Carb set-up

[Dinoswede]

Hi everybody!
I have read all the latest comments re "the flat spot" in our engines. One detail nobody has discussed - so far - is the absence of a vacuum-regulation of
the ignition timing at the Dino engines. Looking at an average 1960´car with carburettors and mechanical breaker points, you will find a distributor with
both centrifugal weights and a vacuum regulation of the ignition timing. The vacuum regulation system normally worked as follows;
The vacuum membran is getting the vacuum power from the intake manifold. It is set to lower the ignition timing (that is making the ignition fire a little
later than otherwise) when you press your gas pedal down in relatively low engine rpm:s.
If this does not work properly, the engine hesitates and misfires directly, until you manage to get the engine revving up a bit more.
For some strange reasons, our Italian friends choose not to incorporate this feature in the Dino design. Maybe because when using the Dino engines in
racing, you probably were revving the engine well over 6000 rpm:s most of the time, or maybe just because of the rush in homologating the 2 Litre Dino,
and nobody in Italy thought it really was a problem.
I have experienced this "flat spot" both in a 2,4 Coupe, and later my 2,0 Spider. I feel it is more obvious in the 2 Litre cars, mostly because the smaller
engine has less of torque in low rpm:s. Personally, I am pretty sure, an after-market vacuum device designed for the Marelli distributors would solve
the problem...

Dinoswede

[Stefan2000]

I know my needle valves are not the problem but i know they are leaking a tiny bit and i have 1 1.50 and 2 1.75 needle valves in my carburettors....so better to replace.
Your right, assuming is not the way to go, i'm making an apointment for next week to get it on a dyno. They guy over there is working on carburettors for 25 years and can offcourse measure AFR at different loads and so on, hope we can get these webers right. Will put the graphs online when i have them.

I've been thinking about vacuum advance quite often. This was the second reason why i bought an 123 ignition because it has vacuum advance as well and i can programm whatever vacuum curve so i'm planning to fiddle around with it when i'm sure the AFR is right.
The thing is though that in the end i Always find out that our italian friends are not that stupid really. When they do or don't fit vacuum advance it's most of the times for a reason. And you can see that almost every italian sportscar of that era (especially with the dual carburettors) doesn't have vacuum advance while the normal cars of that era does. I think it's partly because by then they did not really care about gas milage in the first place. The other thing is the design of the engine itself, first of all these cars have quite radical camshafts that's causing a really fluctuating vacuum signal and second of all because of the big dual carburettors and most likely factors as compression ratio etc.etc. Because of that you have quite little vacuum (or vacuum drops really quick at the smalles throttle response) which causes you to have a bad vacuum signal as well.
But we'll see, as soon as i'm sure that the carburettors are setup right i will try to fiddle with the vacuum advance and see if i can get it to work and post my results here.

Correct me if i'm wrong but the vacuum advance advances the ignition timing relatively to the engine load (or better depending on the opening of your butterfly valve and so vacuum in your intake manifold) This means that unregardless of your rpm the ignition will advance an additional 10 degrees or something while you accelerate slowly, so with a small butterfly valve opening. As soon as you floor the accelerator pedal your vacuum will be zero and advance will follow your normal mechanical graph.
The reason why you can have more advanced ignition with slight throttle response compared to full throttle is because of 2 reasons, with full throttle your compression is higher so the mixture will be warmer when compressed so it ignites faster and second because a rich mixture ignites faster and with full trottle you have (when everyting is setup right) a richer mixture.
Please correct me if i'm wrong or misunderstand something.

[Dinoswede]

Stefan2000;

Many thanks for an interesting comment! I have always believed the vacuum-controlled ignition systems were used to give engines more
driveability in situations just like the driving situations when Dino drivers notice "the irritating flat spot".
From that, I have got the impression, the vacuum pressure in an engine´s intake manifold normally is at it´s peak in this driving situation
(low rpm:s - high engine load).
Maybe you are totally right, in that this not apply for all types of engines - obviously there are a lot of different factors in an engine having
influence over the engine´s driveability, like valve timing, carburettor adjustments, ignition timing, and so on.
So, I guess we are many Dino owners waiting to hear about your testing your 123 distributor with a vacuum system activated - keep us posted!

Dinoswede

[Stefan2000]

I think vacuum advance is a good thing on every engine (if the engine design allows you to get it working) and yes it does improve the drivability slightly, but mainly it's an eco of fuel efficiency component (but a good an efficient one in my opinion).

Slightly retarded ignition timing normally doesn't cause flat spots or something but more just sluggish respons. But it's true that without vacuum advance and slight acceleration your engine is just not combusting at exactly the right point but some degrees to late. This will cost you some efficiency and to achieve the same amount of output you need to apply a bit more throttle and so your using more gas. Not really a problem in the 60's especially not for people who could afford an italian sportscar.
Now you can see where you can win something on your flat spot with vacuum advance (or a bit more dangerous by giving your ignition a more radical mechanical curve) because if you reverse this story it means that with more advanced ignition with slight acceleration you have to apply less throttle, which means that the transition from your idle to main circuit at a given rpm is going to change because your butterfly valve is at an other angle. It might also be the other way around that your flat spot becomes more noticeable.

But vacuum is at it's max with closed butterfly valves and maximum RPM, because than the engine has to suck in most M3 of air through an almost closed butterfly which causes maximum vacuum in the intake. If the butterfly valves open further, the engine can suck in the air more easy for that given RPM causing less vacuum.
Now you can see that when you accelerate slowly, your butterfly valves are open slightly, which means little engine load, lot of vacuum in the intake, which means you can advance your ignition timing with some 10degrees or something.
When you floor your accelerator, your butterfly valves are fully opened, engine load is maximum, vacuum in the intake is zero, which means your ignition timing has to folow just it's mechanical graph without additional vacuum advance.

Will keep things updated about the dino, the carburettors and the ignition.

[Barnfind]

Been reading on my issue for a while and it seems to be that i'm not the only person but could not find an answer to the question yet.
I have a slight dead spot while accelerating around 3000 rpm. It's a 2000 with the 40DCN 14's. I'm quite sure it's taking place at the transition point between idle and main circuit. It's only while accelerating with slight throttle response.
Ignition is spot on, advance mechanism and advance curve is 100%, valve clearance is good, compression good, carburettors are clean and all jets are the Original size, float level is correct etc.etc.
Who has the same problem or more important, who had the same problem and knows how to fix it?
I have some slightly larger idle and main jets but i'm a bit reluctant to switch them as long as i can't measure my AFR while driving.

Thanks in advance!

Stefan2000.

Maybe is my turn now

If don't know how you measure the AFR, but if you are stuck with that issue, buy a wideband controller and lambda, and attach it to the tail of the exhaust. You will see the AFR while you are driving. There are some makes that offer the stand for the probe. It is not too expensive, and it will point you in the right direcion.

[Stefan2000]

Yes that's what i was refering to in my previous posts. I was even thinking about drilling a hole in my (almost dead) exhaust to put the wideband as close to the exhaust manifold as possible. Did you ever try a wideband on a carburated car?
But i will go to the dyno first because the combination of checking the AFR together with someone that's doing this job for 25 years and is having all the different jets, emulsion tubes etc in stock and has the knowledge will be the easiest way.
If the results doesn't satisfy me i will definitely go for the wideband controller option.

[Barnfind]

Ups.. I missed some of the posts..

Yes. I have tried wideband in a rally twin DCOE engine. Don't expect to get an accurate AFR, specially if you are used to work on EFIs. Expect to work in the range 12-17, sometimes leaner, sometimes richer, with a lot of dispersion between the same repeated test The good news is that you are able to know how the engine responds to changes, and measure that changes..

I have seen in the past flat spots in the transition from idle to main jets on IDFs, mainly due to air paths partially clogged. A throttle shaft worn also has an impact. As a crack in the manifold, or a defective gasket. But I can imagine you have checked all the physical components. My experience with carbs is limited, I have been lucky and managed to run al my carburetted cars with not many problems.

Regards

[Barnfind]

By the way, you are absolutely right about the vacuum advance and its consecuences. I only would like to say that the vacuum port is just before the thottle, so advance is not affected at idle. As you open the throttle, vacuum increases suddenly.. and from that point, all you said is what happens..

[Barnfind]

Back to the carb setup:

I've been reading about the float setup, but I'n not sure if it has to be measured with the needle ball pressed, or with the tab making contact with the ball only..

Regards..

[Stefan2000]

It has to be measured when it's just making contact with the ball. Well that's the theory but in reality it depends on how much fuel pressure you have, the current density of fuel right now compared to the 60's, if your float is still undamaged (and free of leaks in first place), what size of washer you put underneath the needle valve etc. etc.
So keep the measurement as a guideline, remove from all carburettors the emulsion tubes, look down into the carburettors and see in real life if the float hight is equal or not. I can almost guarantee you that you will have to adjust the individual floats differently to get the float hights equal. Been through that.
When they are setup correctly the hight should be approx. 2mm underneath the passage to the secondary venturi (but this 2mm is not scientifically examined so don't take my words for granted).