911T- weber trouble.

[IanK]

I am hoping someone can give me a little help.

I have a nice 72 911T. The engine has been fully rebuilt by a well regarded Porsche guy. The car did not come with MFI, but rather webers. They were not in very good condition and so I had them rebuilt by the only weber specialist in NZ.

However, I have lots of troubles. The car backfires, drops a cylinder reguarly, flutters and smells very strongly of gas. When the mechanic was trying to set up the car following the rebuild of the webers he could not get the car to run very well on the standard needle and so drilled it out slightly. Now it can run great, but only about 50% of the time.

No one seems to know how to fix it, or if the issues may be non weber related. The small of gas is so strong that you cannot stand next to the car while it is running.

I know this is a brief description but I wonder if anyone can help or point me in the right direction?

Many Thanks

Ian

[Noah]

Have you baselined the other stuff -- correct fuel pressure, spark plugs not fouled, points and timing set, distributor advance working, etc? Are the carbs correctly jetted for your engine?

[72targa]

I'm most concerned about your statement "smells strongly of gas". I've seen too many pictures of our early 911s all burned up at the rear. If I were you, I would stop driving it until this one problem is repaired. Only then would I start work on any other adjustments. My .02

[IanK]

Hi Noah, yes I understand that all of these have been checked, over and over again. Personally I do not understand why the jets had to be made larger? I also do not get why no one seems worried about the petrol smell. I can't count how many time I have been told that beers small of gas and these cars burn a lot of fuel. My last 911 never did this..

[IanK]

Hi Peter, thanks this this. Yes it worries me to and I was driving my 10 year old to school today and oddly, that was what I was thinking. I guess my big issue is- is it the carbs or can it be something else. If its the carbs then I would get PMO's, but I am reluctant to do that until I know it will work. And, I have bought a 356 and that is now sucking all my cash...I just love the 911 when its going well. I have just got out of the car and for the last 10 minutes it drove beautifully, for the 30 minutes prior it back fired and seemed to be running on 4 or 5 cyclinders...

[66S]

The most common problem is a plugged idle jet. Learn how to remove and clean them. A piece of debris can be in the idle circuit and intermittantly block the idle jet if it isnt lodged in the jet. Float level is also very important to check before the carbs can be tuned properly.

[76911S]

I would contact Paul here.
http://www.performanceoriented.com/

He is on this forum and may chime in anyway. I would take his advise and word on anything Weber/Porsche over anyone else. He does great work and excellent follow up and advise.

Mark..

[chris_seven]

There really shouldn't be too many issues involved in making the car perform correctly using Webers and if they smell of fuel there are a few simple reasons to look at first:

a) The carbs themselves may leak from either the banjo unions or the accelerators pump housings if these parts have not been restored correctly.
b) The fuel pressure may be too high and causing fuel to escape from the jets as the needle valves are not closing correctly. This can cause leaks and make the engine bay smell of fuel as well are causing the car to run rich.
c)The float height may be set incorrectly causing a similar issue to (b)
d)The float valves themselves can stick again causing problems similar to (b)
e)The idle or main jets may just be too big but this would result in the exhaust being rich and may not cause a fuel smell in the engine bay.
f)Some of the braided hose being used as fuel line may have deteriorated due to Ethanol and this will cause a fuel smell in the engine bay. We have had specific problems with Cohline hose and now use Codan which seems to have resolved this issue.

Maximum fuel pressure should be around 3.5 psi but new needle valves in good condition may seal with up to 6psi - we check all the carbs we refurbish at this pressure.

There are basically five Weber types used on 911s; 40IDT, 40IDTP, 40IDA, 40 IDAP and 40IDS. Each engine has one 40IDx3C and one 40IDx3C1 carburetor for left and right respectively. The ‘3C’ refers to three chokes or intake venturi common to all 911s

While the basic carburetors are very similar, there are subtle differences.

There is fundamentally no real reason why your carbs shouldn't be capable or working correctly but there are some nuances that may cause slight 'stumbles' depending on the individual model of carb and your engine characteristics.

To summarize their original applications:

40IDT3C and 40IDT3C1; 1968 911T (non-USA) 901/03 (901/13 Sportomatic).

40IDA3C and 40IDA3C1; 1966-’67 911 (normal) 901/05 and 911/06 and the 1968 911L (non-USA) 901/06 (901/07 Sportomatic). There were also undocumented early 1968 911 (normal, non-USA) that used these.

40IDS3C and 40IDS3C1; 1967-’68 911S 902/02 (901/08 ’68 Sportomatic).

40IDAP3C and 40IDAP3C1; 1968 911 (USA) and 911L (USA) 901/14 (901/17 Sportomatic).

40IDTP3C and 40IDTP3C1; 1969 911T 901/03 (901/13 Sportomatic) and 901/16 (901/19 Sportomatic). Also the non-USA 1971 only 2.2 911T 911/03 (911/06 Sportomatic).


Basically the differences are jetting and venturi size. Most of the jets (and emulsion tubes) are replaceable brass parts. The venturi are replaceable. There are some non-replaceable jets (idle air correction aka idle air bleed) pressed into the casting. The 40IDS has an additional ‘high speed enrichment’ circuit. There are subtle but important differences in the size and placement of the just-off-idle non-replaceable ‘Transition ports’.

The 'cam' which operates the accelerator pump also varies from model to model.

If the carbs have been correctly restored and tested there should be no reason to replace them with a PMO and if you have eliminated problems of fuel pressure and leaks it may be a little more difficult to track down the issues causing the problem but it is relatively straightforward.

Do you have any idea of the type of carb you have fitted and the sizes of the jets/venturis that are currently installed? Is you engine standard apart from the carbs and what type of distributor do you have fitted?

The restorer may have checked and noted some of these sizes and it may help in tracking down the problem.


From time to time we see carbs that have been quite badly abused and or badly restored and the carbs shown below were considered to be 'scrap' by many 'experts'.







They do, however, now run quite well.



It should really be possible to fix most issues.

[IanK]

HI Chris, thanks so much for this. I was looking over the documents from the various tests etc and I note that:

1 Carbs are IDA3C
2. On dyno testing (pre-increasing idle size from 50 to 70) the test noted- Car is presenting odd symptoms- whilst under load there is black smoke from exhaust, car does not small rich nor does the oxygen sensor read rich, in fact it reads lean"
3. The idles were then drilled out to 70 and the car back fires, smells of gas at the exhaust (not the engine area) loses a cylinder and using a lot of gas- but generally runs better that pre drill out of the idle.

[NZVW]

Ian,
Did you have the engine rebuilt in NZ? If so by whom.
How was it going prior to getting the carbs worked on.
Who worked on the carbs,, There are a few "old boys' who know these carbs in NZ
The intermittent nature of your issues backfire, dropping cylinder smell of gas out exhaust indicate a "spark" issue be it timing or a heat related electrical prob. but Im no wiz on these and my 72 has MFI.
Re "petrol smell" this is always disconcerting and alarming..I always keep a fire exting inmy car at all times,
PM me if you want ,, Im Ak based
Mark