Race Engine Forum UK

Let me start with my engine spec:
1756cc 8V TC
Compression: 10.2:1 (Cranking compression 200 psi in all 4)
Head: 43.5mm in 36mm Ex, 3 angle cut, 35mm ports
Cams: Standard Fiat 131 sport 2L (15/55, 55/15?, 9.9mm)
Clearences: .40mm in .50mm Ex
Exhaust: Copy of 124 Stradale extractors: 4,2,1, Prim: 38mm, Sec: 41mm, The rest: 51mm with 1 straight thru resonator and original 124 sport rear muffler attached by flaring original pipe to accomodate 51mm system.
Ignition: Marelli Electronic. 36* @ 5000 Rpm (pinking only when going up very steep hill with WOT, but have since dropped timing to peak @5500)
Inlet Manifold: 130 Strada Abarth shorty, opened to 45mm at carb mouth
Filtration: Weber long ram tubes, surrounded by cage, covered with Unifilter sponge type element.
Carburation: 2X Weber 45 DCOE 152 9n and 9m (***3 progression hole Type***) made in Italy
Fuel pump: original diaphram type
36mm chokes
Idle: 55F8 (mixture screws 1.5-2 turns from fully seated)
Main: 145/F16/180
Plugs NGK BPR7es ( Insulator is light brown colour in blotches, rim of thread that protrudes into combustion chamber is black soot.
Fuel: 98 RON ( sometimes with 50ml valve saver additive / tank)
Symptoms:

Perfect idle, flat torque curve from just off idle to redline, 6500rpm under full throttle.
When cruising at around 2800 - 3500 rpm i occasionally get backfire thru carburettors, very rare though. I have checked gaskets for leaks and all seems good.
When cruising in the same rev range i do get jerking back and fourth, not as noticable in the higher gears on the freeway, but still there.
Also when cruising in the same rev range and even when stationary and i rev the engine to the same rev range i get constant popping / backfiring from the exhaust. When i back off the throttle when in gear from 3500rpm or so i get popping all the way back down to about 2500 rpm.

I read this to be a lean mixture, so connected a motec ARF meter to the tailpipe and confirmed it was lean. i.e. light load 2000-3500 rpm 20 :1. At about 5000 rpm it gets richer but still only 13.8:1 or so. I havent run WOT to redline with the AFR meter as yet.

I have since drilled out the idle jets to : 60 and the mains to 150. No other changes.
The car still behaves the same, if anything i feel a slight stumble as it comes off idle, but then as before.
I havent had time to check with the AFR meter but i can tell its still lean with popping/jerking etc,especially throughout the progression circuit that i use a lot when cruising.

Going thru Guys book i cannot see where Guy has used jetting bigger than this on any engine, which is why im at a loss to work out whats happening here.

Im suspicious of the 3 hole progression circuit, but dont want to go playing with it, if ive missed something obvious.

The other weird thing is that i have a very similar spec engine, jetted identically and it runs as if factory tuned.

Help much appreciated

I do not think the problem is jetting but it could well be sticking bob-weights in the distributor or a faulty electronic ignition amplifier module. Check the coil is right for the ignition system too. The latter two can certainly cause these problems.

I am assuming you are not using Digiplex.


GC

Thanks for the response Guy.
Im not using Digiplex.
The bob weights move freely, i can see the timing adv/ret as i rev up and down at idle with the timing light.
The coil is correct for the rest of the system, being a matched set from a 131.
The electronic ignition amplifier module has not been changed, everything else in the ignition has.

Re-reading thru my post, sorry if i wasnt clear, the AFR readings were all taken under very light throttle, which is pushing me back to the progression circuit. I should not have gone to 150 on the mains as obviously i wasnt checking under WOT.

Should i try richen the idle circuit even more to compensate if indeed it is the progression circuit? In which case what will be richer 60F8 or 50F9? F9 having a smaller air hole but with only a .5mm fuel hole?

You would be wasting your time reading AFR at anything except full throttle, which, according to the load imposed (eg: by a rolling-road) would determine the rpm.

Idle jets must be set at idle (850)
progression should be left firmly alone
main jets control mixture from around 3000 upwards
air correctors merely adjust the top-end fuelling (from peak power to about 1500 rpom below it in the rpm range)

Pump jets are vital for transient response - you would need 45s, no bigger.


G

An update to all interested.
Seems the Type 152 45DCOE's i have with 3 progression holes are the culprit.
Looking at the throttle plate at idle, when closed, it sits way behind or infront (depending which way your looking at it), the 1st progression hole.
Looking at earlier type carbs and indeed the latest Weber versions, there is a progression hole right under the butterfly, hence when your on minimal throttle there is enrichment provided thru this.
I know the car ran perfectly with such earlier carbs a while back, but now im just playing with jetting as a compromise for a situation that im pretty sure needs a 1mm drill bit, and a mill press to fix.
Ill keep you posted.

ace124,
I also have the type 152 45DCOE's and the issue you have identified with the 3 hole progression holes may well be the problem. On my recent rolling road session (see earlier post "My Guy Croft Fiat 2l 8v Engine Dyno Results" for results) one of the problems identified was lean running at low revs. Northampton Motorsport explained that this was an issue with the 3 hole progression.

They improved the lean running by drilling a 4th hole for the idle progression, unfortunately I can't tell you how big the hole was or exactly where, as they carried out the work. However what I would say, as it was explained to me, is if you get it wrong then it will trash the carb body completely. Therefore this should only be undertaken by someone who knows exactly what is required and how to do it, both the positioning and size of the hole required.

Regards,
Wal

Sorry, but attempts to improve performance by messing with progression drillings will most likely lead to tears.

It is NOT a practice I condone or recommend and I assure you all that the in every case the performance problem will lie ELSEWHERE.

G

Guy, if im constantly running lean with the butterflys just cracked open, and before the 1st progression hole is exposed, what else can it be?
Ive had a look at earlier 45DCOE's and indeed the latest Type "G" version also, and they all have a progression hole right under the throttle plate. The 152's i have dont and its all i can put it down to.

Btw when i say lean i mean AFR's of 18-20:1. The car bucks back and forth and pops constantly at this amount of throttle. I went back to 40 DHLA's and problem gone so im still leaning towards the Type 152's off idle progression circuit.

Any other thoughts much appreciated.

I might make a suggestion here. It works much of the time on moderate performance applications in drag racing.. We always idle slightly fat to avoid any possibility of going flat because of a slight lean transition.

Dead nuts sync your carbs on the idle stops and then make very sure that as you begin to advance the throttle the carbs open in perfect synchronicity.. The slave unit must crack the same as the master. Any delay and the lazy one will lean

Set your idle mixture screws to best then give them all 1/4-1/2 turn rich. bring your idle stop in to return to desired rpm. As you know the screws handle the band from dead idle to the first progression hole.

If you are certain it's fuelling you can tune out a flat spot effectively by changing the idle jet setup, see link:

http://www.fastroadcars.co.uk/shop/inde ... oductId=84

on the dropdown there is a complete list of all the options. I cannot unfortunately tell you which one to use though I have done it in the past I kept no record.

G