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Thread: Carb Tuning

  1. #1
    Senior Member qwerty's Avatar
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    Since the big forum upgrade (thanks, Chopper Charles) we are back to getting a bazillion questions about tuning carbs, or rejetting. It seems many people are intimidated by the thought of rejetting. It isn't vodoo, and it isn't rocket science, so lighten up. The hardest part is getting those %^&^$ float bowl screws off. I thought I'd help prospective tuners out with a bit of material that can be pinned, if a moderator would be so kind as to do so. First, I will type a little about the general concepts of rejetting, then give specific instructions and part numbers about how to go through the process.



    First off, rejetting isn't magic. It's a simple step-by-step process. It doesn't matter of you are tuning a lawn mower or a Kieth black hemi, the concepts are the same. It just takes patience and persistance. Now, don't go by how I have my carb tuned. Odds are my tune won't be right for your bike. So many variables affect mixture one simply cannot go by what somebody else does. Never forget that a carb is almost always slightly out of tune. Racers will rejet between rounds or heats as the weather conditions change. That isn't necessary for our purposes. Electronic fuel injection rejets constantly whenever engine operation parameters change. that isn't possible with a carb. If your bike is running fine, you might not want to mess with it. If there are substantial changes in ambient conditions, say a 100*F change in temperature with the seasons or a 4000 foot change in elevation, you'll be looking at rejetting.



    Remember to make just one change at a time based on the evidence the bike provides. that way if a change makes things worse it is easy to back up and correct the problem. If a change makes things better, make another change in the same direction until things get worse, then back off one change. Remember, good tuners are patient and persistent. Good tuners pay attention to their engine and allow the engine to dictate what changes to try next. There must be a reason, a symptom, before a change is attempted. If the bike is running fine, don't try to fix it--it ain't broke.



    Here are the seat-of-the-pants symptoms of a poor carb tune:



    Too lean:

    Engine dies when throttle opened quickly.

    Engine runs hot.

    Ping (pre-ignition).

    Surging while throttle held steady.

    Popping back through the carb during deceleration.

    Power loss.

    White or yellow powdery deposits on spark plug.

    Possible burned valves or holed piston.



    Too rich:

    Engine hesitates, then accelerates, when throttle opened quickly.

    Black exhaust smoke.

    Soft black deposits inside exhaust.

    Soft black deposits on spark plug.

    Loss of power.

    Possible liquid gas washing oil off cylinder walls allowing scoring and siezure.



    There are three adjustments in fuel metering in nearly all carbs. Some carbs have more. We will be working with 1) the pilot jet and screw, 2) the needle, and 3) the main jet. All three have an effect at all engine speeds/throttle settings. Adjustments to the pilot jet and screw have the most effect at low throttle settings. Needle adjustments have the most effect at mid throttle settings. Main jet adjustments have the most affect at high throttle settings. However, the main jet does affect part-throttle operation, but not much. The pilot jet and screw do affect operation at high throttle, but not very much.



    It is best to give the carb a good rebuild first unless the carb is practically new. You can't tune mechanical problems out of the fuel system. Make sure everything is clean, tight, and seals where it is supposed to seal. Make sure the float moves freely and is properly set, and that the fuel flow needle and seat upon which the float works are clean and in good condition. Make sure any diagphram is pliable and free of holes, tears, and wear. Make sure any shafts going through the carb body seal properly. Make sure there are no leaks in the rubber boots in which the carb mounts. Make sure there are no vacumn leaks. Clean the air filter.



    Once the carb is in perfect condition you can start the rejet process. 1) Start with the main jet first. 2) Then work with the needle height for best part-throttle engine response. 3) Finally, work on the pilot and screw to optimize idle and starting characteristics. Any attempt to work this process in any other order is likely to be VERY frustrating. Rejetting in this order results in making going back to retune a previously tuned component a rarity. Rejetting in any other order almost guarantees going back to retune a variously tuned component.



    Sometimes it is necessary to go back and tune the high speed again. On rare occasions you might need to go through the main/needle/pilot sequence two or three times, but that is rare.



    I like to run the main jet a tad richer than optimum performance to lower engine temperature, but not rich enough to foul plugs or wash down the cylinder walls. I do a lot of highway at sustained high throttle settings. Many miles of the East Texas and Big Bend rides consist of mile after mile of deep, loose mud and/or sand. Maintaining 40-45mph requires 4th gear and near full throttle. A lean main would not have caused a problem with the temperatures in the '40s, but out in the desert at 107*F running WFO for long periods of time a lean main would likely result in burned valves or a holed piston. No fun when 25 miles from the nearest paved road.



    I sometimes choose the main by reading plugs after some full-throttle plug chops. Another way to choose a main is to try several jets and see which gives the best top end. If you get too rich or two lean the bike slows down. Much cheaper than buying a bunch of spark plugs. Other performance characteristics of a too-lean mixture are popping back through the carb when you shut the throttle while decelerating, the engine pings (pre-ignition), or the engine surges, which kind of feels like the throttle is being rolled back and forth. A main jet sized one up from any of these symptoms of too lean is usually as close as you are going to get. Remember though, that if you are perfect at high altitudes or cold temps your mixture will become lean at low altitudes and hot temps. I rejet richer for the summer and leaner for the winter, richer for low altitudes and leaner for high altitudes. I've also taken a 128 main jet into a machine shop and drilled it out to a size halfway between the 128 hole and the 132 hole. That is my low altitude hot weather jet.



    Once I am satisfied with the fuel mixture at wide open, I lean out the midrange until I feel lean surge at part-throttle cruise (35-40mph on a TW). Lean surge feels like a miss to many people, kind of like the throttle being rolled open and closed. I keep adding washers under the needle until the lean surge goes away. If the bike is too rich at midrange, the engine will hesitate, then accelerate, when the throttle is snapped open. With a 128 jet in the winter Tdub needs 2 washers from the hobby shop. With the 130 jet (drilled 128) in the summer Tdub need 0.75 washer from the hobby shop. It takes a few minutes to thin a washer with a piece of emery paper. Place the washer on the emery paper. fold the paper over the washer. Sand away.



    Finally, I adjust the idle mixture screw until the bike cold starts easily and accepts throttle quickly from idle when warmed up. If the bike is too lean at idle it will not cold start easily and will die if the throttle is opened quickly. If it is too rich it will cold start in cold weather without the choke, but there will be a lag when the throttle is opened quickly at low engine speeds. I recommend buying a power screwdriver bit about 3 inches long and wrapping the phillips end with duct tape for a handle. With this little homemade tool it is possible to adjust the pilot screw without doing anything to make the screw more accessible.



    The TW200 late carb main jets are:



    #125 288-14343-63 Used in 49-state and canadian TW200s. $6.84 at stadiumyamaha.com

    #128 288-14343-64 Used in California TW200s. $10.47 at stadiumyamaha.com

    #132 288-14343-66 Used in Australian and South African TW200s. $7.50 at stadiumyamaha.com



    288-14343-65 is a valid Yamaha part number for a 130 main for several models. I'm almost willing to bet it is the missing link between our 128 and 132. $9.95 at stadiumyamaha.com



    There are two needles listed for the late TW200.



    5FY-14336-00 Used in all North American TW200s. $13.13 at stadiumyamaha.com

    5LB-14336-00 Used in Australian and South African TW200s. Not available in the U. S. of A.



    Best I can tell from the parts explosions on the late model carbs the only other differences among the three versions are the external plumbing and venting.



    The TW200 early carb main jets are:



    #114 288-14343-57 Used in 49-state and California TW200s. $9.68 at stadiumyamaha.com



    Other main jets available I think are:

    #116 288-14343-58 $12.05

    #118 288-14343-59 $10.08

    #120 288-14343-60-00 $9.55

    #122 288-14343-61-00 $8.19

    #124 288-14343-62-00 Not available in the U. S. of A.

    The late model jets will probably work if you need to go even richer. Anybody know for sure?



    Regardless of if you have an early or late carb, you will also need a couple washers to raise the needle. Simply carry the needle into a good hardware or hobby store and find a couple washers with holes that fit the needle diameter. No sense spending dollars on microshims unless you can find some local, though microshims are much easier to use to make fine adjustments.



    A word of warning on non-Yamaha main jets:



    There are jets available for a number of different brands/models of carbs that will work in the TW carbs, but their numbering systems do not coincide with that of the Yamaha jets. A 120 #jet of another type will be richer or leaner than a #120 Yamaha jet. These alternative jets tend to be cheaper and do work, but you'll have to figure out how they relate to your current jet yourself.



    Good luck with your tuner experiments.
    7375

  2. #2
    Member drob's Avatar
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    Awesome post qwerty!
    - 2004 TW200 - Jimbo's Shield - Ricochet plate - Cyclerack - SoftBottom pad - wide pegs - siderack for saddlebags - RotoPax - Maxxis 6006 tire

  3. #3
    Senior Member betopimienta's Avatar
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    I tried the late model jets in early carb......... too rich . The 120 is probably as rich as you need to go on an early carb from my experience. I have a 118 in my early carb and it runs good. I salvaged a tiny washer from some old electronics scrap i had laying around [washers came from tiny screws securing transformer to case] the washer was just the right size to fit the needle and fixed the mid range lean issue.

    I have heard of other mods concerning drilling out the "carb slide hole" to improve throttle response. But i think this only works on late model CV carbs as the earlies have mechanical sliders.

  4. #4
    Junior Member TW Yamamoto's Avatar
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    Hello Qwerty,

    This is a very generous and time consuming effort you have undertaken. Thank you.

    Regards,

    TWY

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