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. Chokes that are too large not only hurt tractability but also lose top end power. 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 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 then go 1mm or at most 2mm smaller than the chart suggests.
BHP 4 Cylinder engine |
BHP 6 Cylinder Engine |
BHP 8 Cylinder Engine |
Choke Size
mm |
| 93 | 140 | 186 | 26 |
| 100 | 150 | 200 | 27 |
| 108 | 162 | 216 | 28 |
| 116 | 174 | 232 | 29 |
| 124 | 186 | 248 | 30 |
| 132 | 198 | 264 | 31 |
| 141 | 212 | 282 | 32 |
| 150 | 225 | 300 | 33 |
| 159 | 239 | 318 | 34 |
| 169 | 254 | 338 | 35 |
| 179 | 269 | 358 | 36 |
| 189 | 284 | 378 | 37 |
| 199 | 299 | 398 | 38 |
| 210 | 315 | 420 | 39 |
| 220 | 330 | 440 | 40 |
| 232 | 348 | 464 | 41 |
| 243 | 365 | 486 | 42 |
| 255 | 383 | 510 | 43 |
| 267 | 401 | 534 | 44 |
| 279 | 419 | 558 | 45 |
| 291 | 437 | 582 | 46 |
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 to at least get the engine running from the choke size. Multiply that by 4.2 and divide by 100 to get mm. So a 36mm choke would want 36 x 4.2 = 151 = 1.50mm main jets. Air corrector jets are usually in the 160 to 180 range (1.60mm to 1.80mm). Idle jets and emulsion tubes are very dependent on the engine size and cam duration so let your rolling road sort these out..
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