48cc porting advice

[jaguar]

hi
I just need a quick opinion about porting a 48cc piston port engine. It comes from China and mounts in a bicycle . :ride: From reading Eric Gorr's recommendations for mid-range porting I would guess this low-range engine (6000rpm max) needs 96* ATDC exhaust, 120* ATDC transfers, 75% head squish, 135psi. Is that about right?
But I have no earthly idea whereabouts the intake porting should be. Can you give me a clue? Maybe it should be somewhat close to how the Hodaka 100's were ported. :think:

 

[helio lucas]

what will be the use of this engine? why you want to port it? some more specs will be very usefull...
most of the times porting is not the issue with cheap industrial china engines but really poor carb and exhaust.

don´t use those specs, they are not made for that kind on engine!!!! a 96atdc coupled with a simple box silencer will just kill the engine. not enough torque even to pull the flywheel wheight :laugh:

just to give you just one exemple i have a 30 years old casal boss 50cc, two speed, air cooled. a friend gave me the bike if i made it faster, that was the deal. the bike only runs 60 kph stock and was a good engine... after some porting, modded exhaust internals, modded air box and filter, ignition etc... etc... the bike goes at 90 kph.
great, unless we always start the bike on a way down because the of idle torque wasn´t good and we have to fan the clutch too much even if the bike only weighs some 50kg. after a big sprocket change the power was very usefull but the bike only does 75kph. funny, the bike can pull his top speed on a straight, way up or down.

my father "stole" me the small bike and still ride it every day on some very slow trails on summer and on roads in winter some 2km trips.

to resume, the porting is VERY dependent to the gearbox, bike and the use one give it. :cool:

 

[helio lucas]

http://www.dirtrider.net/forums3/showthread.php?t=180270

 

[jaguar]

Helio, the bike will be used in town mostly and I want low end power because I am going to live in a house up on the hill and don't want to push it up there each time. Normally these engines only rev to 5000rpm. I'll have my hands on it Monday and will map the stock port timing. People commonly raise the exhaust a bit and lower the intake a bit. I have porting experience but only on engines with reed valves. I once rode a friends flat-track Hodaka 100 that was ported for high rpm power. It was fun to say the least! It was a piston port engine.
I have no earthly idea what timing the intake should have for 5000 rpm peak power.
Anyone have a clue?

 

[helio lucas]

the book that rich advise on the link i gave you is very good as a starter. it will enlight you on the very subject of intake porting and timings. it provides a table for suggested timings based on rpm target. is very cheap and will give hours or reading and lots of experiments to do... :cool:
my personal experience with piston ported inlet engines is that they affect a LOT the carburation!!! open a full millimeter on that engine will render it useless. will knock away the bottom and midrange completely to give just a bit more peak (maybe) and more over rev and even may not idle at all!

i´ve seen your link now, is that engine the one with one piece cylinder head/cylinder?
normally those engines are very poorly manufactured and a lot of hp is hidden only correcting the engines. check the air filter system and muffler as well.
if you don´t have a great insight to carb tuning (jetting) don´t touch the engine because most of the mods require jetting. and most of the times the carb don´t allow any tuning.
cut added tranfer ports are usually possible and result well.

i deduce that this is a project to learn other wise leave it alone. GET THE BOOK!!!!

 

[whenfoxforks-ruled]

You 2 know your on the same continent? Probably 10,000 miles apart though, or not? Helio, he likely has ALL the books! Vintage Bob

 

[jaguar]

I've been tinkering and porting and reading and think I finally got it figured out. All my knowledge is now dumped at http://www.dragonfly75.com/motorbike/
You are right about the carb. I have ordered a Dellorto SHA to replace it. I am experimenting with a homemade reed valve now which even with the ****ty carb can climb streets better. And I will replace the cheap China CDI and coil to try to get a boost in the ignition system although the magneto is a bit on the weak side of voltage generation. Thanks for the attempted help!