Home Forums General Karting Discussion HPV KPV First Build- Spec Review

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    • #50902
      Andres Malovos
      Participant

      Hi,

      So new to karting and two strokes, but have been playing with cars for a little while.

      I bought a HPV powered kart, and picked up a spare engine to learn on. I tore down the top end and measured things along the way. Some things I should have, but so I learn.

      The main bearings and lower rod bearing felt good. A fair amount of lateral play on the rod, but no discernible vertical play. Dial indicator is saying only a few thou lateral play in the main bearings. I skipped any bottom end work. It has less main bearing play than the engine on my kart now for what it’s worth.

      The bore was good, round and straight enough to skip a hone go from green to red dot piston. 52.62CR Red dot to be specific. I was reading a lot of back and forth on what flex hone to use, I ended using the weisco flex hone to call it safe.

      Ring gap was at the tight end, .008″. Piston clearance too was thin. I figure it should be a tight engine.

      Two stroke timing is all new to me, so I didn’t think of checking how it was timed as I took it apart. I found the piston was about ten hundredths shorter than the new piston. I did not trim the new piston. Not sure how to translate piston height to intake duration other than checking how many degrees of rotation it takes the piston to travel ten hundredths, then map from there.

      The engine had a .025″ base gasket and no head gasket. First gasket I ordered was .020″ so I tried that and this is what I came up with. I’ve seen some remarks about duration based off port exposure relative to BDC. These are just the degrees at which ports are exposed.

      Exhaust port exposed at 90 degrees, closes at 265 for 175 duration

      Transfer ports exposed at 115, closed at 240 for 125 duration

      Intake port exposed at 275, closed at 80 (after TDC) for 165 duration.

      With no head gasket and a .020″ base gasket, squish was .027″. So on the thin side, but safe enough I assume?

      I didn’t touch the timing, but found it at .070″ BTDC.

      I haven’t leaked down the engine, but did a quick compression check, and it was pumping 120psi, which I thought was low. It was pumping 105 before I pulled it apart.

      I really don’t know where these specs fall in the spectrum of HPV KPV builds, but I’d love to get a lot of practice time out of this engine. If there are changes I can make with base or head gaskets, timing, etc, that would move the engine in the practice engine direction, then I’m open to suggestions.

      Thanks

    • #50931
      Walt Gifford
      Participant

      Flex hone is the fastest way to ruin a 2 stroke cylinder I know of.

      Port timing is usually measured in piston travel from TDC. I would get a WKA rule book and use those specs.

      Ignition timing is mild, you could go to .085 no problem. Might help it run cooler.

      I would jack up the head clearance to .035″ for a trouble free practice engine.

      Gif

      FAA certified jet engine and aircraft technician,
      Nicholson Speedway class champion 2001,
      Yamaha KT100 Service Center,
      41 years karting experience

    • #50932
      Andres Malovos
      Participant

      Thanks Walt,

      That’s why I went with the Wiesco one.

      Image result for Wiseco Nylon Honing Brush

      I don’t yet have 2″ dial indicator and bridge to measure in piston travel “.
      I’ll likely get one at some stage.

      OK, so what impact will expanding the squish have in terms of longevity, combustion stability.

      I should also mention I have HPV 3/4 pipes, and am in California, so usually running our pump 91.

    • #50952
      Walt Gifford
      Participant

      Wiseco flex hone, says it’s safe for 2 strokes but you will need sand paper to put in the cross hatch. What does it do if it won’t even put in a cross hatch?

      If you want to fit pistons to cylinders you need something like an Ammco 3950 small cylinder hone.

      Gif

      FAA certified jet engine and aircraft technician,
      Nicholson Speedway class champion 2001,
      Yamaha KT100 Service Center,
      41 years karting experience

      • #50953
        Andres Malovos
        Participant

        Well, I wasn’t looking to hone to size, that I’d leave to the machine shop. Instead I was looking to de-glaze and prep for new rings in a more even way that a scotch bright job would provide, without the risk of damage of a dingle berry hone. Given that, it did it’s job.

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