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KB Article #6: Carb removal and jetting changes

Old 09-19-2006, 06:41 PM
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KB Article #6: Carb removal and jetting changes

This is a How-to on pulling the carbs and making jetting changes. This thread been a long time coming but is basically a copy of another post I made some time ago. Ready, set, go....

You need to pull the tank, airbox, and carbs. If lots of gas comes out of the petcock when you take the hose off you will need to shut the petcock with an 8mm wrench. Carb removal requires removal of choke cable (gently, it's a plastic fitting), both throttle cables, carb heating hoses, and I think the fuel hoses can stay on. Loosen the clamps on the boots. Rock carbs side to side while pulling them out.

Take the carbs to a nice clean workbench. First take off the diaphragm covers (black). Screws can be tight. Slowly remove covers as to not tear the diaphragms. They are fragile. There's also a long-*** spring that'll pop out at you. Slide the diaphragm and carb slide out. Now take one of the cover screws and screw it into the middle part of the carb slide. This is a retainer that holds the needle in. Pull it with a pair of pliers and it'll come right out. Inspect the o-ring. Now remove and inspect the needle. If it has slots in it this is a good thing. That means it has an aftermarket jet kit, and is also adjustable via the little clips. If it has none then you're going to want to buy one. Stock needles can be raised, but not lowered. Lowering the needle will get you leaner. Raising it makes it richer. Needles control part throttle mixture from about 1/8 throttle up to about 3/4 throttle. It's also based on RPM due to the constant velocity design. Vacuum raises the slides. If the needle is aftermarket, then move the clip to the desired position. Put everything back together. To make assembly easier, compress the spring into the hole in the cap, and then slip the slide assembly over where the spring is. Install the 3 parts together on the carb. Diaphragm should be concave down. Careful not to pinch the diaphragm when installing and tightening the cover.

Next, the float bowls need to come off. These screws are super tight from the factory. I recommend using a really good screwdriver and a hammer (as a sort of impact driver) as to not strip the screws. Now once the bowls are off, the main jets are the ones that screw in with a flat bladed screwdriver and will have a number like 178, 180, 182, etc. These will need to be replaced with ones of a different size, optimized for correct mixture and max power at full throttle. They come in sizes that change by 2 or 3, as above, or if it is a Dynojet kit then they are multiples of 5. The front and rear have different sizes, BTW. Richer in the rear to help cool that cylinder.

There is a pilot jet, also flat blade, that will have a number like 45, 48, or 50 on it. This should be removed, inspected, and replaced. Most will prefer the stock #45 jet. The pilot jet controls mixtures at idle up to 1/8 throttle and is the "coarse" adjustment for this range of carburetion.

There are tiny holes in the emulsion tubes that need to be clean. The front and rear emulsion tubes are different. Do not get them mixed up.

After all that is done, on the bottom of the carb you'll see a little "D"-shaped brass screw. If you can figure out how to get that out and then slot the end for a flat blade then it'll be easier to adjust. If you order a jet kit then it should come with a D-shaped tool. You want to adjust these to about 1.75 to 2.25 turns out from fully seated, depending on your setup. The more turns open the richer the mixture. The mixture screws are the "fine" adjustment for this range of carburetion.

Now it's time to put everything back together and test ride. The small vacuum line does NOT go to the bottom nipple on the petcock. It goes to one on the back of the petcock. Common mistake there. Also, if the petcock was shut, reopen it.

When you test ride, use the choke as a testing tool. If you pull the choke out a little bit in a particular throttle/rpm range and there is more power then you are running lean. If it dogs down a little or gets "soft" then you're running rich. This is a basic way to determine seat of the pants improvements. If the bike feels good all around then there's no need to use this method of testing.

DISCLAIMER: I did this about 6 times when I jetted my bike, so don't expect this to be the first time you tear the carbs down. I can provide you a decent baseline based on my extensive knowledge of jetting the VTR, but by no means should you expect perfection on the first try. Each bike is slightly different.
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