For Amateurs like me thinking of installing a Big Bore Kit,
by: steve - May 29, 2015 16:28:32 GMT -5
ital likes this
Post by steve on May 29, 2015 16:28:32 GMT -5
Here is your opportunity to learn from my mistakes.
First of all, you will see some kits on ebay, with the valve head, rocker arm assembly, camshaft, roller weights, cylinder and piston for $70-$
. This looks like an amazing deal, almost too good to be true. It is too good to be true. You can't get good quality parts for that price. I got one of those, and it ran great, for like 30 miles. I have ended up replacing every single part that came in that kit, except the main carberator jet. I'm sure the people that sell stuff that cheap are good people, they probably don't even know what they are selling.
You should expect to pay $100+ for the cylinder, piston, and big valve head. Buy from John here at NCY, or Scrappydog, or Scooters to go, or another one of the banner ads here. I have held the $20 cylinder in one hand, and the good quality one in another, and the difference is huge. The cheaper ones are literally like a coke can compared to the good stuff. The cheap one I bought started leaking compression from everywhere after about 2 days. The valves rattled from the get -go, and the springs were probably going to snap shortly. Pay the money, and do it once. Trust me on this.
Also, take your time. Use colored tape to mark which wires go where. Unbolt the 2 bolts that hold the motor on the frame, and the 3rd one on the shock, keep the back tire and kick stand on the motor, and roll it away from the bike. I suspended the bike from my garage door frame, and rolled the engine out from under it, with the back tire still attached. Kept it up on the kick stand, and had all the room in the world I needed to work on it. Just remember to unhook all the ground wires, the rear brake cable, the throttle cable, spark plug wires, and all that. Don't rush, and end up snapping something that you will have trouble putting back together. Just taking the seat off did not give me enough room. The frame was all in the way just trying to take the valve cover off. No way I could have changed the cylinder on my scoot, with how the frame is. Getting the rings into the cylinder requires gentle precision! Don't try to do it without having access to every inch of them! Trust me on this.
Remember the spacers/dowels! After I took it apart the 3rd time, replacing cheap parts, I forgot the spacers that slide over the studs, and have lost compression again. Those are there for a reason!
Buy a new starter to install with your BBK. You are going to need a BRAND NEW one to be able to turn it over at first. The compression is very high, especially right after you put it together! The starters suck oil into the coil fast. The O-rings on those things suck. You will not notice how much torque your starter has lost until you try to turn over a brand new BBK with it. Again, trust me on this.
Buy 5-10 main jets. You want the largest jet to be 10# larger than the largest you think you will possibly go. Like for a 50cc, if you are installing the 50mm cylinder, get #, 
, 100, 105. Start with the , and keep increasing them until it is too rich. Then,take the insert out of the intake nozzle on your airbox, and it will most likely work fine. If you want me to explain my way of jetting a carb, without having to pull the plug 50 times, I will share. You want your new motor to run rich. If you run that thing lean, like they run them as 50cc motors coming out of the factory, you will lock it up, or burn a hole in the piston very quickly. Running them as rich as you can, without losing performance, is the way to go. Rich = cooler temps. Lean = hotter temps. I put a 50mm BBK on mine, and am running a #100, with the little 4 hole insert removed out of the intake nozzle of my factory airbox, and it performs pretty much the same as running a #, with the insert in, which means it runs great. I also drilled a hole in my muffler, so that probably plays into it.
Finally, buy the upgrade carb. For mine, I bought the true 20mm Keihin(or however you spell it). It is worth it. One, or all of the guys I listed will have that.
Finally, every time I said "trust me" in this novel of a post, it means I have made the mistake, and paid for it. Take your time. If you get stuck, ask one of these guys on here, and wait for a couple of responses. Save yourself the headaches. Buy quality parts. Spend the money. The merchants I mentioned are not selling the stuff at that price to rip us off. That is how much good parts cost. I have come to terms with that.
Be thorough. Buy a new Gates kevlar, or the equivalent material belt to install with the BBK, also.
Any questions? Feel free to ask. I am sure I have screwed up what you are asking about, and can tell you what NOT to do. One of the smart people on here can then tell you what to do.
Good luck!
First of all, you will see some kits on ebay, with the valve head, rocker arm assembly, camshaft, roller weights, cylinder and piston for $70-$
. This looks like an amazing deal, almost too good to be true. It is too good to be true. You can't get good quality parts for that price. I got one of those, and it ran great, for like 30 miles. I have ended up replacing every single part that came in that kit, except the main carberator jet. I'm sure the people that sell stuff that cheap are good people, they probably don't even know what they are selling.You should expect to pay $100+ for the cylinder, piston, and big valve head. Buy from John here at NCY, or Scrappydog, or Scooters to go, or another one of the banner ads here. I have held the $20 cylinder in one hand, and the good quality one in another, and the difference is huge. The cheaper ones are literally like a coke can compared to the good stuff. The cheap one I bought started leaking compression from everywhere after about 2 days. The valves rattled from the get -go, and the springs were probably going to snap shortly. Pay the money, and do it once. Trust me on this.
Also, take your time. Use colored tape to mark which wires go where. Unbolt the 2 bolts that hold the motor on the frame, and the 3rd one on the shock, keep the back tire and kick stand on the motor, and roll it away from the bike. I suspended the bike from my garage door frame, and rolled the engine out from under it, with the back tire still attached. Kept it up on the kick stand, and had all the room in the world I needed to work on it. Just remember to unhook all the ground wires, the rear brake cable, the throttle cable, spark plug wires, and all that. Don't rush, and end up snapping something that you will have trouble putting back together. Just taking the seat off did not give me enough room. The frame was all in the way just trying to take the valve cover off. No way I could have changed the cylinder on my scoot, with how the frame is. Getting the rings into the cylinder requires gentle precision! Don't try to do it without having access to every inch of them! Trust me on this.
Remember the spacers/dowels! After I took it apart the 3rd time, replacing cheap parts, I forgot the spacers that slide over the studs, and have lost compression again. Those are there for a reason!
Buy a new starter to install with your BBK. You are going to need a BRAND NEW one to be able to turn it over at first. The compression is very high, especially right after you put it together! The starters suck oil into the coil fast. The O-rings on those things suck. You will not notice how much torque your starter has lost until you try to turn over a brand new BBK with it. Again, trust me on this.
Buy 5-10 main jets. You want the largest jet to be 10# larger than the largest you think you will possibly go. Like for a 50cc, if you are installing the 50mm cylinder, get #
, 
, 100, 105. Start with the , and keep increasing them until it is too rich. Then,take the insert out of the intake nozzle on your airbox, and it will most likely work fine. If you want me to explain my way of jetting a carb, without having to pull the plug 50 times, I will share. You want your new motor to run rich. If you run that thing lean, like they run them as 50cc motors coming out of the factory, you will lock it up, or burn a hole in the piston very quickly. Running them as rich as you can, without losing performance, is the way to go. Rich = cooler temps. Lean = hotter temps. I put a 50mm BBK on mine, and am running a #100, with the little 4 hole insert removed out of the intake nozzle of my factory airbox, and it performs pretty much the same as running a #, with the insert in, which means it runs great. I also drilled a hole in my muffler, so that probably plays into it.Finally, buy the upgrade carb. For mine, I bought the true 20mm Keihin(or however you spell it). It is worth it. One, or all of the guys I listed will have that.
Finally, every time I said "trust me" in this novel of a post, it means I have made the mistake, and paid for it. Take your time. If you get stuck, ask one of these guys on here, and wait for a couple of responses. Save yourself the headaches. Buy quality parts. Spend the money. The merchants I mentioned are not selling the stuff at that price to rip us off. That is how much good parts cost. I have come to terms with that.
Be thorough. Buy a new Gates kevlar, or the equivalent material belt to install with the BBK, also.
Any questions? Feel free to ask. I am sure I have screwed up what you are asking about, and can tell you what NOT to do. One of the smart people on here can then tell you what to do.
Good luck!
