General 650 carburetor upgrade

[PointFiveO]

Hi all, I have a 650cc engined 500. The engine has a 'sports' camshaft (not sure of the profile) and aftermarket exhaust and is running 123 electronic ignition. It has recently been off the road for and engine rebuild and also the cylinder head is having some polishing and porting done and possible larger intake valves installed. A bit further down the line I would like to upgrade the carburetor to compliment the other upgrades. I'm not going to go wild and although I would love a dellorto fzd I can't stretch to the cost so have narrowed it down to a choice between a weber 30 dic or a weber 30 or 32 iba. I believe the former is a twin choke and the latter 2 are single chokes. Anyway I was hoping for advice or opinions as to which would be best for my engine. If anbody has experience of either maybe you could let me know what your experience has been. Also I was wondering if anybody has been in contact with Middle Barton Garage in England as I believe they have a wealth of experience and are willing to offer advice over the phone or email.

Thanks,

Colm.

 

[Bambino]

Colm,
The Dell'Orto FZD is a nice carby but horribly expensive now though it is a little gem and looks good to boot
The 30DIC can be made to fit but I'm not sure what needs to be done to the inlet manifold and I don't know much about the IBA carbies - sorry.
Both should be much easier to find than the FZD - I think the DIC was commonly used on 850s and the IBA on 127s. They should also be considerably cheaper.
Also, Middle Barton Garage are well worth contacting - they gave me a fair bit of help and I'm on the other side of the planet.
Finally, larger inlet valves are worth considering. To my mind, it's a bit pointless fitting a larger carby if you can't actually get the mixture into the combustion chamber efficiently.
Chris

 

[PointFiveO]

Chris, thanks for the input. I see nanni Ricambi have both the 30 dic and the 30 iba listed and also stock the appropriate manifolds to mount them, both are quite a bit cheaper than the fzd, although I believe they can be a little difficult to communicate with. I've ordered stuff from axel Gerstl in Germany and although quite expensive their delivery has been prompt. They also stock your dellorto fzd but the price of it is absolutely eye watering. I see they also now stock a remanufactured solex 34 pbic and combined manifold /valve cover. I know this is the carburetor originally used on the 595ss and 695 abarths. I wonder how good this carb is. Anyway thanks again, I will try to get in contact with Middle Barton Garage for advice. In regards to larger valves I would like the fit larger intakes, even 34mm ones as they seem to be easier to get than 35mms but I'm dealing with a mechanic who is dealing with the tuning shop that are doing the head work and he says they think it isn't worth fitting them, so it's a little difficult as they are the experts and I don't really want to **** them off by telling them what is best! Anyway we'll see how it goes, will update you as things unfold.

Colm.

 

[Bambino]

Colm,

I've tried several times to get in contact with Nanni Ricambi and have largely given up in frustration. Not to worry, there are plenty of other parts retailers.

As to the valves, I fitted 29mm exhaust (up from 28mm) and 35mm inlet (up from 33mm) to a standard 650 head fitted with hardened valve seats. I also had the inlet tract opened up to accommodate the larger carby. The guys who did the work are friends of mine in the local car club and have an excellent reputation. Their advice to me was that larger inlet valves are well worth fitting so the little motors breathe a bit better at the high end of the rev range. The increase in exhaust valve size is of marginal benefit only and if I did it again I'd leave them at 28mm.

I've attached a couple of photos. The first is of the original 650 head that turned out to be cracked - it has 33/28 mm valves. The second is my new head fitted with 35/29mm valves - there is still room between the valve edges. The third is a racing head fitted with 36mm inlet valves and there isn't much room for error.

Chris

 

[PointFiveO]

Thanks again Chris, I did talk to the mechanic who is going to be putting the engine together. Both he and the guy who is doing the rehoning and cylinder work are experienced tuners of race engines so I do have confidence in their abilities so I suppose ultimately I will go along with what they say, neither seem to be big fans of the big valve route favouring fettling and fine tuning and properly setting a car up for relability and drivability. They did strip and put back together a friends Mk 2 8 valve golf gti only replacing worn parts and managed to get more power than standard on an engine with 280,000 miles just by careful setting up, so I will go with what they favour for the moment. I suppose I just have this thing in my head about having a 'big valve' cylinder head!

Colm.

 

[Bambino]

Yeah - I guess a few horses always escape over the years as well as the ones that were missing because of necessary compromises in initial engine setup so companies can market cars to the masses. Finding them is always an interesting challenge and one with many routes to the desired result.

The advice that I had was that an increase in valve diameter was safe, effective and could be done at a time when the engine was apart. I don't for a minute doubt the credentials of your engine tuners - it's the old cat skinning thing.

If you end up with an engine that revs well and delivers the goods, then I wouldn't let a couple of millimeters of valve diameter worry you.

Chris

 

[usafstud]

Why wouldn't opening the exhaust as much as the intkae be beneficial?

 

[Bambino]

The pressure difference driving the fuel mix into the cylinder is usually, at most, atmospheric (unless you super/turbo charge) and the main obstruction is at the level of the intake valve. Bigger valves admit more mixture, more mixture = bigger bang - in theory, at least.

On the other hand, the exhaust gas is pushed out under considerable pressure by the rising piston and whilst the valve area and exhaust tract are important when designing the engine, increases in exhaust valve area are not as critical for performance than similar proportionate increases in inlet valve diameter.

This is my understanding - does it make sense ?

Kind regards,
Chris

 

[usafstud]

Thanks Chris

 

[Toshi 975]

I did some internet research into engine tuning and found a site on racing engines that had a formula for the optimum ratio between inlet & outlet valve sizes. Working it backwards from the 28mms exhaust valve the calculations pointed to a 34 to 35mm as being the best combo. I went for a 34mm Panda 30 inlets. I had the work done by a bike engine racing specialist who also ported the head , modified valve heads & seats and combustion chambers. The engine ran so well that I blew up the standard fan within a week. It went at over 6,000 rpm and pieces of the fan pierced the steel fan housing.

 

[Bambino]

When I delivered the blank head to the guys who did my work, I left it with them and they rang me later in the day and said '29mm exhaust, 35mm inlet - that'll do it'.
I don't know how they came up with those values but they did say that the standard head was designed to maximise economy ahead of power and that this combination would give me more power without destroying my fuel economy.
I didn't argue, I just bought the required bits and they did the rest.
I haven't revved my engine much yet as it's still running in but as it is also balanced, it should be a screamer
Chris

 

[the hobbler]

I know the Dellorto FZD is rearded by most people as THE single choke carb to fit on big (well bigger) engine conversions to the Fiat 500, but as one Forum member commented, its price, if you can find one, can be eye-watering. Has anyone thought of fitting that other classic single-choke carb--the SU. Many years ago, as a young mechanic, I worked at Radbourne Racing and was given a drawing for the required inlet manifold for the fitment of a HS2 SU carb--1-1/4 inch semi-downdraught. I have since found out that the flange measurements of the HS2 and the FZD are EXACTLY the same. I don't think the very slight difference in the fitment angles (18-1/2 degrees for the FZD and 20 degrees for the HS2) is an insurmountable problem, so it should be possible to use a FZD inlet manifold. I am beggining to collect the parts to build a '695' replica and will be looking seriously at this carburettor installation---it will be intersting to hear of other forum members comments

 

[Bambino]

There is no reason why you can't use an SU-type carburettor - I guess people (me) don't consider them because of the imperial measurements and the fact that they seem to be associated with British cars.

All of the carburettored cars that I've owned over the years have been fitted with single or multiple constant venturi type carbies (Webers, Dell'Orto, Solexes etc.) and I guess I tend to associate them with European (Italian) cars.

I've attached a photo of an SU mounted to a 500 engine, so it seems possible to fit one in the engine bay though I don't know where you'd get tuning data - maybe the Mini people might be able to help.

It's a good thought - SUs are certainly cheaper than Webers or Dell'Ortos.

Let us know how you get on,
Chris

 

[the hobbler]

Thank you Chris-the photo you posted has given me a lot of encouragement as looking carefully at the picture it would seem that the FZD inlet manifold has been used, just as I thought it could--as to technical info, I have just purchased the SU catalogue from Burlem Fuel Systems (the makers of SU carbs these days)--cost is just under £6.00p inc of vat (but plus postage)--it is a superb source of information; in fact I would say essential to anybody using SU carbs. Just as a matter of interest (and encouragement), Triumph used a SU (albeit the 1-1/4 inch MC2) on their 6Ts--about 35 bhp engines. I will keep people updated as to progress on this front.