Will a 48mm piston work in a 49mm steel sleeve cylinder?

CuriousJojo100

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So I think I ruined my 49cc cylinder. I shaved the piston skirt on the intake side and shaved the top of the piston on the exhaust side to create a ramps. I’ve already port matched the steel 49cc cylinder. It’s now very hard to start and doesn’t make much power. It does sound like it’s running very lean now but the RPMs don’t take a while to drop when I let off the throttle like I would expect. It barely vibrates anymore due to the piston being lighter though. Will I have an issue with blow-by from using a high hole 48mm piston?
 

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So I figured out why it was very hard to start and didn’t make much power when it did. I trimmed the edge of the top of the piston because I notice the entire skirt was scuffed. I sanded down the side of the piston until I could barely see the scuff marks. The problem is the fact that the iron sleeved 49mm cylinder’s intake port was casted a little high. All I did was remove the lip from the top of the inside of the intake port. Both rings were visible at bdc so sanding the lip on the top of the piston made intake port slightly visible from the cylinder at bdc. I am using a 2 part putty to line the ports so I can make corrections. I also noticed that the transfers are not the same distance from the exhaust port so my air/fuel mixture was going almost straight out the exhaust on one side. I’m using this cylinder as a test unit so I can make the necessary corrections when the new jug comes in. I don’t think I’m gonna ramp the piston in any way and i don’t think I’m gonna raise the exhaust port on the new jug. I’m running a 28 tooth so I don’t need the motor to rev any higher. I just need usable power up to about 6500 rpm. I really don’t want to be spinning any faster because I want it to last. Lastly, I’m going from a Delorto clone back to the bofeng. I went and ordered a size 68 jet in case I can’t dial in with stock jetting. I will post results when it all comes in
 

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Do you know exactly how to port one of these engines? there is a lot that goes into port timing and being off even just a tiny bit is enough to ruin an engine- been there, done that with nitro model engines.

Sloping the piston head effectively killed your compression and directed combustion gasses where to blow-by.

If you don't want to go past 6500rpm, there is no reason to port it because you are only going to see a power improvement past stock RPM, just clean the ports.

Also unless you are a very lightweight rider, 28t is probably lugging your engine and wearing your clutch, typically you don't want to go below 36t, maybe 34 if you are lightweight.
 
I’ve never used a degree wheel before. I never understood the concept until very recently. I’m sure I’ll be A LOT better after I get and use one. I have a good understanding of motors in general 2 and 4 stroke. I’ve done mechanic work since I was a teen and I was a mechanic in the military for 9 years. I don’t want anyone thinking that I’m saying any of this to say that I automatically know these happy time motors. That part is me saying I am a visual thinker and I understand the internal workings of motors. I will say though that this has been my hobby/passion for 20 years now. I can’t leave well enough alone so I’ve tinkered and porter these motors before I even knew there were forums. I’ve messed up enough to have a good idea of what not to do. I have done exhaust ramps a few times in the past and every time I noticed a noticeably harder pull from starting from lower-mid range up to near 5500rpms. This time I know the ramp is too big. I’ll admit I had tunnel vision going when I did it this time. I took the jug apart yesterday and realized my exhaust port was A LOT bigger than the intake. I have no idea how I missed that but that would explain the really high torque but fall flat on its face in the upper mid-range rpm’s. I’m not sure if that answers your question but I’m here to learn.
 
I’ve never used a degree wheel before. I never understood the concept until very recently. I’m sure I’ll be A LOT better after I get and use one. I have a good understanding of motors in general 2 and 4 stroke. I’ve done mechanic work since I was a teen and I was a mechanic in the military for 9 years. I don’t want anyone thinking that I’m saying any of this to say that I automatically know these happy time motors. That part is me saying I am a visual thinker and I understand the internal workings of motors. I will say though that this has been my hobby/passion for 20 years now. I can’t leave well enough alone so I’ve tinkered and porter these motors before I even knew there were forums. I’ve messed up enough to have a good idea of what not to do. I have done exhaust ramps a few times in the past and every time I noticed a noticeably harder pull from starting from lower-mid range up to near 5500rpms. This time I know the ramp is too big. I’ll admit I had tunnel vision going when I did it this time. I took the jug apart yesterday and realized my exhaust port was A LOT bigger than the intake. I have no idea how I missed that but that would explain the really high torque but fall flat on its face in the upper mid-range rpm’s. I’m not sure if that answers your question but I’m here to learn.
I'm a mechanic too, and thanks for your service!

Like I mentioned, I've ruined a lot of stuff pushing the envelope, which is totally fine as long as we learn from our mistakes.

There are certain people who know the black arts of carbs, 2 strokes and more with no formal training and likely not the correct tools, but they are a dying breed, and I'm sure they have their fair share of mistakes.

I'm not going to pretend I know exactly what happened (aside from compression loss) but I would gather the engine is breathing easy hence the torque, but it's not scavenging the way it should, hence the face-plant at high revs.

Some of that MIGHT be fixed with a jet/needle adjustment or maybe just replacing the piston, or the jug us fubar.

I know a trick to lighten a piston is to keep the skirt length the same but make it skinny.

Again, I don't want to come off as a duche know-it-all, I was just responding as if this was your first forray into engines, so I apologize if I upset you.

There are quite a few that want to start the world on fire, but don't know which way to hold a screwdriver!
 
No sir you didn’t upset me at all. I was simply trying to be as transparent as possible. Like I said, this has always been my hobby that I’ve been passionate about since before I started building. I stared rebuilding bicycles from the frame on up by the time I was 11. I’d take the 20” bicycles and swap the front forks for one with suspension from a 26” mountain bike. I’d still use the 20” rim for weight reasons. I’d do this so I got enough high to use mountain bike pedals and sprockets. The added pedal stroke helped me apply more torque to the shekel. I always wanted to go fast then I started juggling the idea of having a motor on a bicycle. We were poor and the internet wasn’t what it is now so I had no idea that this already existed until I got on eBay for the first time and the first thing I did was type in “bicycle motor”. I’ve been living the dream ever since. Point is, I’m very passionate about learning. This is the last motor I will have ported using guess work. I normally just clean the ports and ride like that for a few months. After that I raise the exhaust roof by roughly 1 to 2mm, Lower the intake floor by roughly 1mm. I widen each port by about 2mm. I usually never touch the transfers and i always use one base gasket. That has seemed to work for me because the results have been consistent using that method. This time I shaved some metal at an angle on the top side of the transfers from the back of the cylinder wall while making sure the transfers were the same shape no matter which angle I looked at them from. The goal was the direct the intake charge toward the top of the cylinder instead of one of them losing a good bit of charge to the exhaust…especially after widening that exhaust port. I ran the BoFeng carburetor with this setup with stock jetting and a f2 “expansion chamber”. I weigh about 185 pounds and this is on a beach cruiser frame. Besides having to pedal a little faster to get it going, I would barely have to apply throttle and she would pull with ease. It felt like a rocket up till about 45mph then fall on its face. The pull would quickly stop instead of gradually like I would have expected based off other builds with the same gearing but using the smaller 66cc motors with the same port work, carb and exhaust, and ignition timing. I went with the Delorto clone and the improvement in the low to upper mid-range of the rpm range was night and day but she would still fall on her face once I got around that certain speed. I was on the way to work and I had a good trail wind so she was going faster than normal. An 18 wheeler passed me up but he wasn’t going much faster than me so I got in his draft not long after he passed me and I made sure no one was behind him. The bike took off like it had nitrous. It started vibrating at what I would guess is around 500rpms for a few seconds. The engine note got noticeable raspier like it was hitting a powerband. The bike had a second wind and started speeding up like crazy and the vibration subsided more and more the louder it got. I passed that truck and one car before I decided to let off because that road has a 65mph speed limit and I’ve never heard any of my builds scream at those kind of rpm’s. I’ve had one scream to about 7k with everything identical minus the jug size. That was the 66cc. This was much much higher. Honestly it scared the crap out of me and I don’t plan on making runs at those speeds often at all but I do want to confidently build until I have something that can reach those speeds because the build was planned that way instead of guesswork
 
No sir you didn’t upset me at all. I was simply trying to be as transparent as possible. Like I said, this has always been my hobby that I’ve been passionate about since before I started building. I stared rebuilding bicycles from the frame on up by the time I was 11. I’d take the 20” bicycles and swap the front forks for one with suspension from a 26” mountain bike. I’d still use the 20” rim for weight reasons. I’d do this so I got enough high to use mountain bike pedals and sprockets. The added pedal stroke helped me apply more torque to the shekel. I always wanted to go fast then I started juggling the idea of having a motor on a bicycle. We were poor and the internet wasn’t what it is now so I had no idea that this already existed until I got on eBay for the first time and the first thing I did was type in “bicycle motor”. I’ve been living the dream ever since. Point is, I’m very passionate about learning. This is the last motor I will have ported using guess work. I normally just clean the ports and ride like that for a few months. After that I raise the exhaust roof by roughly 1 to 2mm, Lower the intake floor by roughly 1mm. I widen each port by about 2mm. I usually never touch the transfers and i always use one base gasket. That has seemed to work for me because the results have been consistent using that method. This time I shaved some metal at an angle on the top side of the transfers from the back of the cylinder wall while making sure the transfers were the same shape no matter which angle I looked at them from. The goal was the direct the intake charge toward the top of the cylinder instead of one of them losing a good bit of charge to the exhaust…especially after widening that exhaust port. I ran the BoFeng carburetor with this setup with stock jetting and a f2 “expansion chamber”. I weigh about 185 pounds and this is on a beach cruiser frame. Besides having to pedal a little faster to get it going, I would barely have to apply throttle and she would pull with ease. It felt like a rocket up till about 45mph then fall on its face. The pull would quickly stop instead of gradually like I would have expected based off other builds with the same gearing but using the smaller 66cc motors with the same port work, carb and exhaust, and ignition timing. I went with the Delorto clone and the improvement in the low to upper mid-range of the rpm range was night and day but she would still fall on her face once I got around that certain speed. I was on the way to work and I had a good trail wind so she was going faster than normal. An 18 wheeler passed me up but he wasn’t going much faster than me so I got in his draft not long after he passed me and I made sure no one was behind him. The bike took off like it had nitrous. It started vibrating at what I would guess is around 500rpms for a few seconds. The engine note got noticeable raspier like it was hitting a powerband. The bike had a second wind and started speeding up like crazy and the vibration subsided more and more the louder it got. I passed that truck and one car before I decided to let off because that road has a 65mph speed limit and I’ve never heard any of my builds scream at those kind of rpm’s. I’ve had one scream to about 7k with everything identical minus the jug size. That was the 66cc. This was much much higher. Honestly it scared the crap out of me and I don’t plan on making runs at those speeds often at all but I do want to confidently build until I have something that can reach those speeds because the build was planned that way instead of guesswork
Thats cool! it's a fun hobby that I just got into a few months ago, but I've always had Nitro R/C cars to tinker with.
45mph is respectable for a basic mill, usually you need an Avenger, Phantom or Minarelli to get to and over that speed.

There is a lot of buildup on the head, what plug and fuel mix are you running?

Ever think about a 212 or 224 build? it'll put a smile on your face if you're looking for grunt and speed. Engines themselves are only $110 to $180, getting them to 10-17hp is another $250 or so.
Here is my finished bike I nicknamed Spitfire.
PXL_20240329_205045909.jpg


Unfortunately for me, it killed my desire to push 2 strokes to the limit, I'm 300lbs, and I don't benefit much from a 2 cycle's powerband.
 
I’m running the NGK BR6HS. I have a 4 liter tank so I do 2 dollars at the pump and half a little container of oil. I was doing a full container to one tank for the first 8 months of having this motor but it smoked more than I wanted it to before it fully warmed up…more than normal so I went to 3/4 of a container and now half seems to do the trick. Light smoke on startup and for the first 1/4 mile or so. I’ve thought about the predators but I don’t want it to be so obvious that the bike has a motor from a distance. I ride slow and relatively quiet despite the porting and exhaust while I’m in town. I open it up on the long country roads though
 
I’m running the NGK BR6HS. I have a 4 liter tank so I do 2 dollars at the pump and half a little container of oil. I was doing a full container to one tank for the first 8 months of having this motor but it smoked more than I wanted it to before it fully warmed up…more than normal so I went to 3/4 of a container and now half seems to do the trick. Light smoke on startup and for the first 1/4 mile or so. I’ve thought about the predators but I don’t want it to be so obvious that the bike has a motor from a distance. I ride slow and relatively quiet despite the porting and exhaust while I’m in town. I open it up on the long country roads though

Do you know what ratio comes out to be?

Synthetic helps with deposits too.

You might be running a little rich, or too much oil.

You can make a 4 stroke super quiet with a scooter muffler, much quieter than a 2 stroke, but yeah, you're not going to hide it!
 
I’m thinking it’s quiet when I’m cruising around town because I’m barely on the throttle. With the long gearing ( 28 tooth on a 26”rim and a tall tire from a Trek) the RPMs stay low. Yes it’s very loud when I open it up though. All that torque makes it fun to drive around town. The most I usually take it to in town depending on the road is between 25 to 30mph. I’m guessing the mix is around 32 to 1
 
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