Trying to make a CHinese 359 top end run

[David B]

So I bought a rebuilt 2159 aka 359. Top end is a $40 Ahlborn China kit. Didn't have any power, only 100psi compression. I got a new 357 top end for it but in the meantime I decided to play around with it at work (machine shop) and see if I could get it to run decent. Squish was .050" so I took .028" off the base and got it down to .022". Of course that lowered the ports and turned it into a low RPM torquey tractor. I don't have much frame of reference for a 60cc saw but it's defintely better, plug looks good 4 stroking at 12,500RPM. I want to raise the powerband. Initial numbers with .018" gasket after taking .028" off the base:

(47mm bore)

Int : 82* = 156* duration. Don't really want to try to raise it. Widened to 28mm from ~22mm

Exh : 104 - raised .020" to 102 for 164 duration, thinking I need to raise more for more RPM and even out duration ratio to intake - 100? Widened Exh to 30.5mm = 65% of bore - could go 33mm.

Xfer : 120 - think they need raised and widened, but don't have tiny tooling to get in them. Might get some.

Blowdown is 18* get - closer to stock numbers by raising exhaust?


A factory cylinder with no base gasket:

103 exst - 154° duration
79 intake - 158° duration
Blowdown - 20°

Figured out since I don't have removable xfer covers, I could blow the lower transfer ports out to gasket size like the 357, they are wide open now. Also knife edged the leading edge of the port divider. Die grinders I have are a big too big for this, bull in a china cabinet. Rough, but quick.


Compression is now 150psi below sea level. Little disappointed in that. Cheap ring methinks. Found a Caber for $8, on the way.

Swiss cheesed the muffler baffle and made it dual port exh.

Any and all input appreciated!

[mike_belben]

I dont mean this as a diss on the FF.   the place for you to find the info youre after in abundance is the hotsaw forum of arboristsite.  There will be plenty of threads discussing timing, and its a rabbit hole for sure but youll like it.   Read things written by BLsnelling.


What i can add to the things youll find there,  is that the power isnt just in the top end, its also in the crankcase.  Two strokes have a compression ratio above the piston and below.  The transfer ports arent the only conduit that needs smoothing and volume reduction, so does the case.  Aluminum tig rod and devcon A2 are your friend. And no i dont mean weld the case,  I mean anchor the epoxy with it.  I have welded a few pistons with great results. 

[mike_belben]

Btw, never knife edge any inlet.  Does a rain drop have a knife edge? Does a wing or helicopter blade?  No.  Edges shear air and make turbulent wakes where we really want laminar flow.  We want uniform, calm boundary layer adhesion free from collisions.  Look at interstate layout.  Nice round broad sweeping on and off ramps have the least accidents.  Abrupt changes to traffic flow go against the inertia stored in the vehicle.  Same as the inertia in a fuel/air molecule.  We dont want to smash them into guardrails, that makes a pileup.  

Half sphears and trumpets, study those shapes.  Ideal inlet radius is .5 x diameter.  So a 2inch tube should have a bellmouth with a 1" radius arc swung from the edge of the tube.  Cant always fit it in practice, but as blunt and round as possible wins for air entrances.  Sharp is fine for air exits.

[David B]

Mike, your input is much appreciated! I will relocate my post on the other forum and knock off my knife edges. :-)

At this point, I'm planning to machine to squish band a bit because there is a little ring around the outside .010" tall x about 1/8" wide which is limiting my "true" squish. I want to get rid of it and get the "roof" flat to increase compression. Then machine the base a hair more to get squish to .018"-.020". Then I will have to go ahead and raise intake. I'm seeing a trend while researching of recommendations to not go over 80* on the intake port. Probably 120-122* upper transfers and widen a little. Going to have to measure and map it all out, I may be getting close to freeporting with the exhuast port at that point.

[mike_belben]

For removing that band in top of jug i make a carbide insert holder out of a wooden towel and do it by hand.  If you have a lathe turn an OS dowel down until its a nice fit.  Then just cut a slot and hammer in an insert.  Stuff it up there and twist, itll peel right out.  Make sure you dont score the chrome. 


I almost always delete base gasket and use 3bond for base seal.  Just make sure it has sat for a legit night before yanking starter.   Its been too long for me to quote timing off hand.  If youve got torque to spare and struggle for rpm, bring the exhaust and transfers up a hair at a time.  But be warned a peaky saw is no fun to buck big rounds with.  Thats more a limbing powerband.