I'am having trouble with my 66 911 equipped with webber 40 ida 3c's. i cannot get it to run right, the symptoms being; its running extrodinarily rich, and back firing occassionally. I just replaced the ignition system and coil with pertronix ones, and put in ngk spark plugs, and while this helped, it has not solved it. I am using a 7 psi fuel pump, but i have a fuel pressure regulator in line limiting it to 3.5 psi. This car has been sitting for 7+ years without running. I have already dissassembled cleaned and put back together both carb banks. As i said, the car runs very very very rich, yet it idles fastest with the mixture adjust screws the way they are, leaning it out only seems to kill it off. carb 1 runs fastest with the mixture adjust in all the way [shouldn't this cut off the fuel?]
any advice would be much appreciated, because I am almost out of ideas. I do know that the person who worked on the carbs last managed to install the floats upside down, so who knows what else they may have messed up. My next step would be to totally tear down all carbs, and insure that the jets are the correct ones, can anyone tell me what the jet numbers should be for these carbs?
also, the engine was rebuilt 7 years ago and has zero miles on it, could this be a factor?
really makes you appreciate fuel injection....

 

Have you verified the inlet fuel pressure is 3.5 psi?

Have you checked the float levels with an external float gauge?

Can you observe fuel dribbling out of each discharge nozzle at idle?

I've answered your questions with my own questions.

Jet settings are variable depending on your engine and who did the jetting. Provide some engine details and folks should give you some ball park or exact jetting suggestions.

Sherwood

 

Once again, Sherwood offers sterling advice.

Follow that to the letter, and your car will run a lot better.

 

I have verified the fuel presure, i have checked the flot levels, and I am unable to detect fuel dribbling from a discharge nozzle. the engine number is 907996, and the carb numbers are 40ida-3c. I tried tuning it again today, and while it no longer emits a plume of smoke, it still backfires with suprising regularity, and is clearly running about as rich. the idle is also strange. it likes to run at aboug 1800 rpm, regardless of setting, but, given any resistance [slipping the cluth], it quickly falls, and unless throttle is applied, will keep dropping until it is barely turning, or even stops altogether. If throttle is pplied, it will go to 1800 and stay there. playing with the idle screws seems to have frustratingly little effect.

 

I'm going to assume that your dwell and timing are correct, and that your engine is sound. (1) Remove and clean the (6) idle fuel jets, which are located along the outboard side of each carb ( 3 per) near the top. There is one jet per cylinder, and each has two parts. Remove them one at a time, pull them apart (make sure that your hands are clean and you don't have any rags nearby - no lint allowed) and hold the thin section up to the light to verify that the orifice is clear. Blow off all parts with compressed air and re-install them. (2) With the air cleaner removed disconnect both carb drop links (operating push rods) and use your Uni-syn to get an average left to right balance. (3) Clean and lube one drop link and install it, clean the second link and install it on the bottom. With the throttle at its stop attempt to connect the remaining drop link ball cup, it should go on without pre-loading the opposite carb. If there is any preload loosen the ball cup nuts and adjust as necessary. (4) Turn each volume control screw clockwise until it stops (these are the screws near the bottom of the carbs that are fitted with a spring). DO NOT GET MUSCULAR WITH THEM - just turn gently until they stop. Now, back out each screw five half-turns. Start and warm the car. (5) Attempt to use the idle speed screws to obtain an idle near 1,000 rpm - do this with the drop links disconnected at the top, and use your Uni-syn. (6) You should have a fair idle at this point. Re-connect the links. One at a time, turn the idle speed screws clockwise - slowly and only 1/8 turn at a time - until the engine just begins to labor. Back out the screw 1/2 turn and go to the next. If the engine won't labor clean the idle jet again - this time with the engine idling. (7) If this procedure fails you will have some sort of problem either in the carbs, linkage, or other. (DO NOT vary the order of these instructions!) Good luck!
Pete

 

thanks for the assistance, but a few newbie questions; when you say "turn the idle speed screws clockwise until the engine begins to labor" do you mean the volume control screws? and would the compesation screws [with the nuts on them] have any affect on this? pardon the ignorance..
i'll give it a shot tommorow, thanks for the hints!
-Bob

 

Syncrometers are just fine since Uni-syn's are NLA.

 

wow, i just love the response time here.
i also edited my earlier message with 2 new questions: when pete says "turn the idle speed screws clockwise until the engine begins to labor" does he [do you] you mean the volume control screws? Also, would the compensation screws [with the nuts] have any affect here?
-Bob

 

Bob: Use the compensation screws with lock nuts to equalize air intake on each carb (using my obsolete Uni-syn) - I should have included this in step # 2, then turn the "volume control screws" in very slowly until... Sorry, I glitched in my step # 6! There are only two idle speed screws, and six volume control screws...

Steve: I tried one of those other things - I hated it and bought a backup Uni-syn!
Pete

 

LOL,..I use both as they have complimentary qualities.

Longer duration cams such as "S" cams or greater, really make accurate readings with the Uni-syn challenging and thats why I like my Syncrometer,...

 

well, as an update, I tried Peters method with no luck. I couldn't find any junk in the jets, and when I replaced them, it ran the same, even after tuning attempts.
After that I simply tore them down, looking at every jet, and using plenty of carb cleaner. I did find some gunk in 2 mainjets.
put it back together, ran it, tuned it, same deal. the idle wont do anything below 1800 or above 450 rpms, and cylinder 1, 4, 5 all run the best with the control screws [with the springs] all the way in. [shouldnt this cut off fuel to the cylinders? I was not able to detect any sort of dripping from the accelerator pump/jet. I have jet numbers if that would help. any ideas that might help a newbie....

 

Hello,

did you get your carb problem fixed? I had a similiar issue on a 1966 911 with carbs and the issue was crud deep down in the idle circuit. My car sat for 26 years, so the crud was to be expected. Using a digitial temp guage, I found two cylinders not idling.

I took off the carb tops and with the bowls full, pumped the carb. The two cylinders that were not hot, had a blockage deep down behind an aluminum plug. No amount of brake clean or air could clean out the passage. I had a good machine shop drill out the two plugs and using a drill bit, cleaned out the crud.

Now the crud is gone and the car accelerates fine.

As I did my research, I found with Webers that the first one third of the acceleration comes from the idle jets.

Good luck, Tom

 

Thanks for the response Tom, glad to hear your carbs are working for you
My car has been running much better, but it has yet to be completely solved. Turns out my issue was primarily with my ignition: not enough spark. Despite the new coil/pertronix ignition, and new ngk plugs, it needed even more. I went to an msd unit and NGK bp4es plugs. using those I was able to get all cylinders to fire. From there I did much more tuning [the adjustments screws work much better when that cylinder is actually running!]. Still, not all difficulties have been solved. Right now, the car runs like a rocket from about 3000 rpm's up to redline [and pulls like a train!], but I am getting fuel mileage in the single digits, not sure quite what fuel mileage, but by approxamation, significantly lower than 10 mpg, and likely much close to 5. The midrange is also crap. From about 2k to 3k the car just shutters and lurches very violently. I'am not sure what my next step will be. Having taken the carbs off and apart many many times, I am quite certain I have no clogged passages or jets. I do know that the jets are incorrect though [according to factory settings]. here's what I have now:
venturi : 30 mm factory setting: 27mm
main jets:125 factory setting: 110
air correction jets: 180 factory: 185
idle's: 55 factory: 50
emulsification: f26 factory: f2
it is possible the f26 e-tubes may be f 2.6, but they are scratched enough I can neither confirm nor deny this.

Anyone have any idea what could be causing my problems? I've been through the carbs very thuroughly at this point, and besides the re-jetting, they should be perfectly functional. How much should these settings affect my running?
thanks everyone for the help so far, I kinda thought this thread was dead till tom brought it back!

 

Is your engine a stock 2.0 liter, or is it bigger? What about cams, compression ratio, etc? My advice would be to call Richard Parr at PMO carburetors in Santa Monica. That guy is a whiz; you call him up and tell him the specs of your engine and he'll set you up w/jetting that is pretty much dead-on. We have installed his beautiful PMO carbs on several customer engines and most of em have only needed minor tuning to run perfecty. But, he will be able to sell you the correct jets and anything else you need to get running properly . . .

If your engine is otherwise stock, I don't see any reason to be running bigger venturis and jets . . . .

---Chris A.

 

Richard Parr has a business to run, therefore he does not freely give out his phone number.
You probably need idle jets, main jets, a float level glass tube, a synchrometer, and a maybe a gasket.
Get his email and send him a note with all of the specs you have presented here.
He will email back suggested parts. Order his parts. Then as a customer you have earned the right to ask for help.
I'm sure you understand this, some don't.
The factory settings would be a great place to start.
My experience is the floats are very sensitive to the correct height setting.
The glass tube gauge used to set the floats is indispensible.
One reason PMO carbs come with a sight glass in each bowl.
I would get the proper jets before doing any further work.