The first thing to get right when setting up sidedraft or downdraft carbs is the choke size. Chokes that are too small cost top end power but are good for low rpm tractability and economy. Chokes that are too large not only hurt tractability but also lose top end power and the fuel consumption will be terrible. Get the choke sizes right and you can achieve 30 mpg or more on twin carbs in normal driving. Go too big on them and under 20 mpg is very easy to find.
The Weber tuning manual gives a choke size chart based on cylinder size and the expected rpm at which peak power will occur. It isn't the easiest thing to use if you have no idea what rpm the engine might run to with a given set of tuning mods. An alternative way of choosing the choke size is based on expected engine bhp. Of course once the engine is on the dyno or rolling road different sizes can be tried as part of a thorough calibration session if you have the money to pay for that but the chart below will put you very close. It applies only to setups with one choke per cylinder. It can be used for any sidedraft or downdraft carb such as Weber DCOE or Dellorto DHLA. The chart is based on best top end bhp. If you want to sacrifice a few bhp at the top end for better low rpm power and economy then go 1mm or at most 2mm smaller than the chart suggests.
4 Cylinder engine
6 Cylinder Engine
8 Cylinder Engine
A good rule of thumb is that if the suggested choke size is within 7mm of the carb size then move up to the next sized carb. So a 40mm carb would be good up to a 33mm choke and 150bhp. A 45mm carb up to a 38mm choke and 200 bhp etc. Always use the smallest carb this rule allows. For a 30mm choke you'll get more power and tractability with a 40mm carb than a 45mm one.
Main jet size can be estimated from the choke size on which it's almost totally dependant. For a standard or mildly tuned engine with a mild cam multiply the choke size by 4 so a 30mm choke would need a 120 main jet as a starting point. For engines with very big cams or big chokes selected for maximum power at the expense of tractability the mains might need to be a bit bigger than this because there won't be a very high depression across the chokes to get the fuel moving. Sometimes you might even have to go a bit smaller but 4 times the choke size will always get the engine running reasonably well. Air corrector jets are usually in the 160 to 180 range but they really only affect high rpm fuel mixture and can be left until last to calibrate. Idle jets and emulsion tubes are very dependent on the engine size and cam duration so let your rolling road sort these out.
The key is getting the choke size right first though. Too many rolling road operators just leave the chokes as is and try and optimise the jetting around them because they're expensive and they don't always have a full range of sizes in stock or simply because they never think to check or have no idea what size to change to. If you've bought your carbs second hand and they were originally jetted for a completely different engine the chokes could be anything from 30mm or less for a standard road engine to 38mm or 40mm for a 2 litre race one. You're wasting your time and money trying to tune any engine if the chokes are more than a couple of mm away from the ideal size.
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