The Thread Spread

- Oval Boring -

The Hole's Not The Same
The question - What is oval boring carburetors? Can it be done a stock 250R Keihin? What are the benefits?
The response - It is an attempt to make a large carburetor behave as though it were a smaller one (at lower throttle speeds) and a smaller one behave like a larger one (at higher throttle speeds).
By making the bore oval (taller vertically) you are able to give the carb increased air flow at the fully open position and less flow at
partially open positions. Since you didn't cut a round hole in there, the stock slide will still cover the throttle bore. Most carburetors can handle an increase in bore in the neighborhood of 1 to 2 mm. If you can add another mm vertically - you have gained something. Not much - but you have gained some increased air flow.
As far as whether this feeds the reeds or crankcase any better... I would guess that perhaps it does at low flow (1/4 to 1/2 throttle). But the margin must be very close. If the actual calculated oval area is the same as the actual calculated round area (of 2 different carbs), there would be no detectable difference at moderate throttle positions - therefore they would both be making the same power. There may be an advantage below and above this area to oval boring - if the engine is jetted correctly.

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