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Bbore porting question

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  • Bbore porting question

    Just got a big bore kit for my 47cc bike and just wanted to know how much could I port the exhaust as far as how many mm's could I raise the roof of the exhaust port and how wide could I go. Any input would be greatly appreciated.

  • #2
    umm, how long is a piece of string? The width should not exeed 65% of the bore for the brittle cast iron rings, if you had some nice chrome steel ones you could go as wide as 75% but not with our rings. If you have an old ring bend it a little, the brittle cast iron snaps so fricken easy it's scares me, the chrome ones can be tied in a knot and wont break.

    Ok so the height, have you degreed your ports? or done a McDizzy port map on them? do you know what time areas are? When i degreed my little 43cc engine i was mortified as to how the thing can even run with these port timings, i was even more shocked when i checked the squish gap, basicly these engines are so far from reaching their full potential they make me drool with ideas.

    Ok so you got the cylinder there and your ready to go right? I would suggest
    you don't raise it but only widen it to 65% of the bore width and be happy with that, go out and enjoy it. Meanwhile go to macdizzy's and read up on port mapping and ta's also learn about 2 stroke scavenging and how expansion chambers all that, buy, steal, borrow, some good books, Jennings (hard to find, kill for a copy) Bell, Irvings and Blair ect ect do all that and then you would know how high to port your exhaust.



    • #3
      dont raise the port to much 1 or 2 mm at most, it increases rpm but lowers compresion ratio if you have a cag with the 47cc reed engine you can raise 2mm, widen to about 70% of bore and get rid of the base gasket to bring the C/R back up


      • #4
        what are you running on your pb? is it a 40cc or a true 48+cc pb?

        i did the following on mine.. (bb kit, FCC) raised the roof about ~2mm, widened the exhaust port to 30mm, matched the floor to BDC of the piston. i am getting around 229 psi from that setup.

        hope that helps


        • #5
          Mega, with using that much psi have you found any probs with the woodruff key on the crank?


          • #6
            no i havent. the only time the key is on stress is when you start the bike. the intial torque from pulling the pull start puts a load on the key.

            as long as you are just past TDC, you can give it a good pull.

            what has broke on my pb was the starter paw. since it was plastic it was the first thing to go. i ended up getting a metal paw, and it solved that issue.

            hope that helps.


            • #7
              Thats what I was talking about. We ran the no base gasket option and burred over the keyway and hammered a seal in the process.


              • #8
                ah ic. i used the metal spacer provided with my bb kit. but since i have a full circle crank, i didn't want to stack gaskets bc of the inconsistancy of the seal and thickness. my squish is set at 0.3mm TDC giving me the 229 psi, (also with very little removal off the roof on the exhaust port)

                i achieved 0.3mm by sanding the metal spacer down to the optimum thickness. not a lot of ppl do that bc of the teadious work involved (fitting, measuring, removing, sanding... etc.. over 15 times..)


                • #9
                  Thank's for the info. I'm not running the full circle crank. that may have had play in what happened. That and I'm not too sure if the bearings were seated properly either as it wasn't my engine that had this malfunction.


                  • #10
                    Zameil, a few drops of loctite "super stud lock" will stick that sucker down no worries. If it keeps breaking keys it may be a poor fit between the two tapers, some bearing blue would show any high spots. As for rocket keys, bah, we should figure out a way to have fully adjustable timing, there's not a lot of metal in that area but i was looking at it some time ago and it can be done. Even better, a retrofit of a cdi with an ignition curve from something roughly the same cc. I want to make my dyno first and then i will start playing around with these things.

                    229 psi, love it


                    • #11
                      Timing problem fixed. Slot the magneto mount holes. A bit of trial and error and you should be fine.
                      Clockwise = advance .

                      The remains of my rocket key.


                      • #12
                        Originally posted by zameil
                        Timing problem fixed. Slot the magneto mount holes. A bit of trial and error and you should be fine.
                        Clockwise = advance .

                        The remains of my rocket key.
                        zameil, theres not a lot of room on the sides of the bolt holes to get much movement, problem is the further out the radius the more you have to move it to get the same degree, i was thinking of chopping the mounts down and screwing on an adjustable plate, will see when i get to it.

                        You guys know you can just file your own rocket keys right? just get the right diameter rod, hacksaw and a file and make it any degree you want. a timing light and a degree wheel could check your work no problemo. If you made it from 4140 it would last for ever.

                        btw, you guys ever put a digital speedo on your bike without a resistor plug in it, whaaa i started it up and the speedo went mad, put in a NGk-BMR7A and now it's sweet.
                        43cc bog stock, but a bit lean i got 47kph with my 6'2" of 129kg not so lean arse on it, get in ma belly