Author Topic: jetting for big bore  (Read 1482 times)

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Offline phi

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jetting for big bore
« on: July 24, 2011, 04:05:00 am »
My apologizes, if this is commonly know.
Recently, I searched this forum to find a "how-to" for jetting an engine with big bore. I did not succeed, which is the reason for this post.
I had a stock engine with pods and 4-1 exhaust. The carbs had allready been jetted for these mods. I then installed a 900 cc. cylinder, and needed to jet for this.
To my suprise, it ran perfect with the current jetting. All I did was to install a main jet two sizes bigger, sync the carbs, and it ran perfect. Have been checking the plugs, and they are more perfect tanned than David Hasselhoff. I would have expected much more tinkering to get it right.
My theory is this: when one increase the displacement, the speed of the air through the carbs also increase. When the air speed increases, so does the amount of fuel drawn through the jets, making the proportions just about right. It is not until the top end RPMs the engine needs more fuel, hence the bigger main jets.
As a side topic: I considered carbs with a bigger bore, but the stock carbs does a fine job keeping up. The result is much more torque and a somewhat hotter running engine, but no other significant changes.
Hope this may be helpfull for other "big block" fans.
wow, did you see that old Honda?