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72 AT-1 engine conversion to CT-2/3

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  1. AnnabannanX

    (multiple images make me divide it up in several posts...)
    Hello all
    Some time ago I bought a engine out of a 1972 DT125 (I believe). And the exhaust.
    Bought It from the US so It was quite price'y to get it here.
    Well I wanted a project. So I got it.
    I knew It could be a lot of work since it was in a unknown condition.
    When I got It, the first thing I noticed was that it was stuck. Could not turn it on the kick pedal.
    So I took the head off. The piston was almost at the top. Gave the piston a couple of medium hits to try to loosen it. And it worked.
    That was when I got to see the damage to the cylinder.

    So either I had to get it bored out, or get another cylinder.
    And the engine was filled with a gray mix of oil and water...

    Posted this on another Yamaha forum and got some info.
    Like I did not know back then what engine I had. Got informed that the cylinder and head from and CT-2 or 3 would fit.
    I know now that the AT-1 and CT-1 is piston port 2 strokes.
    And that the CT-2 and 3 are reed valve engines. A huge plus. And they are also 171cc's

    Also the electrical on the engine was a goner. Water + time = usaly rust...
    And I believe it was 6 volt...

    So I got my hands on a 1973 CT-2/3 cylinder, head and carb (reed and intake ofc).

  2. AnnabannanX

    Here is it cleaned and painted

    Well this cylinder also needs to be bored out. Not as much as the original one tho.
    And its a 171cc so it is worth more of the work

  3. AnnabannanX

    Like I said the electrics was shot...
    So I had some spare electric/ignition parts for my DT50R

    Had to modify it a bit

  4. AnnabannanX

    And "make" a new woodruff key on the crank

    Have ordered a new small end bearing, gaskets and some other stuff.

    Was thinking about modding the reed cage, but found out that the one of my Origami C1 Pocketbike could fit (with modifacations).
    It even has a "reed stuffer" and bigger openings.
    Gonna cut out the center bar on both sides and install Polini carbon reeds.

    Anything I can do to get a bit more low end power?

    Also got a tip about cutting the spark arrester off the baffle.
    Would that increase the sound volume much? And what can I do to make it more quiet while not loosing power?

    Was also thinking about having the crank rebuilt. And maby make some crank stuffers for it.

    Any constructive input is appreciated


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