Fiat 128SL 1500cc jetting question.

[speedyg]

Hi,

I am after a starting point for the jetting on a single DHLA carb I have fitted to my car. The car is driven everyday by my wife (when I'm not pulling it to pieces) and it is also used for a few trackdays a year.

Car:1975 Fiat 128 SL

Engine:
Strada 1500 with 1500 head skimmed .016"
128SL Cam (euro spec)
128SL 4-2-1 exhaust manifold
2-inch Exhaust
Single Dellorto DHLA 40

Current carb setup

Chokes 29mm
Main jets 140
Emulsion Tubes 7772.5
Air correction jet 180
Idle jet 55
Air bleed 7850.1
Pump jet 50
Needle valve 1.50mm
Aux vent 7848.1


Attached are a couple of pics of the engine and one of the car. The airbox shown is to be replaced with a K&N oval filter.
Thanks for your help.

Gareth
New Zealand

[Guy Croft]

Gareth, hi,

nice to see some photos, thanks!

I'd first keep the chokes at 29mm and maybe go down to 125 main jets, leave everything else the same and see what happens.

You could try it as-is but I'd expect it to be too rich in the mid range due to the large main jets.

Once you have the main jet sorted for the 3000-5000 range you can start fine tuning the air corrector. It controls the top-end mainly, and you may need to go bigger (say 185-190) to lean that region out.

You should print off in its entirety:

http://guy-croft.com/viewtopic.php?t=64 ... sc&start=0

That thread has lots of carb tuning info in it.

I like the alloy fabricated airbox, trust it has rampipes with at least 1" clearance to box roof and cold air ducting - you probably know already about the detonation risk of inducting at engine bay temps in that car.

There seems to be some pipe connecting the inlet runners - what is it?

GC

[speedyg]

Thanks for the quick reply, I have ordered the main jets but the closest they had to 125s was 128s. I will start with that and see how it goes.

The airbox needs modifying as the inlet come out right on the engine support brace. I am still trying to track down some rampipes.

The inlet manifold was made by a machinest friend who had it and the carb fitted to a 128 3p, which has now been supercharged. He said the pipe was some sort of balancing pipe?

Thanks for your help and I will report back when I get it running.

[Guy Croft]

Hi,

OK, thanks. But I'd definitely get rid of the 'balancing' pipe.

GC

[speedyg]

Sorry it¢ž¢s been a while since I last wrote but I have been having a few issues with the car.
Here is what I have found out.

With the 140 jets installed.

Starting the car cold.
The car is hard to start but I can get it going by pumping the throttle a few times and turning it over (no choke). On about the 5th time it¢ž¢ll fire and after a few more times it will start with a very light throttle. If throttle is depressed at all it will cough and die. If I let it idle for a minute or two (<500rpm!) I can increase rpm and eventually get it running. Once warm if I give it a quick throttle pump it will cough a bit.

Starting car warm is fine.

Low range running.
Getting the car off the line is a bit jerky on/off.
Will cough on a quick throttle pump.

Mid range running.
Feels ok but really starts to pull well at around 3750rpm.

Top end
Will rev fairly easily to 6500rpm and feels quicker than the previous standard carb.

With 128 jets installed.

Starting the car cold.
Same as 140 jets, if I try any choke it won't start and if I try any choke when it first starts it will die.

Starting car warm is fine.

Low range running.
Getting the car off the line is a bit jerky on/off, a little worse than the 140 jets.
Will cough on a quick throttle pump same as 140 jets

Mid range running.
Feels really flat when floored unless above 4000rpm feels ok on ‚½ throttle but not as good as the 140 jets. Carb is also a lot noisier on full throttle.

Top end
Feels down on power.

So not sure if that tells you much or not. The main problem is the cold starting and it being a bit touchy to drive low down.

GC I see why you recommend only to buy new carbs as I have found quite a few problems with this one. Needle valve had been drilled out, no pump jets at all, and various other problems which all seem obvious now but for a while there it was causing me grief.

Thanks for your advice I really appreciate it

[speedyg]

I am wondering if I would have been better to go for twin dellortos as opposed to a single setup as there seems to be alot more info out there on twin carbs.
In theory and when tuned properly, should a single dhla work fine and be ok for pottering around town and doing an occasional trackday?

Gareth

[Guy Croft]

Hi,

I don't supply twin sidedraft inlet manifolds for that installation, very sorry, but yes if you can find one - twin carbs on a head like that with one inlet port per cylinder will be better in every case than one single carb of any kind.

No reason at all, however, why a well set up single sidedraft carb should not perform prefectly adequately for uses you describe. If it is set up right and the power is inadequate for your needs, defintely go twin carbs.

GC