Race Engine Forum UK

Hello

Since end of April I I use my Fiat 128 in historical races (french hillclimb championship)

My car is going relatively well, but I want to improve carburation
it seems too rich ...

I would like to have your opinions on jettings for my carbs ?

I have 2 Weber 40 DCOE
venturi choke 34mm

My first idea is to choose:
Main jet 140
Air corrector jet 200

Do you have any idea in order to start improvement

regards

Philippe

A picture of my car

One could not suggest jetting based simply on the engine size and type.

The engine setup determines the jetting:

1. Valve size
2. Gasflowed including manifold
3. Exhaust manifold type (4-2-1 or 4-1) and lengths and diameters of pipes
4. Tail pipe bore to silencer
5. Silencer type (straight thru big bore?) and how many
6. Engine CR (compression ratio)
7. Camshaft type (make, lift - hopefully a billet/blank grind not a regrind).

And certainly the air intake has a major bearing: if you are inducting direct from the engine bay you are losing power and will need weaker jetting. If you duct the air via a sealed system and remote filter and have an airbox to seal off the carbs you will get more power, less risk of detonation and need richer jetting due to the higher air density.

Anyone who builds a competition engine and inducts direct from the (very) hot engine bay needs their head examined (their head, not the one on the engine..). Temperatures at the carbs can be 80 deg C or more.


GC

The only real answer to your questions can be given with a wideband lambda probe on the road, or on the rolling road.

Hi Philippe, Great looking car.
Tuning a car requires specific tools that most people don't know how to use effectively. Required tools include a lighted magnifying scope to inspect the spark plugs, vacuum gauge, compression gauge, carb synchronizing tool, timing light, and a Weber carb tuning book.
If you have these tools then you have already spent more than the cost of a good dyno tune. It is up to you to decide if you want to be a driver or a mechanic. Some of us want to be both.
I don't know how much experience you have with tuning engines but finding a professional to tune your car might be the less expensive way to go.
Tim

Hi

My experience is based on historical races but with Alfa Romeo Giulia 1600 and 2000 ( you can have a look on my website if you are curious :))

It's because I have no reference on Fiat engine, I search primary informations

I have suffisant tools to improve and to work on my cars.To start improvements, I like to share informations between Fiat users.

A modification will be done on my exhaust in order to fit a lambda.

Few informations:
Exhaust 4-2-1 inside diameter 42 mm
I have not diameter of the pipes at the moment (Alquati manifold)
Adm valve: 40 mm
Exh valve: 35 mm
compr. ratio: 10.6
Silencer Ø 50 mm
Camshaft Catcams 1900203
299 / 299°
258 / 258°
10.45 / 10.45

I repeat my question is only to begin with a base of settings.
Of course, I will improve by testing

regards

philippe

Philippe wrote:

I have 2 Weber 40 DCOE venturi choke 34mm. My first idea is to choose: Main jet 140 Air corrector jet 200

Looks like a high comp big valve conversion 1300 with comp cam? 140 mains would be way too big (rich) for an engine of that cc.

Try this:
Mains 125
air correctors 200
emulsion tube F11
idle jet 40F9
pump jet 40

The main jet controls the fuelling thru the entire range under load from 1/8 throttle (approx) to full power, the air corrector just adjusts the top end (upper 2000 rpm). Make sure the fuel pressure does not exceed about 4psi at the carbs.


G

Thank you a lot Guy for these informations !!
Have a nice day

Philippe