HT mileage and performance comparison

Discussion in '2-Stroke Engines' started by mountain80, Dec 6, 2008.

  1. mountain80

    mountain80 Member

    So for interest sake after taking a stock HT and doing a port and polish, matched manifolds, taper bored carburetor,higher compression, rejetting the carb, cutting the baffle tube changing the gearing from a 40T to a 36T, retapered needle I now have a huge increase in mileage. Granted when I did the first mileage check it had about 600 km and was still a little tight but I now am getting a huge increase. After measuring out a predetermined amount of fuel (4 litres) and having an increased tank size(4l) I ran the tank till she died. Here is the difference:

    Stock 75 miles/Imperial gallon
    improved 114 miles/Imperial gallon

    I ran the tank bone dry and used my electronic bicycle speedometer which I checked against a garmin gps. With my new 4Litre(actually 4.4 litre) tank I ran out of fuel at 160 km or 100.8 miles.

    It has more power AND mileage so one can get their cake and eat it as well.

  2. Flapdoodle

    Flapdoodle Member

    Impressive. That is a 52% increase.

    Did you modify the carb, or is it a different one?
    Did you change the needle taper yourself? How is this done?
    Is the bike considerably louder?

  3. mountain80

    mountain80 Member

    I modified a stock carb. If you look closely at the filter side of the intake you will notice a step or shoulder as it is called. The shoulder is now gone and the bore is tapered inwards for a smooth flow. Yes I changed the needle taper myself. It took a month of weekends polishing and measuring with a digital micrometer as well as testing. I did a lot of throttle chops and marked where the needle position was and marked with a marker while inside th carb bore.Basically from idle to 1/4 throttle it was lean and I have to run it at the leanest needle setting for 1/4 throttle and up for 2 reasons; when you increase compression an engine is more efficient and I live at 4400 ft altitude so I ended up taking the stock.028 jet and dropping it down to .025 thou. Dropping the needle leaned everything out nicely with a tan color on the plug. As far as loud it actualy is respectable, I cut the baffle tube in the cap so it is barely sitting inside the baffle chamber/plates but no more. Another reason I feel for the mileage/power increase is I had custom machined a boost bottle and had carefully include the length of hose to the intake so it matched within 1 cc of the engine displacement. The difference in bottom end torque was huge. Their was a fair amount of porting/polishing as well as the piston skirt being cut to exactly match the intake port. I did not go radical on my porting but made sure everything was matched as perfect as possible. As a last note I used 2 layers of scotchbrite pads for a filter and opened up the filter housing, but soon I may go to an after market filter such as the one from sick bike parts. When I did my mileage test It did include start/stops and full out running for 3/4 of an hour nonstop.
  4. Flapdoodle

    Flapdoodle Member

    I am even more impressed. But then we won't get the return unless the effort is put in.

    I would be afraid the Scotchbrite abrasive would come loose and become ingested. I have some furnace filter pad that looks very much like Scotchbrite. For these filters they make a spray that is supposed to make them stickier. The ingredients say it is water and ethylene glycol. Easy enough to make.

    After your success, I will remove about 1/2 of the step on the carb intake and see how that goes.

    Thanks a bunch for sharing all your hard work with us.