Ported 272 cylinder

[motosierra]

Finally got up the courage to enlarge the exhaust port on my 272. Removed about 3/8” from both horizontal ends of the port on the cylinder side. Installed a new piston /ring….Initially she had good compression and was running with lots of torque. Then I had a problem with the flywheel and she would not start. Exchanged the flywheel and now she is running but with reduced compression and a rattle in the engine. I pulled the jug and the piston was showing scuffing on both (front and back) aprons. Anyone have any idea what could be the cause of this?...thanks

 

[Lawn Masters]

Did you increase the fuel input when you modifed the ports? when you do anything that will allow the saw to breathe better, you have to add more fuel to allow it to run rich enough not to sieze up.

 

[MacDaddy]

You might have made the ports to big and the piston skirt will wear realy fast if that happened Look in the port with the piston in the jug and see if the port is wider than the skirt

 

[timberwolf]

Widing the port 3/8 of an inch on each side sound like a whole bunch too much. How do you now attach the muffler? Or did you just open op the exaust hole in the muffler 3/8 of an inch?

As stated above force on the skirt must be high, likely getting short circiuting between Xfers and exaust ports and reduced scavenging, likley going to need more fuel.

Also if you dropped the exaust port any amount you could be hooking the skirt on the bottem of the port depending on port shape and chanfering.

 

[motosierra]

Did you increase the fuel input when you modifed the ports? when you do anything that will allow the saw to breathe better, you have to add more fuel to allow it to run rich enough not to sieze up.

thank you and yes the saw required much more fuel on the high adjustment to run smoothly and I adjusted the carburation on the rich side.

 

[motosierra]

You might have made the ports to big and the piston skirt will wear realy fast if that happened Look in the port with the piston in the jug and see if the port is wider than the skirt

The port is not wider than the skirt....I may have exaggerated when I said 3/8" on each end...and thank you for your reply

 

[motosierra]

Widing the port 3/8 of an inch on each side sound like a whole bunch too much. How do you now attach the muffler? Or did you just open op the exaust hole in the muffler 3/8 of an inch?

As stated above force on the skirt must be high, likely getting short circiuting between Xfers and exaust ports and reduced scavenging, likley going to need more fuel.

Also if you dropped the exaust port any amount you could be hooking the skirt on the bottem of the port depending on port shape and chanfering.

The port was only opened on the cylinder side and tapered back midway to the muffler end. Muffler end was left intact. There was no material removed from the top or bottom of the port so I do not believe the timing was affected.... the saw ran super..great for a while.....until the woodruff key in the flywheel(key was cast with the flywheel) wore away and the saw would not start...... I do not believe the saw had the correct flywheel .... I replaced it with a flywheel with a separate key (not cast with the flywheel)

 

[HiOctane]

The port was only opened on the cylinder side and tapered back midway to the muffler end. Muffler end was left intact. There was no material removed from the top or bottom of the port so I do not believe the timing was affected.... the saw ran super..great for a while.....until the woodruff key in the flywheel(key was cast with the flywheel) wore away and the saw would not start...... I do not believe the saw had the correct flywheel .... I replaced it with a flywheel with a separate key (not cast with the flywheel)

You must use a flywheel with the key cast in,because if you use a flywheel from,lets say from a 266 ,162,630,they have a separate key and probably different timing.Dont forget also if you use a separate key,the crank on your 272 is not built to receive a separate key,the keyway is kind roundish each side opposed to square keryway on 162,266,630.

 

[ErrolC]

Does the scuffing appear more prominent near the top of the piston..or the bottom..when you're opening ports up like this its a good idea to put a small radius around the lip of the port to help prevent the piston skirt or ring from lipping (catching the port edge)..also, in plan profile, the top of the port chamber (not the port at the liner wall) needs to be given more volume to assist quicker blow down to, in a sense, give the port shape a bit like an onion so appears bulging out (wider in the port tunnel) near the liner so the port wall exits the liner at a right angle to it..the port floor plan profile shape near the cylinder can stay the more conventional shape as the flow characteristics there, is quite different once the port is fully open..by widening the port window you increase the risk of the ring saging or springing into the port itself (thats why some engines have bridged ex ports)..radiising (say .0025" rad depending on the bore dia) or blending the the cylinder into the port (on the cylinder side) helps the ring slide back onto the liner and helps prevent the plating from flaking off...theres a whole lot more to it but the idea is to not make the window any bigger than it needs to be, to maximise the cylinder wall area for the ring and piston to bear their loads..just a little bit at a time..these small engines are easy to over do..a 5% cutout (in area) could give you a 10% gain..it could also give you an engine with horrible characteristics, or dead..in some modern 2strokes its simple to get 20% or more increase in ex flow without altering the port window at all just by modifying the port tunnel in critical areas and doing nothing else..remember though that changing the ex port will change the behaviour of the transfer ports and likely make the engine run hotter..good luck

 

[motosierra]

Does the scuffing appear more prominent near the top of the piston..or the bottom..when you're opening ports up like this its a good idea to put a small radius around the lip of the port to help prevent the piston skirt or ring from lipping (catching the port edge)..also, in plan profile, the top of the port chamber (not the port at the liner wall) needs to be given more volume to assist quicker blow down to, in a sense, give the port shape a bit like an onion so appears bulging out (wider in the port tunnel) near the liner so the port wall exits the liner at a right angle to it..the port floor plan profile shape near the cylinder can stay the more conventional shape as the flow characteristics there, is quite different once the port is fully open..by widening the port window you increase the risk of the ring saging or springing into the port itself (thats why some engines have bridged ex ports)..radiising (say .0025" rad depending on the bore dia) or blending the the cylinder into the port (on the cylinder side) helps the ring slide back onto the liner and helps prevent the plating from flaking off...theres a whole lot more to it but the idea is to not make the window any bigger than it needs to be, to maximise the cylinder wall area for the ring and piston to bear their loads..just a little bit at a time..these small engines are easy to over do..a 5% cutout (in area) could give you a 10% gain..it could also give you an engine with horrible characteristics, or dead..in some modern 2strokes its simple to get 20% or more increase in ex flow without altering the port window at all just by modifying the port tunnel in critical areas and doing nothing else..remember though that changing the ex port will change the behaviour of the transfer ports and likely make the engine run hotter..good luck

Thank you for the great information....I have not had a chance to pull the cylinder and check the location of scuffing...I thought it was throughout the apron faces but will double check soon. At that time I will put a small radius around the lip of the port.

How much material are you talking about removing from the exhaust channel at 90 degrees from the lip of the port (cylinder side)?....and do you know where I can learn more about this?.....some people recommend raising the top of the exhaust in this area(or dcking the piston) to facilitate the escape of hot exhaust gases. Are you familiar with this?

Do you think that a worn cylinder or a piston to small for the cylinder could be part of the problem?...thanks again for the great info...