Utah Newbie making motor mods

Discussion in 'Introduce Yourself' started by misterww225, Dec 4, 2009.

  1. misterww225

    misterww225 New Member

    Hi, I have modified half a dozen of these chinese motors for my son and his friends. I tried several mods to find combinations that worked well. We have no dyno or other fancy tools, like software. These are cheap chinese motors, but they do respond to specific mods. We modified the ports, head, intake, and the exhaust. The intakes are composed of Breather filters from Checker Auto, Walbro 19mm carbs, and an adapter block made from phenolic. This adapter blends the 19mm diamater carb to the oval shape of the port. The heads are slant heads. These heads have higher compression than the others to start with, so less machining is required and this retains structural integrity. The heads are centered on the cylinders by putting machined rods through the head/cylinders. Two holes on either side of the bore are then drilled through the head into and through two of the cylinder fins. Roll pins are then pressed into the cylinders. The heads are re-drilled to clear the roll pins. This provides adequate alignment for these cheezy motors. The compression ratio is checked by sealing the piston/cylinder with grease, locking the piston at TDC, bolting on the head, and filling the head with oil to the bottom of the plug threads with a graduated 20cc syringe. The compression ratios (UCCR or CCR) can then be calculated. We machined our heads on the lathe (4-jaw chuck) for about 7cc combustion volume, or 10.4/1 UCCR, but raising it higher seems to be a good thing. The cylinders are ported. The exhausts are raised to 160 degrees duration (Open 100 degrees ATDC), and widened to a 1-1/4" oval. The transfers are only cleaned up NOT raised. Normally they are at about 114 degrees stock. This is about 23 degrees blow-down, which seems to make them "happy". The intake piston ports are lowered to provide about 130 degrees duration (Opens 115 degrees BTDC) and widened to 1-1/8", being careful not to get near the ring-end travel line. The port passages are opened up by cutting about 3/4" of the bottom of the cylinders and sharpening the bottom edges. This leaves about 3/4" of port passage wall left. The bottom passage side edge of the transfers are radiused by beveling the 90 degree corners slightly. This combination improved bottom/mid range torque while increasing top end over 7500 RPM. By the way, the RPM can be calculated by using the MPH and the motor/bike gearing and tire diameter. All edges of the ports were carefully filed to remove burrs and prevent ring hanging. The exhaust outlet was re-shaped to blend with the new port shape. The same was done to the intake piston port. The exhaust header has an oval shape to match the exhaust port and is a 2" length of pipe to blend from this oval to a 1" round. The exhaust tubing is 1", and is connected to the rest of the exhaust by using 1" high-temp hose and clamps. The muffler is from a Banshee, cut almost in half and re-welded together (Banshee owners usually replace the original pipes/mufflers anyway). The ignition timing is verified by gluing a small degree wheel onto the flywheel, making a pointer from a small piece of safety wire under the nearby screw, and using a timing light. The timing is initially 20 degrees and reaches a max of 30 degrees at high RPM. The fuel we run is premium mixed with Klotz BeNol at 7oz per gallon which prevents bearing and piston damage. The compression ratio can be raised quite high on these 2-stroke motors unless you have a tuned pipe. The tuned pipe is a "sonic supercharger" capable of providing up to 5 to 7 lbs boost. As a result, the compression ratio will need to be low and the ignition timing will need retarding in the "boost region" (on-the-pipe) RPM. We have not balanced out motors as of yet, but plan to in the future. Here is a balancing we are proposing;

    by: DIYMark MBc Member (www.motoredbikes.com
    edited by: JW

    1) First decide on your balance percentage. The recommended is 55%.
    2) Now you will need to weigh the total piston assembly - piston, rings, gudgeon pin, and record it.
    3) Now weight each end of the rod with the corresponding bearing inserted into its race. Weight each end with the rod in a horizontal position. 2 accurate scales are ideal, but using one will work too and record it.
    4) Add the piston assembly weight to the little end weight and times his amount by the balance percentage (keep in mined 55% equals 0.55) then add the big end weight to this number and you have balance mass.
    BalanceMass = [( PistonAssym + RodLittleEnd) X 0.55 ] + RodBigEnd
    5) Now make up a BalanceMass that weighs exactly as calculated above. You can make it from anything
    such as nuts/bolts taped together or be fancy and machine a collar.
    6) Inspect and re-machine the crankshaft counterbalances, if required, to obtain concentric components.
    7) Fix the BalanceMass to the crank pin, press together, true, and mount your crank on 2 knife edge bearings that are level to the ground and parallel (2 steel rods will work).
    8) You have to remove enough metal from the crank throws symmetrically (left/right) so that with the balance mass attached to the crank pin, you can turn the crank along the knife edges and no-matter what position you leave it at, it won't roll over to the heavier side. If the BalanceMass itself is too heavy, it will require removing metal from the BalanceMass and then removing the same amount from the piston assembly. This requires careful thought. Although not necessary, if you want to be precise, use the density of steel/aluminum to calculate possible diameter holes and their depth to equal the desired weight to be drilled.
    9) Once you are satisfied with the balance, dissemble the crank, clean and oil all components, press together, and true the crankshaft.
    10) You are done.

    Tools we have used include a lathe, dial indicator with holding fixture, mini 90 degree 1/8" rotary pneumatic grinder, 1/4" grinder, 10" degree wheel, numerous files, graduated syringe, timing light, and magic marker.

    Attached Files:

    Last edited: Dec 4, 2009

  2. professor

    professor Active Member

    Wow! 225, that is a mouthful.
    How do all these mods work out?
    And I am curious how you learned all this stuff. Impressive.
  3. a/c man

    a/c man Member

    Are you still around? MISTERWW225?

    Just dug up an old post from Dec.2009. WOW what a wealth of information with no follow-up posts. I sure would be interested in hearing more about these engine mods, here's a guy who really knows his stuff. If you're there please write us again Misterww225.
  4. Karryhunt

    Karryhunt Member

  5. Fabian

    Fabian Well-Known Member

    Hi misterww225

    Excellent, excellent, excellent work.
    Full marks for using a Walbro 3 needle carburettor - in my opinion, it's the only way to go.
    Can you post a few more high resolution photos of your porting work (intake, transfers and exhaust), posting side by side pics of ported and stock ports.

    Sounds like you have a lot of knowledge and a "CAN DO" attitude.
    What's your opinion on the workability of creating some sort of adaptor to mount a reed valve intake system onto the crankcase, and then grind out a suitable intake port in the crankcase.

    Have you explored the idea of making a stroker crank or a big bore cylinder combination.

    Cheers Fabian
    Last edited: Apr 14, 2010
  6. Fabian

    Fabian Well-Known Member

    What's the part number for the Walbro carburettor?
    How do you hook up the throttle cable to the carburettor?
    Last edited: Apr 14, 2010
  7. a/c man

    a/c man Member

    I dug this 6 month old post up. Mister225 never followed up on his one and only post.
    We need guys like him on this forum. I was hoping we'd here from him again to answer all the questions we would have. If you're out there please write us back Mister225.