2 stroke stuff

well.....
I wrote this whole thing last night & somehow dident post it & it got lost so here goes again
the illustration Ive seen of a 2 stroke isent exactly like ours ours have an intake port & transfer ports so the bottom & top of the piston pressures calcel out each other to some extent I believe limiting the usefulness of a reed valve.

corection
the top of the piston never clears the top of the intake port at BDC so basically it works like a 2 stroke with the carb on the crankcase I just think maybe there is a little more volume so the pulses in the crankcase are mellower

when the piston is at TDC the intake port is only 2/3 open while starting downward even after porting,
on the way down first the exhaust port opens allowing some exhaust to blow out & then the transfer ports open. with the pressure in the crankcase increasing because of the downward motion of the piston the transfer ports help push out the exhaust, then finally the intake port opens & scavenging pulls in the rest of the charge,
then with the piston moing back up when the piston skirt opens the intake port the suction from the upward motin sucks more air into the crankcase again
so raising the roof on the exhaust port shortens the power stroke
lowering the floor of the intake port shortens the time that the presure the transfer ports push out the exhaust with

I need to bone up on 2 strokes because I think there is something we or at least I am missing about porting on these engines
any light anyone can shed on this would be greatly appreciated
have a great weekend
JAK
 
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a couple of pics of a culinder Im working on

you cant see the exhaust side that well
I opened up both ports pretty well they are both larger at the cylinder side of the port then they were at the manifold side before... does that make sense?

I widened them considerably as well
take care
JAK
 

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skipu

Member
Aug 6, 2008
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Re: a couple of pics of a culinder Im working on

have you run that motor yet ? looks like its going to be better in higher rpms to me ! i do alot of cr kx and yz 250 cc engines port jobs are all different depends on what your trying to get out of it to big can actually hurt performance considerably you lose velocity thus slowing the air fuel charge down you start losing the atomizing effect which in turn creates insufficient burn and can be really hard to tune im working on three of these motors now in my spare time all three are different port jobs the reed setup im working on is at a hault untill wed. power converter on my mill smoked but ill let you know my results with all three after i test them and try to help ps let me know how that motor runs and throw the carbs that come with those kits away you cant ride wheelies with them they fall on there face PEACE !!!
 

Goat Herder

Gutter Rider
Apr 28, 2008
6,248
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If the piston skirt is blocking the intake port any at top dead center trace the area blocking the the intake side. Then pull the jug off shave just that off the skirt, clean up any burrs reassemble.Some of these ch put the intake lower clearing the piston skirt, and they run better.
 
Re: a couple of pics of a culinder Im working on

have you run that motor yet ? looks like its going to be better in higher rpms to me ! i do alot of cr kx and yz 250 cc engines port jobs are all different depends on what your trying to get out of it to big can actually hurt performance considerably you lose velocity thus slowing the air fuel charge down you start losing the atomizing effect which in turn creates insufficient burn and can be really hard to tune im working on three of these motors now in my spare time all three are different port jobs the reed setup im working on is at a hault untill wed. power converter on my mill smoked but ill let you know my results with all three after i test them and try to help ps let me know how that motor runs and throw the carbs that come with those kits away you cant ride wheelies with them they fall on there face PEACE !!!
yeah I realize that too big isent good
the cylinder on my bike now is a tad less agressive & definatly dosent have the torque of the stock porting, but the mid range is much better
& because of the carb pretty much has no top end either but I was able to get one of these to spinn pretty well & the engine pretty much fell apart
I think they need to be sorta low to mid range engines
Ive seen some stock scooter cylinders with HUGE stock ports I guess on european scooters with a torque converter so they can keep it up on the pipe
I have allot of expierence with 4 stroke performance but this is a whole new thing
I hope your weekend is good
JAK
 

Saddletramp1200

Custom MB Buiilder
May 7, 2008
1,439
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Houston, Texas
The smoother the surface, the faster fuel and air come together. Your carb provides the fuel, the intake when polished works much better. Getting rid of spent fuel is not an issue. Expansion pipe is a factor tho.
 
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skipu

Member
Aug 6, 2008
184
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New jersey
in a flow chamber this isnt true as polished will tend to create droplets at low rpm thus losing atomization on our tests with dirt bike simulated vaccuum pressures causes fuel to dew up. but at high speeds when vaccuum is greater its not as bad try a 100 grit roll of is a perfect finish for these low vacc motors it keeps the fuel slightly turbulant so you dont lose that perfect mist ive had excellent results with this method to polish the exhaust is good evacuation is key and less tendencie to carbon build up i have also raised the tranfer ports a bit with good results but dont get greedy hope im helping here peace !!!!!!!
 
hey :)
I wanted to ask
with the stuff I have on my engine so far it does seem to have more of a power band in the upper mid range & a tad flat on the bottom where before it was Ok on the bottom & then sorta petered out so I like that better because it pulls hills at higher speed

I noticed in the video of the suspension bike build that those things really have a power band like a turbo kicking in
what makes the engines so different
I know ours are more designed for a broad low RPM power curve
I guess its all that tuning with the EGT temp, swept volume ETC that need to really be done to get it to work where you want it too
mine is getting better & better
I actually had to re ring the one in my bike because I over revved it a bit during break in
honing took the scuffing out & with the cylinder a little looser it revs more freely & seems to run better every day as the rings seat
the porting on the cylinder on the bike is about the same as the one in the pic
you all have a great weekend
JAK
 

Prasinos

New Member
Dec 1, 2008
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Another thing about the polished surfaces. Many of the surfaces on these motors are very rough for a reason. With a certain amount of texture on the intake and exhaust there is actually less friction with the air, it works much like the surface of a golf ball. Many of the surfaces on your engine are supposed to be rough so just know what you are getting into before grinding away.