2017 - 2-Stroke Minibike

[Ltdkc]

Full crank cylinder

 

[David Ham]

yeah that looks pretty much like another half crank I have, you should be able to make the pipe work for that. but there is a lot of porting work involved! and you'll probably break through in the end. but you can seal that in the end and it'll be worth it.

 

[Ltdkc]

Yeah I've done a bit of porting to other cylinders. I've opened up the height of the the exhaust port by about 5mms, I've widened it by about 4mm, I've deepened the transfers and a made them transfer into the crankcase, also the inlet is opened up to 24mm. The inlet on the crankcase reed valved engine is quite a bit bigger

 

[David Ham]

chain and sprocket

 

[David Ham]

Ok so the half crank is going back on the mower, now that I have a full crank thanks to Ltdkc. Its the same brand and still 160cc but its a lot better. I've also started on a stainless fuel tank. the weldings not great but it doesn't leak and thats the main thing!

 

[Ltdkc]

Hey mate, how are you going with that little motor

 

[David Ham]

so far I've engaged the exhaust port by about 5mm in height and 4mm in width(8mm altogether). Inlet has been widened out to about 21mm to fit a 22mm carby. I cut off the end of the exhaust port and shortened the inlet, I didn't want to but the exhaust needed to be 27.5mm in diameter; it was 25mm so it would be impossible to widen out such a long piece of pipe so I just cut it off. it also made it easier to machine. The inlet was 22mm wide on the outside so I just cut most of that off. also anticipating breaking through on the exhaust port I welded a bit above it. Some disgusting welds that you can see on the inside of the port. if they were good welds I shouldn't have noticed anything but theres some grey there so its probably dirt or slag lol. just cleaned it all up as well. I'm thinking I should probably raise the transfer ports a couple of mm?

also ordered new gaskets, rings, spark plug and boot

 

[Desertduler]

Hi, may I ask how you came up with the port timing for your engine?

 

[David Ham]

It wasn't actually me, I had someone design the pipe for me and he calculated a max of 8100 rpm for the original ports that were 9.5mm high, and a max of 2100 rpm for the exhaust port. along with a restrictive carby this kept the revs down for lawn mowing purposes.
to get much more power he advised me to open up the exhaust port to 20mm high, giving it a max of 6000rpm, along with an inlet duration of 120 degrees. on my new engine the transfer ports are about 8mm high so I'm not sure if I should take them up (its difficult to get the Dremel in there) or just leave them and hope they aren't too restrictive.

 

[Desertduler]

It wasn't actually me, I had someone design the pipe for me and he calculated a max of 8100 rpm for the original ports that were 9.5mm high, and a max of 2100 rpm for the exhaust port. along with a restrictive carby this kept the revs down for lawn mowing purposes.
to get much more power he advised me to open up the exhaust port to 20mm high, giving it a max of 6000rpm, along with an inlet duration of 120 degrees. on my new engine the transfer ports are about 8mm high so I'm not sure if I should take them up (its difficult to get the Dremel in there) or just leave them and hope they aren't too restrictive.

It would be interesting to know what the blow down period is now after raising the exhaust port as far as you have. The blow down period is measured in crankshaft degrees. The period is measured from the beginning of the exhaust port opening to the beginning of the transfer port opening. These measurements should be taken with a degree wheel attached to the crankshaft.

 

[Ltdkc]

You can also increase your inlet duration by trimming a few mm off the bottom of the piston skirt on the inlet side. But don't go too far because you'll get blowback through the carby, then you'll be in reed valve territory. You don't take out any material from the top of the inlet port either. How are you going to mount the carby and exhaust on it now that you've trimmed down the inlet and exhaust spigots? I've got an idea of how to get around it if you want.

 

[David Ham]

It would be interesting to know what the blow down period is now after raising the exhaust port as far as you have. The blow down period is measured in crankshaft degrees. The period is measured from the beginning of the exhaust port opening to the beginning of the transfer port opening. These measurements should be taken with a degree wheel attached to the crankshaft.

I can measure that... I'll measure everything else while I'm at it.

You can also increase your inlet duration by trimming a few mm off the bottom of the piston skirt on the inlet side. But don't go too far because you'll get blowback through the carby, then you'll be in reed valve territory. You don't take out any material from the top of the inlet port either. How are you going to mount the carby and exhaust on it now that you've trimmed down the inlet and exhaust spigots? I've got an idea of how to get around it if you want.

I dont know how I'll mount those yet, I'm sure I'll think of something. whats your idea?

 

[Ltdkc]

Victa

A possible way to mount both inlet and exhaust flanges

 

[Desertduler]

6.5 I admire that you are using a two cycle engine which is a rare engine choice for most people. You will learn more from what you are doing than just buying the same old cookie cutter engine and installing it. It takes more guts to be different than follow the rest of the herd and then you will have something unique when you are done. I too am a fan of two cycle engines and I have many variations of them.

 

[David Ham]

that idea had crossed my mind, to mount something off those bolts.

6.5 I admire that you are using a two cycle engine which is a rare engine choice for most people. You will learn more from what you are doing than just buying the same old cookie cutter engine and installing it. It takes more guts to be different than follow the rest of the herd and then you will have something unique when you are done. I too am a fan of two cycle engines and I have many variations of them.

Yeah its definitely a learning experience for me as I have never really modified any engine significantly. But I'm really liking it so far, I like how two strokes can be mounted in any position without the need to worry about a cumbersome oil sump. and they're a lot more compact

 

[Ebrownie]

that idea had crossed my mind, to mount something off those bolts.

Yeah its definitely a learning experience for me as I have never really modified any engine significantly. But I'm really liking it so far, I like how two strokes can be mounted in any position without the need to worry about a cumbersome oil sump. and they're a lot more compact

Plus its always a bonus that they smell and sound ten times cooler than any old industrial four stroke
And, as I like to say, mix gas, haul azz

 

[David Ham]

Plus its always a bonus that they smell and sound ten times cooler than any old industrial four stroke
And, as I like to say, mix gas, haul azz

you're not wrong

how do these look:
Full crank timing
Exhaust opens 98° ATDC, duration 164°
Transfer opens 129° ATDC, duration 102°
Blowdown period 34°
inlet opens 58° BTDC, duration 127°

 

[Ltdkc]

Victa

Awesome mate. Are those timings close to the timings that your expansion chamber is designed for? Also when you checked your port timings did you have a cylinder base gasket on or did you do the timings without one? Coz just thinking that the timings might be slightly out if you didn't have the gasket on. And also, after having put some heat into the cylinder when you welded the the top of the exhaust, you may need to check that the cylinder is still nice and round and isn't warped.

 

[David Ham]

Yep, I'll check again once the base gasket and everything is on. the pipe was designed for an exhaust duration of 162.5 degrees so its pretty close. I'll see what I can do about the transfer ports...

yeah the cylinder is all good I checked as I was welding
worst welding experience of my life though... the rods were probably decades old... I just found them lying around in the shed and they said 'cast iron' so

 

[Ltdkc]

Yeah good stuff mate. To enlarge and raise The transfer ports on my Victa I got a small ball end carbide burr and a drill and attacked them from the top of the cylinder. I also deepened them and ground them down so that they transfer into the crankcase. I'll pull the cylinder off of mine tomorrow and take some pics and show you. Also, if you can, try and make sure that on your inlet, exhaust and transfers, that you try and make things smoother for the gas flow side of things, coz Sharp edges make for average flow and turbulence. In my engines I've done this as best I can and I'll show you that too. I'm no porting professional but what I've done is far better than how it left the factory. Lol.

---------- Post added 04-22-2017 at 12:04 AM ---------- Previous post was 04-21-2017 at 11:55 PM ----------

Oh yeah. Also, before you do any port work inside the cylinder, do your best to protect the cylinder walls from an excitably jumpy Skippy burr. They can something jump around away from where your working and you can accidentally Nick the bore which is **** thing. So to help protect the bore in case of this happening, I clean the bore from any oil, then run 3 or more layers of that fibrous 100 mile an hour tape. Cheep insurance ay!