Jetting twin choke carb

[PointFiveO]

HI All, I'm the owner of a fiat 500F with a tuned 650cc engine. When I got the car it had a 'fast road' cam (not sure the exact spec) a sports exhaust, a 3.5L sump and was running a 123 electronic ignition system. Since then it needed the bores rehoned. At the time the cylinder head was polished and I bought a weber 30 dic2 carb and had it fitted to replace the standard weber 28 imb. Initially it ran very rich with huge flat spots when accelerating unless the pedal was used very progressively. Even gently revving when stationary would cause it to splutter and almost cut out. Comparing the specs of the weber 30 dic2 to the very similar weber 30dgf used on the panda 30 (twin port version of the 650cc engine) I saw the main differences were that the 30 dgf had smaller main jets than my 30dic2 (1.10 & 1.00 instead of 1.25 & 1.15) and larger air jets (1.90 & 2.00 instead of 1.80 & 1.70). Initially I had my mechanic change to the smaller spec main jets which made no real difference to the running, so I then had him change to the larger air jets. The combination of the two made a big difference. The engine runs much more sweetly and accelerates with little in the way of flat sots unless you mash the pedal strait to the floor. However I still feel it is running a bit too rich. It starts fine when cold but when hot needs the accelerator to the floor to start, it will turn over for a good 7 to 10 seconds before starting emitting a big plume of smoke from the exhaust before settling down to a normal idle. Also fuel consumption  (not a huge priority) never seems to better low 20s (with weber 28 it never topped 33mpg). Anyway my question is when rejetting to lean it out a bit more and to make it run a bit better would I be better reducing the size of the main jets a bit more or increasing the air jets or doing a combination of both? or maybe I should just concentrate on restricting the secondary choke and leaving the primary as standard? Any advice from anyone used to fiddling with carbs would be greatly appreciated, I have heard it is somewhat of a black art and that a twin choke carb on these engines will always have a bit of a flat spot somewhere in the rev range. Overall I am happy with the engine and how the little car runs. Great site by the way.

Colm.

PS; Have tried to attach some pics but think they might be too big so here's an old clip from some years back when I first bought the car.

https://www.youtube.com/watch?v=_vHfRnH9k3E

[Pete126]

Hi Colm welcome to the club forum,

My own take on your problem is the carb you have has the wrong size chokes and emulsion tubes if you look at the base of the carb you should find a carb number, if you do a google search this will tell you what the carb is off, I suspect that its off an 850 coupe?

In this day and age its simple to adjust the different parameters on new cars by plugging them into a laptop with the relevant software, The only way to get the maximum power output from your engine is to have it tuned on a rolling road, they will be able to to swing the distributor and check the mixture throughout the rev range changing jets and emulsion tubes etc to give you the best performance and economy, by changing parts willi nilli  without a gas analyzer connected you don't know if the engines running rich or lean, if its too lean you can overheat the engine.

Have you seen this thread:

http://club126uk.co.uk/smf/index.php?topic=7694.0   

[PointFiveO]

Hi Pete thanks for your input, you are right the weber 30 dic2 was used on the 850 sport which had a 903cc 4 cylinder engine, however matching the main jets and air jets to the sizes used on the 30 dgf on the twin port panda 30 engine did make it run much more smoothly although I do realise that the differences between that engine and the standard 650 engine mean it won't be a perfect match. I was going to get it set up on a rolling road but stories from friends of how the engine is run to high revs for long periods put me off the idea. I will reconsider it now as I think you are right about it been the only way to get the fuelling correct. I was just thinking that the 30 dic seemed to be a popular aftermarket modification and that there was a chance that somebody else had done the trial and error work before me!

Cheers,

Colm

[Pete126]

Setting up the carb is what pulls all the other engine mods together IE: Cam, Compression, bigger valves, and ignition timing etc, They all need to work together to get the best out of the engine, If its a standard engine its easy to set it up as per the manual, but as you don't know what cam has been installed and other mods that have been done on the engine, a rolling road tune is the only way to go As long as the alternator/fan belt is tensioned correctly everything should be OK it cools the engine if your doing 60-70mph on the the motorway so there shouldn't be a problem, The the Weber 30 dic2 is an excellent upgrade when setup correctly    

[PointFiveO]

Thanks again Pete, seems rolling road is the way to go then.

Colm.

[Rusty's Uncle]

I am just putting together a kit of parts to try this conversion and found this thread most useful  
Having rebuilt a number of Weber 26IMB, 28IMB & 30DIC carbs I realised that the 30DIC twin choke is very much like two single 500 or 126 carbs sharing one carb body with some components the same as the 500 & 126 carbs. Also the Venturi sizes match up. I am currently looking at using an early 30DIC which has a primary venturi of 23mms (same as a 126) & a secondary of 21mms (same as the 500). So to get the mixture somewhere near right my theory is to get the main jet / air jet ratio the same as the single carbs and consider each venturi separately . The big unknown is the emulsion tubes so again it seems like a plan to go for 500 / 126  ones to match.
Any thoughts or experience ?  

[Pete126]

Think i would start at this point, Bis 704cc

[Rusty's Uncle]

Thank you for that Pete . It all adds to the knowledge or probably lack of it in my case 
So the Bis carbs have the smallest main jets than any carb ever fitted to an air cooled engine back to the very early 500s. But that makes sense as they also have the smallest venturis so it seems to me that the Venturi size to main jet ratio is another thing to take into account when working out the best mixture jet set up. I remembered that I had a Panda 30 twin choke carb that I had not investigated so going to dig that out and see what I find as they were 650ccs and put out 30 hp I believe 

[Rusty's Uncle]

So it seems logical that the main jet size is related to the venturi size and that the air jet size is related to the main jet to set mixture. Also looking at the stats for the twin choke carbs the secondary inlet runs a much leaner mixture than the primary venturi. To try and get some sort of idea about jet sizes and mixture I divided the air jet by the main jet size to get a ratio for every carb ever fitted to the 2 cylinder engines. Sounds a bit nerdy I know but I then plotted all the ratios on a main/air jet graph and I realised that I could select a main jet size for a given venturi , select a jet ratio for mixture and it would tell me the air jet size. I may be talking the biggest load of "spheres" ever but it seems to make sense to me. So I have set this Weber 30DIC 1 carb with 1.12 main & 1.9 air on the 21mms primary and 1.00 main & 2.00 air on the 23mms secondary. F8 emulsion tubes on both. Got some bits of hardware to sort out before I can fit the carb but will report results 

[PointFiveO]

Hi Rusty, I think if you have a weber 30 DGF from a Panda 30 you are better to use that than down spec the 30 Dic. I have a 30 Dic2 carb and noticed the spec was almost identical to 30 dbf as used on the Panda except for the main jet and air jet sizes so I changed the large main jets down and the smaller air jets up to match the 30 dbf ones to make the engine run better. It might still be a bit too rich but runs well enough compared to before. I notice that the specs of the 30 DGF as used on the 126 BIs seem very different to the panda 30.

Cheers,

Colm.

[Rusty's Uncle]

Hi Colm
I guess if I was sensible I would just fit the Panda 30 carb but I have been wanting to try the 30 DIC for some time now. They have been used a lot over the years and there are a couple of cars in the UK running the carb with no problems from what I hear. I suspect that the bad feedback about the carbs are from guys that have not got the set up right. Out of interest the Panda carb I have is stamped 30DGF1 /253 and had a smaller 1.8 air jet on the primary choke.

[PointFiveO]

That's interesting, there seems to be many variations on the spec of the 30 DGF depending on the application. It probably would have been easier as you say to fit the 30 DGF as it was used on such similar engines to the standard 650 whereas the 30dic was used on the fiat 850 coupe in 850/903cc guise from what I've read. In my case a weber 30 dic carb came up for sale on the 500 owners site so I hopped at it. THey do also seem to be more expensive than the 30 DGF. I know where you are coming from in wanting to use a 30 dic though. Back in the late 80's in my teens I read an article in Classic and Sportscar (still have the article somewhat tattered) that featured a mechanic who converted his 500 into an abarth with the 650 engine up specced with an aftermarket camshaft, exhaust, stage 2 cylinder head and weber 30 dic carb. Back then I decided one day I might have a similar 500., which I now do, the only think I wanted which I didn't get so far were larger intake valves and the cylinder head ported. Still it's a work in progress.

Colm.