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Carburettors

Let's start by reminding you of one of my earlier statements, an engine is an air pump, the faster and easier you can get the air in and out, the faster the engine will go. You must remember, though, if you are running a standard carb and change the air filter for an aftermarket type which will flow better than the standard VW unit you will need to increase the main jet on the carb. Note: More air needs more fuel. You can buy adjustable main jets for the Solex 31 PICT-3 or 4 and the 34 PICT-3 as used on the 1300 and 1600 twin ports respectively. It is a better idea to buy an adjustable main jet, as trying to guess the jet you need could be expensive before you finally get it right.
Carb choices are getting smaller. The 48IDA was always used with the big engines, but Weber don't make them any more which is making them difficult to get hold of. The Americans used to use the Holley Bugspray for the street engines whilst in Europe we used the Weber 32/36 DFEV. The Bugspray is no longer available new, whereas the 32/36 DFEV is still going strong. If you can find a Bugspray it will help a small capacity engine run much better, and an engine up to 1800cc will like the 32/36 DFEV. The Weber is a progressive twin choke, this means that the first 32mm barrel does all the main work until you floor the accelerator and then the 36mm cuts in, which you will notice. There was, and this guy thinks there still maybe, a set of Kadron Solex carbs, these are two single 40mm Solex carbs on individual manifolds with a crude linkage (which often loses one of its retaining clips). Having said that, they are good value for money, as are any other twin single carbs sets.
After that comes the best of the rest, 34ICT, 40IDF and 44IDF. Dellorto also make an equivalent range, 34FRD, 40DRLA and 45DRLA, the only difference is the Dells are usually a little more expensive (due to a smaller demand and, in this guys’ opinion, better quality), but if you intend using a blow through turbo system you can forget the Webers, they can't handle the pressure as well as the Dells. The 34ICT/34FRD are great for a small engine up to 1800cc, it gives a good response and is much better on fuel consumption than the 32/36 DFEV. The 34FRD, Baby Dels, are now no longer made. If you can get some, do, they have two separate single carbs, one above each head. The 40IDF/DRLA and 44IDF/45DRLA are twin chokes, which can be used as either single centre mounted or as two side mounted carbs. The single centre mounting doesn't work well 'out of the box', but if you can get it jetted correctly it should run well, although icing is more common with this set up. The best way to run these carbs is as a twin set up with one either side of the engine. They usually come as a complete set with manifolds, air filters, linkage and fuel line. They run well on any engine from 1600cc up, although this guy would use the 40IDF/DRLA for 1600-1800cc and the 44IDF/45DRLA from 1800-2000cc.
In his opinion the Dellorto has a far superior linkage system that the Weber, it uses a hexagonal rather than a round bar, which means the arms don't need bolting up so damn tight, but have a look at them both and see what you think.
One point to note, you are likely to have problems using your standard fuel pump with these dual twin carb set ups. They work well until you reach about 4,500 rpm then can start to struggle. Electric pumps are available from many outlets and some are designed to replace the existing pump and sit on an adapter next to the distributor, but most pumps should be run from as near the tank as possible, as they are designed to push not pull the fuel. There is a nice recess on the opposite side to the master cylinder along side the front of the floor pan framehead.
You can use 48IDAs on the road, but you will get better mileage and easier town running from a set of 40 or 44/45s.
If you plan to run a set of 40s on an 1800, do ask the shop for a set designed for this engine. The set-up the 40s come with is designed for a 1600cc engine, using 28mm choke and 115 main jets, where an 1800cc engine will require 32mm chokes and 128 mains. Use these calculations for your carbs:
Determine carb size: (Square root of (cylinder cc X maximum rpm)) divided by 40 (I would suggest a maximum rpm of 6,500 to 7,000)
Determine choke size: (Carb size X 40) divided by 50
Determine main jet size: Choke size X 4

 

 

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Last modified: February 11, 2007