Weber overhaul and set up advice please

[Captaintremendous]

Gents.
I have had my 1969 911T for a while now and to honest, much as I love the car, I've never been 100% happy with how the Webers have performed at low range. It has always had an annoying stumble when trying to accelerate in 2nd, for example having come down through the box to negotiate a sharp bend, when I then try to accelerate away. I often have to dip the clutch and boot it a little to get it to clear and start pulling smoothly. Anyway, long story short, I have taken the plunge and have the carbs on the bench. They are 40 Ida 3c. The previous owner used the car for the track and I have not changed the set up to date as the car performs well at speed. I would like to improve drivability at lower speed however and lose the maddening stumble.
This is what I have found.
Venturi 34
Main jets 145
Idle jets 60
Air correction jet 185
Emulsion tube F3
The only ident I can find on the accelerator pump nozzle is a number one.
Everything is clean and i have replacement gaskets, o rings etc for when it goes back together, float height is good and accelerator pumps have new diaphragms fitted and function well.
Any thoughts on jetting, Venturi size etc and tips on setting up when it comes time to break out the Synchrometer?
Cheers

[MikeB]

Any wear on the spindles, have you replaced the Teflon bearings at each end?

[mycar]

Did you check the timing before you took the carbs to pieces ?

And the spark plug gaps ?

Just me then.

Is the engine stock ?

If the engine is stock you could try returning the carbs to stock and see how things run then.

http://www.racetep.com/weber/40IDA_46IDA.pdf

Before you reassemble the carbs It may be a good idea to clean out the galleries in the main body with carb cleaner/acetone, compressed air and pipe cleaners. (wear goggles) Have you considered removing the lead plugs to gain access ?

As Mike says, air leaks at the spindle ends can cause low rev stumbling.

Edit.... Just a thought. You can never be sure that the existing jets are what they say they are. I've had jets drilled out at rolling road sessions to 'free things up' and without a set of jet gauges, nobody would ever know. At least with new jets you know where you are.

[Captaintremendous]

Thanks for the responses so far. Timing and dwell were checked just prior to carb tear down, all good. Everything has had a good "blow through" with the air line and to be honest, everything looks nice and clean. My worry is that the carbs have been set up with top end "track" use in mind as opposed to a balance of performance and driveability. I don't want to faff around too much with chopping and changing jets etc, so need a pointer to a good average starting point.

[Captaintremendous]

Sorry, forgot to say the car runs S cams, compression etc is good and engine is pretty much stock apart from that. Have not replaced the Teflon bearings but that sounds like a good idea as they're on the bench anyway and it can only help.

[mycar]

My worry is that the carbs have been set up with top end "track" use in mind as opposed to a balance of performance and driveability. I don't want to faff around too much with chopping and changing jets etc, so need a pointer to a good average starting point.

Probably your best bet then is to put everything back together and find a man with a rolling road who is familiar with these cars and comes recommended.

It's reasonably straightforward to set the carbs up and get them running* but what you've got is a bit of an unknown quantity. the only way to find out what it's doing through the rev range is to run it on some rollers with a sensor up the tailpipe.

* http://www.performanceoriented.com/spec ... cedures-1/

[Lightweight_911]

You don't say whether your car is a 2.0T or a 2.2T but in either case the jetting is far from std.

As a guide Factory settings settings were:

2.0 911S

Venturi - 32
Mains - 130
Idle - 55
Air correctors - 180
Emulsion tubes - F3


2.2 911T/S

Venturi - 30/32
Mains - 125/130
Idle - 55/55
Air correctors - 180/180
Emulsion tubes - F26/E3


.

[Captaintremendous]

Thanks for all the advice so far gents. Car is a 2 litre Andy.

[fetuhoe]

There are a few issues with this set up.

If it has S cam but T pistons it will tend to pink as the compression is too low to suit the overlap of the cam so I guess something has changed in this area as well.

It would also be worth looking at the different advance curves of the 69T and 69S distributors to see what is fitted.

I would put it on a rolling road and do a simple power run and look at the AFR across the range and see where it is wrong.

Our local rolling road does these simple power runs on a Sunday morning for £40 a pop on a 'first come' basis with no booking needed.

You can then try to sort out the idle and main circuits and get them running correctly.

If neither of these areas are correct you will never sort out the progression.

The idle circuit looks way too rich as the idle jet is 60 and the idle air corrector is 110.

A 1969 911T ran an IDTP carb with a 45 idle and a 140 idle air corrector. They also had very different progression drillings to the 40IDA.

34mm venturis also seem too big and should be good for around 240BHP they will delay the operation of the main circuit and give the progression system a tough time. A 1969 T had 27mm venturis

The pump jets also changed design over the years and these have an impact in how fuel is discharged as does the shape of the pump operating cam.

I would also measure the volume being discharged by the pump and try to make sure that it is in the right ball park, some carbs have adjustable pump rods and others don't.

[Captaintremendous]

Hi Fetuhoe and thanks for the response. Dizzy is an S unit. Car has not suffered any pinking issues, so as you say, pistons are probably appropriate aithough details from the previous owner about engine spec are a bit thin on the ground.Would the idle circuit being rich cause the 2nd gear acceleration stumble do you think? The car goes like a rocket once on the main circuit, the problem lies earlier on so i would like to try and maintain the higher end performance and just improve the low end.
There is noone near to me with a rolling road as i am in rural France,so it will have to be trial and error.i am tempted to go back to original factory settings and accept that it is what it is, but it seems a shame to sacrifice the performance the car has at present rather than persevere with the idle circuit (assuming thats where the problem lies)

[Captaintremendous]

Thanks to mycar for posting the link to performanceoriented. What an amazingly informative piece. Lots of food for thought there and no mistake.

[mycar]

If you're a bit lost in France without a rolling road then the Performance Oriented big guide to carburetors is your best bet.

Have a go with the lean best idle tuning and see how you get on, it's a great way to get to know your carburetors, I love Paul Abbott's introduction.

'This is an iterative and inter-active process where each adjustment affects all the others, this means the procedure for adjusting idle mixtures and air flow balances requires repetition until no changes are realized during the course of adjustment.'

That's long hand for 'This process may unhinge you.'

I recommend you get a Colourtune, it's a bit fiddly but it allows you to see whether a cylinder is actually firing and how the fuel is being burned at differing revs.

Good luck !

Oh and you'll need one of these....

[Captaintremendous]

Shiny new Synchrometer is poised and ready. Still trying to make a decision on initial jet changes, but think i will use near standard S spec as a starting point and go from there.
Does anyone have an email address for PMO in the States? I only have phone and fax numbers. Thought i might try for an opinion off them as well.

[fetuhoe]

The problem you are going to have is understanding and the point at which you are suffering from the transition flat spot.

I believe that you are suffering from a very common 'fudge' used in engines that have been set up for track applications.

Webers do quite commonly suffer from flat spots in the transition circuit and many tuners used to richen the idle circuit in an effort to 'flood' their way past these issues and reach the power circuit as quickly as possible.

If you start with a venturi that is too large it will be very difficult to make the system function correctly as the entire progression circuit will start to high up the engine speed range hence the large idle jets.

The lean best idle technique was mainly developed for setting up 'progressive' carburettors and some care is needed when applying this technique to 'one barrel per cylinder' designs.

It is important to ensure that none of the throttle shafts are binding and that all of the throttle valves are closing correctly as this is the main reason for unevenness in any set up of this type.

The position of the idle mixture screws is also a good guide to the idle jetting in absolute terms and I would tend to disagree with the comment that as long as all the screws are in the same position all is good.

Traditionally idle mixture screws used to be set by the factory at around 'one turn out'. Early Weber manuals suggested that if optimum running was 1 1/2 turns out increase the size of the idle jet by half a size ie 55 from 50.

If optimum running was only half a turn out then reduce the idle jet size by half a jet size ie 45 from 50.

You also need to be aware that if the idle speed screws are open more than half a turn to obtain the correct idle speed then you are holding the throttle plates slightly open and uncovering the progression drillings.

This will result in a idle speed that will be engine temperature sensitive.

PMO don't respond to e-mails on a routine basis so fax is the only way to make an initial contact.

[chris68]

I would definitely check your distributor advance. Map it right through the rev range and check back against the curve in the book. When first setting up my engine I wasted a long time chasing carb' issues that actually turned out to be poor low speed advance.