Want to get things straight in BBK

[thunderbolt]

Just want to get some clarification on BBK's
1. On a stock 49cc I should not get a kit over 80cc , any kit over that will wear out the bottom of engine ?
Other things I'm unsure of :
2. Replace the carburetor ? Or can my stock carb be used unmodified ?
3. Use same camshaft or replace it ?
Please forgive my ignorance any advice be appreciated .

[tortoise2]

Oct 12, 2018 9:39:46 GMT -5 thunderbolt said:

any advice be appreciated 

4. Accept my 50cc mistake, and replace with at least a 125cc scooter.

[thunderbolt]

Doesn't it make more sense to buy a thousand dollar 50cc, spend hundreds of dollars in parts and labor to make it 6 mph faster ?

[90GTVert]

1. The bigger and more powerful; the more stress. So, stock will likely last the longest and above there it's always some compromise. 47mm bores are usually thought of as the limit for long lasting builds.

2. Any 18-20mm unsealed carb can work. If stock is not sealed and has an adjustable needle, it should be fine. It will need to be tuned when a BBK is added, as will any carb you buy.

3. Your choice. You can use the stock cam. You may get a little more from a performance cam.

Don't forget that the CVT may need attention too. Likely just roller weight swaps to keep the RPM where it needs to be.

[tortoise2]

Oct 12, 2018 10:38:57 GMT -5 thunderbolt said:

Doesn't it make more sense to buy a thousand dollar 50cc, spend hundreds of dollars in parts and labor to make it 6 mph faster ?

That is the typical misguided consensus, at least on this forum, until rational reasoning evolves.

[islandscrub]

Licensing and insurance requirements, along with higher registration fees are why a lot of folks stick in the 'moped' range.

[ratdog]

There is no magic number for the over bore that will last. No matter what, you are putting more strain on your engine. But to build a 65 cc engine to give the preformance of an 80 cc engine will require more RPMs. So you would have to build a 8500 to 9000 rpm 65 cc engine to equal a 7500 rpm 80 cc engine. There is a lot to be said for big displacement engines running at lower RPMs. I think how you build the engine is more important then the displacement you build to.

The problem is that very few go to a 80 cc BBK, and keep the engine a “7500” rpm engine.

If you are building a “7500” rpm engine a stock size carb is great. But it would be best to get an aftermarket 18 mm carb that you can tune. The main advance of a larger after market carb is that it moves the hp peak up the RPM range. Makes an engine that maybe runs best at say 8500 rpm, and buy increasing the RPMs we are increese go the HP. So, IMHO, and aftermarket 18 mm carb is a very good way to go. You should only go bigger if you want to increese rpm/hp.

Cam. The aftermarket cams on the market are not high preformance cams. Normally, if you were going to move the hp peak up the RPM range, a cam with more duration, more lift, and more overlap would be a ticket to making that bigger carb work for you. The
“Best” cam for these little scoots is the “A9”. But the A9 cam dose NOT have more duration or more lift. I will be doing some expermiting with real (custom) cams to see where we can go on these little engines, but again that will be for engines built for more RPM.

The other thing that kills these little engines is heat. Many go to overbore kits and stick with a stock head. This increases compression which means more heat and higher octane gas. The stock engine has a 10.5 to 1 compression ratio. The compression ratio is a comparison of the displacement to the head volume at top dead center. Now if we incresse the cylinder volume by 30% but keep the head volume the same, that is a big increese in compression. For most purposes, a max drive around in traffic and stop and go on pump gas max compression ratio is about 11.5 to 1 on 93 octane gas. From a racing engine, we can run up in the 13 to 1 range.

To help combat the heat build up, we need to run more gas thru the engine. The gas evaporing off the piston helps cool the engine. So jetting is a really big deal.

[thunderbolt]

I've decided not to do any engine mods . I've found out how much fun it is even with my low powered machine ,think I'll replace it with a 250cc scooter hopefully springtime . Also considering a bike comparable to the Honda rebel . I do fear I'm probably going to have to replace the carburetor . But until then I'll keep it the way it is . I'm to old to be broke down on the side of the road this winter . I had to push it 2 miles last week due to a flat tire . That got me to thinking .

[90GTVert]

Sometimes 50s just aren't the best choice, stock or modified. If you do go with a ~250cc scooter and you want a greater expectation of reliability, try to stick with Honda, Yamaha, Piaggio, Vespa, Suzuki, and even Kymco. The big Chinese scoots do seem to be more reliable than the 50s, but they're still Chinese scoots. Lots of nice stuff out there from around 150-400cc that still feels light and easy to handle around town, but it capable of 50MPH cruising or even highway use; depending exactly what you get. Honda Rebels and Nighthawk 250s and especially Kawasaki Ninja 250s are pretty plentiful in the used market if you want a small motorcycle that you can likely find cheap. There are tradeoffs between scoots and motos though, so you have to decide what you want. Scoots are going to win for ease of use with the auto trans, are easier to get on with step through designs, and there's no contest for built in storage space vs small bikes. The small bikes are going to offer more speed and power though. Similar displacement motorcycles tend to be quicker and faster than comparable scooters. I have enjoyed owning both 50s and a 500cc scoot, because they each have their own place or are more fun for certain situations.