XT250 air box & air induction mods
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Thread: XT250 air box & air induction mods

  1. #1
    Senior Member SportsterDoc's Avatar
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    XT250 air box & air induction mods

    I realize that it is the general consensus of this forum that modifying the air box of the TW 200 has little if any effect.
    However, most of my 33 vehicles with 4 wheels or 16 motorcycles had intake mods.
    None on the TW or Moto Guzzi, but my 2012 Triumph Bonneville had snorkel removal, resulting in a top speed of over 112 MPH from 865cc. Also, it had dual air induction, but the air source was plugged to reduce bluing of exhaust headers.

    Side note:
    1. A week ago, I plugged the air intake (rubber stopper in the short hose from air box) to the air induction to stop back-popping on decel. 2014-2015 models had a recall of almost 5,000 XT250s, due to the air induction rubber hose burning through at sustained speeds, resulting in damaged wiring. 2016 up have longer metal pipes to valve and a thicker wall rubber hose.

    2. When I went to the dealer this week, I asked for a listen to my XT250. I’ve heard some sounds that I could not identify. Mgr rode it and said sounds were normal but did not identify. My theory is that they may be related to the air induction valve (or maybe the balancer gear). Air from the air box feeds a vacuum operated reed valve, allowing air to flow from air box to exhaust valve. I am all for reduced emissions and the unit has no effect on horsepower, but I do not like unidentified sounds, nor the idea of blowing cold air on an exhaust valve. A block off plate for the 250 Raptor would allow air induction removal, but would prefer to leave as much intact as practical.

    So, today, I capped off the 3/16 vacuum port on the top of the throttle to head manifold and capped off the 3/16 port to the air induction valve assembly (just to keep crud out). The short section of 3/16” vacuum hose was stored.

    When I was out running errands, I looked for a 1 1/8” hole saw, which is the maximum diameter usable on the top of the air box. Only 1” or 1 ¼” hole saws available locally, so I used a butterfly bit, instead. Not as clean a hole, but just took more cleanup after cutting. Caution: Using 1 1/8" leaves no room for positional error. XT250 riders would do fine with 1".

    Stock intake: ~1” x ~ 1 ¼” = 1.25 square inches (less than throttle body diameter)

    Filter area: 1 5/16” x 3 3/4” or 1.3125” x 3.75” = 4.9219 square inches, with flow improvement by deep pleats.

    Throttle body: 33 mm = 855 sq mm or 1.32 square inches (larger than the stock intake area)

    Also, the snorkel directs air to a limited surface of the air filter, which you may be able to see in the filter photo (blackened area more towards bottom and to one side). Might be my imagination, but altitude power drop on the XT250 seemed to be more than on the TW200, at elevations to 8,500 feet.

    By adding the 1.125 diameter hole, that adds 0.9935 square inches to the stock 1.25 square inches, totaling 2.2435 square inches of intake. Still under 50% of the filter area, but now considerably larger than the throttle body, which should lessen flow restriction.

    This last tank of fuel went 89 miles on 1.1 gallons = 81 MPG. I was surprised, as I had several highspeed runs on the freeway. Typically run to 65 MPH, wait a few seconds, increase to 70, wait a few seconds, increase to 75. Even at 75 (indicated) there was 1/4 to 1/3 throttle left…but that extra throttle does not do much.

    At idle and gentle reving, the added intake made no difference in sound. The bike is cornered in the garage, in preparation for a community garage sale tomorrow, so I won’t get a chance to run it until Monday. If there is any negative effect, the hole can easily be blocked, partially or completely.

    Hope to see past 1,000 miles on the odometer by end of day, Tuesday. It is over 40 miles from home to Cold Creek and about 23 miles is a 70 MPH highway.

    1-air filter at 843 miles.JPG
    2-1.125 diameter intake added.JPG
    3-added intake with filter cover .JPG
    4-843 miles.JPG
    Last edited by SportsterDoc; 10-13-2017 at 06:41 PM.
    2014 Honda CB1100
    2018 Yamaha XT250
    2016 Moto Guzzi V7 II
    2017 Yamaha TW200
    2012 Triumph Bonneville
    2002 H-D Sportster 1200 Sport
    2003 H-D Sportster 883
    1976 Honda CB750F
    1975 Honda CB360
    1970 Yamaha CT1
    1972 Yamaha CT2
    1972 Yamaha AT2/CT2
    1970 Honda SL350
    1970 Honda CL350
    1967 Honda CL160
    1967 Honda CB160
    1962 Honda CA110

  2. #2
    Senior Member SportsterDoc's Avatar
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    159 miles today
    break-in/run-in complete
    modified air box had no noticeable effect on running below ~5,000 feet elevation
    however, at 6,000 to 8,500 feet, either it made a difference for maximum speed (65) pulling a 6+% grade and/or it improved with break-in
    at 2/3 throttle, last run, it would not do much more than 55-57 in the same area

    72 MPH comes reasonably quick
    72 to 80+ is much slower in acceleration

    Up to about 68 MPH is a comfortable cruise...mirrors are clear

    106 miles back from Indian Springs was 75.8 MPG (rated 76).
    Some portions of Cold Creek are more than 6% grade (ran about 55-60 at less than WOT).

    10-Run-in complete.JPG

    https://tw200forum.com/forum/general-...-hot-cold.html

    Probably not much new to post after this.
    Last edited by SportsterDoc; 10-16-2017 at 06:55 PM.
    2014 Honda CB1100
    2018 Yamaha XT250
    2016 Moto Guzzi V7 II
    2017 Yamaha TW200
    2012 Triumph Bonneville
    2002 H-D Sportster 1200 Sport
    2003 H-D Sportster 883
    1976 Honda CB750F
    1975 Honda CB360
    1970 Yamaha CT1
    1972 Yamaha CT2
    1972 Yamaha AT2/CT2
    1970 Honda SL350
    1970 Honda CL350
    1967 Honda CL160
    1967 Honda CB160
    1962 Honda CA110

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