Over Boring Carburetors 101

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Over-Boring Carbs 101

The key to a successful carb bore is your skill at eyeballing and having the right drill bits, picks and sand paper. It is also nice to have a Vernier caliper, a metal lathe or drill press is mandatory, as you want a cylindrical bore, not a cone!! (hand drills and electric drills just won’t cut it!)

First, you need to completely strip your carb to its bare body and clean it. (Needle jet, choke plate, the whole works!) It is essential to clean the pilot circuit well so as to avoid future complications after the bore. (a clean jet is an easy jet to unclog!!)

Next, it is crucial to measure the slide diameter, as this is what determines your maximum bore. It is best to stay .75 of a mill under the slide diameter to ensure a proper slide seal. (-so air cannot bypass the slide.) My slide ended up being a touch over 13mm so I took a risk and bore mine out to a 12.5mm, but I don’t suggest you ever go over half a mill in any situation! (better safe than sorry)

Second, find a drill bit that’s the right diameter. Don’t go by the # posted on the bit as the bit may be slightly out of round or just off measure. I suggest you drill a hole in some scrap first and then measure the hole to be certain.

Once you’re ready to bite the bullet, set your drill press table perfectly level and at 90 degrees to the bit for a perfectly straight bore. Make sure you have a deep enough bit and set your press so the bit will drill all the way through and past the venturi. (the narrowing at the jets)

Now, place the carb body on the drill table, intake opening down, so that the back end of the venturi (the end that clamps / bolts to the intake manifold) is facing up. Try to line up the center of the carb bore with the center of the bit as best you can, measure it if you have too. You will want to clamp the carb to the table, or get a few hands to help hold it down as you bore. Lubrication is a must during the boring process to help cool the bit and flush chips from the carb.

As you begin boring, press very lightly into the venturi so the bit finds its center and slowly allow the bit to bite in. try to not allow the bit to get too hot as you continue through the venturi, or the aluminum can possibly start to smear into the pilot jet orifice, welding it shut.

Once you have finished the bore, it is now time to detail it. Take some files, picks and sand paper and begin sanding down and picking out ridges that have built up in the bore, and at & in the mouths of the venturis. It is now that you must locate the pilot orifice and unclog it. I used a pick, but by all means, use what ever you feel necessary to unclog it. You can check it with a spray of WD40 up the pilot circuit pickup tube, which runs into the bowl. If it sprays up out the pilot orifice, you’ve been successful.

Attention to detail is the determining factor of your successes, so maintain it at this stage. If it's a Bing don't forget to trim the end of the throttle slide so that it clears the venturi completely when retracted.

Once you are happy with your detailing, clean the body, reassemble the carb, and try it. Chances are, it will run lean and sound like it’s getting too much air. This is were some of the guesswork starts, as you will now have to either open up the main jet orifices with tiny drill bits, or buy a bigger jet. Either way, after this, its all up to you. Experiment with different settings on a running bike, and over all, jet it rich!! (Especially on 2 strokes!!!!)

Messed up? Don't lose hope! Read this first: Salvaging a ruined Carburetor


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