6:34 PM 12/31/2005
DataMite Project Sec 2

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The system

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Performance Trends

Testing the Hyosung Comet GT250S


Was asked to explain how to richen the fuel mixture and about re-jetting the Gt250, this will cover the GV250 and is relevant to the GT/GV650 bikes as well, but I will use a GT250 for all the images unless marked with a deferent bike.

Image 1

MIKUNI BDS26 Type Double Carburetor as used on the 2004 GT250 in the USA and Korea.

  • Idle r.p.m. 1,450 ~ 1,555 r.p.m.
  • Float height 17mm or 9.67in
  • Throttle cable play 0.5 ~ 1.0 mm
  • Main Jet (M.J.) Front 92.5 Rear 95
  • Main air jet (M.A.J.) Front and Rear 90
  • Jet Needle (J.N.) Front and Rear 2ND
  • Needle jet (N.J.) Front O-3 Rear O-4
  • Pilot jet (P.J.) Front and Rear 20
  • Throttle valve (TH.V) Front and Rear 130
  • By-pass (B.P.) #1-0.9 #2-0.9 #3- 0.8 #4- 0.8
  • Valve seat (V.S.) Front and Rear 1.2
  • Starter jet (G.S.) Front and Rear 22.5
  • Pilot outlet (P.O.) Front and Rear 0.75
  • PV Stroke (P.V.) Front and Rear STD

NOTE: The above data is from a printed Service Manual # 99000-94710, many of the terms are translated in several ways and some are swapped and do not correspond to the terms used elsewhere in the service manual or the parts book.

:bomb: TURN OFF THE FUEL :bomb:
:bomb: REMOVE THE BATTERY CABLES :bomb: Image 4 Fuel Valve

This is the early Fuel valve, may not be the same on all bikes.

To remove the tank:
First turn off the fuel and remove the fuel valve if like the image # 4,this can be removed by removing the 10mm bolt (only one bolt), disconnect the fuel filter and plug the fuel line. You will also have to un-plug the electrical cable to the fuel gauge.

Then open the rear and pull the RED knob to relies the seat, remove the seat and you will see a 12mm bolt that holds the seat on, remove and lift the tank, if you are very careful you can turn the tank and rest it in the seat area to work on the engine.

NOTE-2: Safety, I assumed you have read all or at least some of the many safety notices and referenced section 1-1 of the above referenced service manual if you have not, it is at least funny to read and you just might not burn up your bike or hurt yourself.

TOOLS and how to remove Metric Screws on the Comet bikes.
Some images we will be referring to in this procedure, please do not do any of this unless you have read and understand all, this is a work in progress and we do not want to make expensive mistakes.

Image 2
Right side

Raising or lifting the needle, will mask or correct the [? lag/miss?] at the 3,600 r.p.m. spot. This is done by inserting some washers below the “C” clip on the needle. This will make the mixture richer.

Image 3
Left side :bomb: TURN OFF THE FUEL :bomb:
Image 5

Carb removed for photographs, you can do this with the tank and carbs on the bike.

NOTE: The MSRP on a set of MIKUNI BDS26 Type Double Carburetors in the USA is $US 796.20 I am told, also it is not possible to get parts, only the set. So please do not brake or lose any parts, also as the needles are hard to tell apart so only work on one at a time.

The carburetors and most everything else on the bike is assembles with a permanent “Lock Tight” that must be broken to remove the screws, nuts and bolts, see the section on removing screws, nuts and bolts.

Image 6

Note this image does not agree with the Parts Book, but is from a factory installed Carb?
Imag 7

Shims and Jets
Imag 8

More to come
Mike on the rock