires, new stock air filter.

If you guys have zenith 40 tin, can you list your: jet setups, idle screw turns out, fuel psi, fuel deadhead or return - type of fuel pump, valve clearance last noted, altitude you are at, and if you have the stock air box or dual aftermarkets? Any and all information is helpful. I know its not going to be exact, but your information does make for easier trouble shooting.

karlekroth · 04-18-2015, 09:12 PM

Just heard 8 PSI might be too much for these carbs. Could be a problem. Switching fuel pumps to a lower 2-4 psi, 3.5 being optimal supposedly. I was afraid that putting a regulator on the current pump might burn it out. Here's a few pumps: Carter P60504 is one. Electric Fuel Pump Low Pressure 12 Volt Universal 2-4 PSI Solid state is another. If the new pump + epoxy coil don't fix the issue, then i know 100% that it is carbs or major exhaust leak. Then begins the correct jet settings. Please list your set up as requested if you have zenith.

pors1968 · pors1968's Garage

You need 3 to 3.5 psi

Jose_JGC · Jose_JGC's Garage

Just guessing, it could be a loose fuel pump fuse. It is easy to test, can fuse #6 be rotated easily?
Jose

bgyglfr · 07:26 AM

Take a look here for initial settings. You will need synchro meter and a float bowl gauge for carb tuning. An afr sensor takes most of the guesswork out of it.

Zenith 40 TIN Carburettor Manual

Quote:

Just heard 8 PSI might be too much for these carbs. Could be a problem. Switching fuel pumps to a lower 2-4 psi, 3.5 being optimal supposedly. I was afraid that putting a regulator on the current pump might burn it out. Here's a few pumps: Carter P60504 is one. Electric Fuel Pump Low Pressure 12 Volt Universal 2-4 PSI Solid state is another. If the new pump + epoxy coil don't fix the issue, then i know 100% that it is carbs or major exhaust leak. Then begins the correct jet settings. Please list your set up as requested if you have zenith.

karlekroth · 06-11-2015, 01:58 PM

I put in a cheap fuel pump that puts out 2-4psi as a test. The car runs amazingly better after warm up. The previous 10psi was drowning the zenith 40 tin carbs. I don't expect this cheap fuel pump to last long, so I'm already looking for my better quality pump. You guys helped me fix a HUGE issue. Now I can finally tune the carbs properly.

Dirty Marty · Dirty Marty's Garage

Karlekroth,

I've got a 70 like yours with a Marelli distributor as well. You may want to check your timing..........mine was doing the same thing and I was chasing the carb settings all over the place and just making it worse.
Out of sheer despair I advanced the timing a a couple of degrees and it took care of it............but make sure you know that your timing is somewhere in range.

Good Luck
Dirty Marty
Fallbrook,CA

karlekroth · 06-15-2015, 09:22 AM

Yeah, we checked the timing many times. We switched to pertronix ignitor + coil and had to get it ballpark to run ok. But it was a pain. That was all before I noticed that the fuel pressure was way too high, causing the car to run well on start up but then terrible after warm up. We should have checked fuel pressure first.

924CarreraGTP · 924CarreraGTP's Garage

I took the advice of some other owners on here and went up to a size 55 for the idle jets in my 40TIN Zeniths. My '70T runs much better, but I still don't have a consistent idle for very long. The engine will idle kinda roughly for 10 seconds or so but then it starts to drop and will eventually stall. The transition between the idle jet and main jet is still not great either. Most people don't realize that these carbs had to be reset with every change in season and only stay in tune for 1,500 miles or so. It's strange to me that before I decided to rebuild my carbs they seemed to idle fine with a 47.5 idle jet. I'm just not sure why I can't make it idle correctly no matter what I do.

wayne robson · wayne robson's Garage

What Dirty Marty says ,is correct , timing is critical , also Pertronix
makes two types, the later version eliminates ,burnt out igniter
which can happen if ,example leaving ignition on for longer than 5--10 minutes,etc,.
timing can be advanced approx one to one and a half degress.
i have been using Pertronix for over twenty years now,not just on Porsche.

karlekroth · 06-22-2015, 10:25 AM

924CarreraGTP. Did you check fuel pressure? That was my main problem at idle. 3.5psi fuel pressure is what I've been told for zenith, though i think i'm even slightly lower than that because of the cheap pump i bought. I've also been told that ethanol free gas helps. I'm still on 47.5 idle jets, but I'm also at sea level in Hawaii where the weather never changes. It will still die, just not as painfully as it did before. I'm thinking about just increasing the idle speed up past 1000rpm as I've had to do on motorcycles to keep it from stalling. As long as you're moving it doesn't matter, you don't want a high idle in traffic for very long on an air cooled. Old cars, old problems. Without this forum I'd really be in trouble.