Weber overhaul and set up advice please

[Captaintremendous]

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.

I am with you on choke size, but Webcon can't oblige so i am so i am on the look out for some 30's or 32's if anyone has some. Going large on the idle jets is clearly what has been done in this case to try and "de fudge" matters, so i aim to try and getb back to as near to original as i can and go from there.

[Captaintremendous]

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.

I am pretty happy with distributor operation Chris. It was recently rebuilt and checks out well.

[Jonny Hart]

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.

I am pretty happy with distributor operation Chris. It was recently rebuilt and checks out well.

We are regularly getting crisper throttle response and more power with a custom ignition map.

Today's fuel isn't the same as in 1969 so the published Porsche curve is no longer optimum. Neither is it optimum if you have done anything at all to your engine.

[mycar]

What's involved in custom mapping a dizzy for this sort of application Jonny ?

[fetuhoe]

PMO Chokes will fit but getting hold of them from the Oil Analysis Company can be a bit long winded but at $105.00 the price is not bad and the quality as good as original parts which are OK but not great.

There is a guy in Italy sells aftermarket chokes at about 90€ a set but the shape of the venturi isn't ideal and its position is a couple of millimetres out in terms of height which will affect jet calibration.

We make our own but only at 34mm and 36mm so not really much help.

We have a 1967S running an '002' distributor with points and a Beru Sports coil and it runs just fine even with E85 fuel in France. it is a standard engine but it pulls cleanly from idle right through to the cut out at 7200rpm.

If you do want to change the curve then re-mapping the dizzy is a pain, it needs to have the stops re-set and the springs changed and really needs a Dizzy machine to see what is going on although you can do the job with it on the car.



We bought a SUN 504 a few years ago.

If you have an early basic Kettering Ignition you could fit an Aldon Amethyst which works with either contact breakers or an ignitor and lets you programme an advance curve from a Java App.

They come with a vacuum or TPS input so they can provide some allowance for load. It will also work with just a basic programmed curve. I think they are around £120 a pop.

All you need to do is remove the springs and fix the bob weights and set up the curve with a laptop.

We have one running on a 3.0 litre motor fitted into a 1967 912 running on Zenith 40TINs that makes around 240BHP.

There is a lower cost system called 'Stealth' which provides a 7 point map and is not something I would be keen on using.

[Jonny Hart]

What's involved in custom mapping a dizzy for this sort of application Jonny ?

Ok, so this is obviously salesmanship but fitting our CDI+ will allow you to do everything you need whilst retaining orignal looks without changing the wiring harness or drilling any holes. You won't need to touch the dizzy.

See our website in my signature.

[mycar]

What's involved in custom mapping a dizzy for this sort of application Jonny ?

Ok, so this is obviously salesmanship but fitting our CDI+ will allow you to do everything you need whilst retaining orignal looks without changing the wiring harness or drilling any holes. You won't need to touch the dizzy.

See our website in my signature.

So in this instance, how would you say your CDI would help the Captains predicament ?

[Jonny Hart]

What's involved in custom mapping a dizzy for this sort of application Jonny ?

Ok, so this is obviously salesmanship but fitting our CDI+ will allow you to do everything you need whilst retaining orignal looks without changing the wiring harness or drilling any holes. You won't need to touch the dizzy.

See our website in my signature.

So in this instance, how would you say your CDI would help the Captains predicament ?

Let me state firstly that I am not an engine tuner by any means so I can't comment on the choice of jets or fueling.

What I have learnt by watching an expert is that the fuel and timing go hand-in-hand. Once the fueling is in the 'ballpark', the advance is brought forward until maximum power is observed on the dyno without pinking. This is done up the rev range in stages, resulting in a custom ignition map being produced. Of course sometimes advancing the timing makes the car yearn for more fuel, thus further carb adjustment and round and round we go. That's my somewhat simplified take on tuning.

Of course this procedure is very tiresome with a standard distributor as each advance change requires the distributor to be taken out, dismantled, the weights filed/welded or the springs changed and then verified on a machine like the Sun posted above. Then refit, set timing and re run the dyno.

Our CDI+ allows the ignition advance to be setup dynamically by altering the advance curve instantly in real time using a laptop whilst the car is on the dyno.

The OP commented on hesitation and lack of engine pickup. Our CDI+ is a dual discharge system offering 2 sparks back to back across the entire rev range. This improves the chances of burning the fuel in the 911s rather poor combustion chamber. This is proving to cure hesitation in a lot of cars even before timing adjustments are made.

Here are a couple of comments from our customers:

"engine run was so perfect, with zero adjust was even smoother than before - esp. in the tricky range of 2000-3000 rpm. "

"I am still waiting for the correct exhaust system for the car to help with the mid range "hole", but thought I would get a quick dyno run in for starters.
I have gained from approx 4900 rpm up and what is really surprising is the bump in power up at the top. Did a few runs and this was always repeatable."

Anyhow, enough of words, here's one of our units in action, turn the sound up!

https://www.youtube.com/watch?v=VkwSsi6 ... e=youtu.be

[brembo]

Jonny, can your boxes be used in tandem in a twin plug setup?
P,S, fuse panel arrived safely.

[Jonny Hart]

Jonny, can your boxes be used in tandem in a twin plug setup?
P,S, fuse panel arrived safely.

Yes, we have two boxes on one of the Larousse STs.

[Captaintremendous]

Damn! Just as I was getting my head round the carbs! Now i need to go and lie down in a quiet room for a while.

[mycar]

What type of ignition does your car have Captain ? Simple points and coil (Kettering) or CDI (capacitive discharge) ?

[fetuhoe]

A late spec 1969 911T would have had a 3-Pin HKZ CD with points and most likely a Bosch Dizzy - 0231 159 008 This should be a 901/16 designated engine.

The earlier 2.0T would have a Marelli Dizzy.

A period 911S Dizzy would be a Bosch 0231 159 007 (006 on a 911E) with a substantially different advance curve.

As it appears that the engine runs well at high rpm then the question is whether it if fundamentally carbs or ignition that are causing the flat spot.

Presuming the distributor is as stated a 'S' unit and that the cams fitted are 911S and that it doesn't pink so compression is OK then surely the first starting point will be to make it run smoothly and well on the existing ignition system.

There is no real reason this engine should not run acceptably and have a smooth progression using stock ignition parts.

It is a given that the correct dizzy needs to be fitted but the real culprit in this case is almost certainly the carb settings which are at best 'unusual'.

Trying to optimise ignition and carbs with road testing in 'rural France' will be quite fun but doesn't seem entirely practical.

When we used the Aldon Amethyst in the 1967 car we didn't have an HKZ box, just a Kettering Ignition, we didn't have the correct dizzy - just a unit set up for a K Jetronic and we didn't want to send a few hours re-curving the dizzy.

We simply programmed a curve from a 911S and it runs well.

I am sure that optimising the curve to suit E85 fuel could give improvements but IMHO if you mess with the curve before you optimise the carbs it will almost certainly drive you a little mad.

[Captaintremendous]

What type of ignition does your car have Captain ? Simple points and coil (Kettering) or CDI (capacitive discharge) ?

Points and coil.

[Captaintremendous]

A late spec 1969 911T would have had a 3-Pin HKZ CD with points and most likely a Bosch Dizzy - 0231 159 008 This should be a 901/16 designated engine.

The earlier 2.0T would have a Marelli Dizzy.

A period 911S Dizzy would be a Bosch 0231 159 007 (006 on a 911E) with a substantially different advance curve.

As it appears that the engine runs well at high rpm then the question is whether it if fundamentally carbs or ignition that are causing the flat spot.

Presuming the distributor is as stated a 'S' unit and that the cams fitted are 911S and that it doesn't pink so compression is OK then surely the first starting point will be to make it run smoothly and well on the existing ignition system.

There is no real reason this engine should not run acceptably and have a smooth progression using stock ignition parts.

It is a given that the correct dizzy needs to be fitted but the real culprit in this case is almost certainly the carb settings which are at best 'unusual'.

Trying to optimise ignition and carbs with road testing in 'rural France' will be quite fun but doesn't seem entirely practical.

When we used the Aldon Amethyst in the 1967 car we didn't have an HKZ box, just a Kettering Ignition, we didn't have the correct dizzy - just a unit set up for a K Jetronic and we didn't want to send a few hours re-curving the dizzy.

We simply programmed a curve from a 911S and it runs well.

I am sure that optimising the curve to suit E85 fuel could give improvements but IMHO if you mess with the curve before you optimise the carbs it will almost certainly drive you a little mad.

Agreed. The carbs are deinitely the source of the problem. As i mentioned early on, the car runs well and in fact goes like a missile for an old 2 litre, as was commented on at my last track day. Therein lies the problem. Great on the track and to be fair, great on the road until you find yourself in a situation where you have a slow second gear pull away. if it had a synchro box instead of a 901 dogleg and i could slip into first, there would be no problem as such, but that's not the case.It is a case of getting that low end properly jetted and fettled. No complaints higher up.