Carb differences

[4v6]

Haynes Weber carb manual has reference's (stripdown, jetting's etc) for the ADF series.
ISBN 1 85010 784 X (Haynes publishing.)
From what I can see in my book, you have two 1995 cc TC motor's, both using the 34ADF.

Venturi diameter's are quoted at 24 and 26mm (primary and secondary respectively).
Idle jet's at 50 and 90.
Main jet's at 122 and 130.
Air corrector's at 170 and 175.
Emulsion tube's F20 and F5.
Acceleration pump jet is 45.
Float level 6.0mm+/- 0.25mm.
Needle valve size 1.75
Choke fast idle gap (mm) 1.15 +/- 0.05
Choke pull down (mm) 7.5 +/- 0.5

Obviously the above is for a stock motor only.
Hope that help's.

Tony.

[Guy Croft]

You've checked the carb with care so I venture to suggest at this time that the flat-spot and backfire is likely to be an ignition fault.

...loses the spark ...ex fills up with fuel ...igntion suddenly works again ...bang

G

[MinorTC]

Thanks guys - very helpful info. :)

I shall check it all out...

[Guy Croft]

What kind of ignition system is it?

G

[MinorTC]

It has the standard electronic ignition Lucas 2CE 'Constant Energy Ignition Unit', made in the UK. It appears to have the same basic connections as the Magnetti Morelli AEI 200A Module type. I tightened the connections to the coil.

Today, I removed the lid of the carb and had another good look; cleaned the emulsion tubes, and discovered that one of the two pump injectors on the primary barrel was blocked (not the one that normally squirts fuel as you actuate the throttle - the other one, whatever that does, presumably manifold vacuum activated?), so I removed it and cleaned it out by poking a length of wire brush wire through it and squirting WD40 through it.

I also removed the normal pump jet, and replaced the rubber O-rings prior to refitting both.

This has greatly improved things; giving the car a blat along a dual carriageway, it could at least achieve 80mph fairly easily, whereas previously it struggled even to crack 70! However, there's still some slight flat-spotting and backfiring under hard acceleration, though it is less pronounced than before and doesn't take as long to get over it.

I've noticed that it does this in 'surges', i.e. if you keep the throttle planted, it will flat spot and backfire, clear itself, pick up again, then repeat periodically.

I was wondering if it is worth adjusting the float valve height to 7-8mm, instead of the standard 6mm, as I've heard suggested, because it is apparently a bit critical on these carbs, and also whether high octane 'super unleaded' would work better than normal unleaded?

[WhizzMan]

To answer a post on the previous page, yes, fuel jets do wear in time. Small particles in the fuel that are not stopped by the filter are still abrasive enough to be of influence.

Your suggestion to experiment with the float level could possibly help, but I have no experience with these, so please don't do it just because I said so. The "surging" gives me the idea that maybe you have not enough fuel coming into the carburetor at full speed? Would this be helped with raising the level?

[Guy Croft]

I'm not comfortable with this one at all.

Lucas incl CE stuff was never fitted to 8V TC (or any other Fiat) engines. It is perfectly possible to generate a misfire (which is seems to be with backfiring) with incompatible ignition stuff. You can get damage to the powerpack and the coil. This has nothing to do with the adv curve. All I know for sure about your setup is you've 'strobed' it at tickover and it's got 10 deg. Marelli electronic units are notorious for causing precisely the misfire you have mentioned and eventually they fail altogether.

Take some expert advice from H&H Ignition (see links) on this without delay.

That apart tinkering with the float level is a thoroughly bad idea, whatever the factory setting is that will be right.

If you have not done a compression test do so, hot engine, wide open throttle all plugs out.

G

[MinorTC]

Thanks, Guy. I carried out a compression test last year, and all cylinders returned healthy, even results of 190 psi. All spark plugs show the same tan colouration, so fuelling would seem to be about right.

Today, I removed the Lucas 2CE ignition module and Lucas coil and cleaned all the contacts (which were somewhat oxidised):



I have a spare Magneti Marelli coil for an 1800cc Fiat Twin Cam, and did some multimeter tests of the windings resistance; while the Lucas coil had low impedance of 1.9 ohms and high impedance of 4.7 kilohms, the Magneti Marelli coil had low impedance of 4 ohms and high impedance of 10 kilohms - double that of the Lucas coil! I couldn't, however, fit the Magneti Marelli coil to the Lucas ignition module as it is physically larger.

[TomLouwrier]

mornin'

It's one thing to fit a Lucas kit to your engine, but don't go swapping parts between Lucas and Marelli systems. They are meant to work together and have their properties designed for that.
Combining incompatible systems will work badly (if at all), or damage one or all of your parts.

You can draw no conclusions by comparing those two coils 'as is'. In order to find out whether one may be faulty you need the right values as given in factory specs.
Remember that coils and power electronics in general have a tendency to go on the blink once they get hot. They may do well on the workbench but give problems once you've been running for a while.

regards
Tom

[MinorTC]

Good advice there, Tom!

The car came fitted with the Lucas 2CE system, which as Guy says, isn't standard equipment for Fiats.

I just happened to have the Magnetti Marelli spare coil, so thought I'd measure them to compare, out of interest.

[WhizzMan]

Given the comments and suggestions from previous posters, I'd be putting a stock ignition on myself now, just to eliminate a few things. Do you have a complete kit to try it out?

[MinorTC]

Alas, I don't have an original Magnetti Marelli system. I was thinking that fitting a 123 distributor is probably the best solution, but they're a little on the steep side.

The Lucas system fitted works well as far as I can tell, with easy starting and good running for the most part. I'm not convinced it's at fault, though it would be nice to eliminate it from the possibilities.

I have a spare 1800cc Fiat twin cam, with what I believe has a conventional points type distributor. Would this work on a 2000cc one, or would the centrifugal advance weights be hopelessly out?

[Guy Croft]

The 1800 distr is fine for your motor, same curve.

G

[MinorTC]

Thanks, Guy, that's good to know. If the problem persists, I'll try swapping in the points type distributor.

[Marc]

The best carb to use is a 32/36dfev as far as progressive single carbs go...