Stuffed HT case impression

Discussion in 'Performance Mods' started by Steve Best, Nov 19, 2016.

  1. Steve Best

    Steve Best Well-Known Member

    So this is going to be an impression, not an accurate test because I stuffed these a month or more ago and got busy. Didn't write down any data about this engine. I'll never make it as a scientist.

    It is my old black unknown 66cc Happy Time China Girl. My first engine.
    I bought the entire bike and engine for a song but it worked well enough to get me hooked.
    I cannot remember what the top speed was but I improved it a bit with a head mod, new muffler and cleaning the flashing out of the transfer ports. No porting, no timing changes, maybe a little more compression.

    I stuffed the bearing retainers and spread 1-2mm of JB weld over the entire sides of the cases much like this:
    I also filled in the through under the transfer ports where it went below the crank cheeks, as shown in solid red. I drilled an oil flow port to the bearings on each side, because I could see the oil had been baking in there. No pictures I am afraid, lost my darned camera again... (old guy, losing it!) New seals and bearings while I was in there.

    So not a huge gain. Nothing in top speed, in fact I think I might have lost a bit. I showed 45kph and I am sure I broke 50 kph before this mod, but cannot be sure. Where it gained is rock steady idle and lots of low end torque. It would now do the steep hillclimb with only a bit of assistance now (this is not the modified Grubee, which will climb with no assistance). Previously it would not climb the hill even if assisted. Throttle response is crisper. Powerband is much lower rpm than it had been originally. I'm surprised.


  2. Street Ryderz

    Street Ryderz Well-Known Member

    great info! did you have any luck fitting the minireli (sorry cant spell) jug? I am curious because i have tried using hocca's 47mm slug and it works but I ran a flex hone through the cylinder to let the new rings seat properly and then it wore through the chrome in no time. so if their cylinder could also be used that would be awesome I cant afford to replate jugs.
  3. Street Ryderz

    Street Ryderz Well-Known Member

  4. jaguar

    jaguar Well-Known Member

    that is weird because usually increasing crank pressure improves power at a higher RPM range.
  5. Street Ryderz

    Street Ryderz Well-Known Member

  6. Street Ryderz

    Street Ryderz Well-Known Member

    makes sence to me now it moves the intake flow faster more efficiently but less of it.
  7. Steve Best

    Steve Best Well-Known Member

    I know, I was a bit surprised too, but then I thought back to good old Gord Jennings, and read up a little on page 95-96:

    I think SR has it pegged:
    While it does raise the tuned rpm (which is a resonant effect like more water in a bottle you blow a tune over) it loses overall capacity for resonant storage. The note goes up but the decibel volume goes down as you decrease case volume. "for reductions in crankcase compression ratio cause an equal reduction in the amplitude of wave activity in the intake tract, which in turn reduces the ramming pressures available to charge the crankcase."

    Old Gord is da'bomb isn't he? 40 years ago he had so much of it figured out. Just amazing.

    This worked for me years ago with an old CR250, and with my son's Yamaha Blaster, but I'm guessing it just has to do with how close to that ideal 1.5:1 compression ratio you are, and really, I over did it. I really stuffed a lot of epoxy to this baby. Man, it starts and idles WONDERFULLY. Rock steady. Much more power in the mid-range. Snappy crisp throttle. No rev.

    So how am I gonna make power out of this thing now?
    There is $22 of epoxy in this baby!

    I say I carry on with port mods that I know have worked on previous cylinders?
    This cylinder ports start out much worse than the Grubee cylinders. It is 30mm intake studs and tiny port.
    I'm thinking I try to optimize the intake and exhaust windows, see what that does, and maybe match transfer ports to case? After all, it will only increase case volume now...

    Still tinkering with the Minarelli, waiting for wristpin bearings and a pipe to arrive.
  8. gary55

    gary55 Well-Known Member

    Do the transfer ports on the minarelli match up to seal on the chinese case? Can't wait to see the outcome on this one.
  9. Street Ryderz

    Street Ryderz Well-Known Member

    maybe a slight increase of intake duration would help.
  10. Steve Best

    Steve Best Well-Known Member

    No they don't. Minarelli is much larger square twin transfers compared to the HT small triangle.
    The Minarelli also has a boost port on the intake side. I made a 1/4" aluminum spacer to adapt them.
    I am kinda excited to try them before the snow comes. I really need a pipe to make it work.
    Engine has been mocked up for a while now, ready to go. Waiting...

    Good point, how to do it so I can back up if it doesn't work? Cut piston skirt? I have spares.
    Last edited: Nov 26, 2016
  11. Street Ryderz

    Street Ryderz Well-Known Member

    That is what i would do yes.
  12. Street Ryderz

    Street Ryderz Well-Known Member

    sounds promising so you made them match with the adapter plate?
  13. gary55

    gary55 Well-Known Member

    So the 1/4" adapter was also ness. for correcting deck hight as well as sealing transfers?
  14. Steve Best

    Steve Best Well-Known Member

    Yes, I needed another 7mm or so height so the plate gives that to me and allows adaption between the 2 ports.
    This picture is the adapter/spacer plate about to be dremeled.
    It was drilled to rough out metal to minimize dremel work needed.
    No finished pictures, I lost my camera again.
    This 6.25mm plate is not high enough yet, it will take stacked gaskets too.
    Where is that darned pipe and wrist-pin Mr Mailman?

  15. gary55

    gary55 Well-Known Member

    I don't mean to bug you with a bunch of stuff you have probably already covered. Your minirelli project is one of those things that once it's in my head it just keeps popping up with new questions. Really cool. Do you remember which thread you discussed this project in? I already looked at all the media on it, And would like to see what you were saying along with the pics..
  16. butre

    butre Well-Known Member

    don't know where you got that idea, if so then the secret to power would just be to cram as much case pressure as you can in there. we all know how much of a balancing act case compression is
  17. Steve Best

    Steve Best Well-Known Member

    For you Gary I put comments on the pictures. Feel free to post questions to them.
    Also do not miss the thread by konix shiwa on this:

    He got me started on this, as well as Four-Squared in the other forum with his Puch hybrid.

  18. LRSimons

    LRSimons Member

    Regarding crankcase pressure, I found the same thing that Steve did. Maaaaybe some more top end, but embarrassingly little torque. I had to pedal up some hills after I did that lol. Keep in mind this is a 48cc with a 36T out back though.

    The minarelli build is awesome, hope it reaches completion. Won't have to deal with chrome crap coming off then...
  19. Street Ryderz

    Street Ryderz Well-Known Member

    Hey Steve is that a starfire gen2 48 in the pics above?
    Last edited: Nov 26, 2016
  20. Street Ryderz

    Street Ryderz Well-Known Member

    looks like it! Here is the one i have.notice the logo on the back of clutch gear case and there is no cable lug or access to the clutch spring adjustment.also best of all it takes 40mm od bearings.

    Attached Files: