50mm BBK advice
 

I'm going to install a 50mm BBK kit on my 50cc scooter this weekend. I have the whole assembly here with me as we speak: new piston, piston rings, big bore shaft, new head, new rocker arms, you name it. I purchased some feeler gauges to set the valves correctly, some Permatex engine assembly lube for the piston to avoid a dry start and some high temp RTV to help seal the gasket.

I also purchased a new 19mm carb with a #90 main jet to help with fuel consumption.

Anything I should keep in mind or any advice I should get before installing the BBK? Thanks in advance

Is the kit compatible with your cases? Going up 11 millimeters on the bore is a pretty big step. I'm not familiar with the maximum size allowed on a 50cc but an increase like that would require case boring on a 150cc

Yeah from all the videos I've seen you don't need to modify anything. It's *almost* a direct bolt on kit. Take off the old, put on the new!

I'm running stock air filter, and an aftermarket "performance" exhaust. There's not much to it, and from what I can tell the engine can breath better.

Any advice on how to tune a carburetor? I'm replacing the old one when I do the BBK install. I have no idea how to dial in the idle speed or mixture screw, or what to listen for. I also bought a new NGK spark plug for the install.

Stock air filter isn't gonna cut it, it's far to restrictive. You'll need an open-element style air filter.

You engine will now be nearly an 80cc, so I'd suggest #80 to 85 main jet, 90 is most likely going to be too rich.

My big problem here, is that you will need to test your compression after installing the 50mm BBK to determine what kind of fuel you will need. It is likely this BBK will make more than 150lbs compression, which means your bike will need 95 or great octane gasoline to run, and a performance starter.

As to getting the carb right, I would have purchased a "jet kit" which has several different jets, as getting the mixture right is gonna take some swapping. Jet size is dependent on not only the displacement, air flow and exhaust flow, but also on your climate and altitude.

The kit I bought came with a K&N style air filter, should I install that instead? I'll try my #90 jet for a day or two and see if it's too rich, then will move down to something leaner.

I have no way of testing compression... Any ideas here? I typically fill up with 93 premium, is that good enough? From all the researching I've done, I haven't heard or anyone upgrading their starter, have you had experience with this?

Sorry for all the questions, but thanks for your reply!

So I think I'll use the open element K&N air filter, I have the performance exhaust, and I'm located in Hawaii (hot and humid). From what I've read a #90 SHOULD be fine...

close off the mixture screw to the right.open it up around a turn and a half to the left this should get you started and running.let it warm up a g0od five minutes.if it doesn't idle on its own turn your idle screw to the right for higher rpms.your idle is the screw that sticks out further than your mixture screw.once you can get it to idle turn it the the left and see if your rpms climb up.if it does go the other way till the rpms climb.you want to set it in the middle.set your idle to what a 4 stroke should be

Alright so I installed the BBK this morning! Everything went well. It was pretty easy, took about 3 hours though as it was my first time doing anything like this.

Initial Impressions: install was easy and straight forward, just messy (oils and lubricants). Took about 4 seconds with the electric starter to turn over by itself (I installed a new carburetor, which I was weary about because that's too many variables at once to go wrong, I think gas needed to circulate through it before the engine could run by itself). Acceleration is amazing! Feels like I have two 50cc GY6's "under the hood" of my scooter. Haven't tested the top speed yet, but it's definitely faster in all regards. Exhaust notes are more "resonating", where it feels like everything vibrates slightly at high RPM's. Definitely sounds cool.

Any recommendations on how to break my "new" engine in? Advice? Thanks

I also plan on replacing the roller weights from stock to 6.5g. Is this a good idea? I heard that weight is a happy medium between acceleration and speed. Any advice on how to do this? Any special tools needed?

I'm not an authority on 50cc weights, but your choice sounds solid. As for breaking it in, I'm from the easy break in school. Chinese scooters are not Japanese. This business of "Ride it like you stole it" never works out well. Heat cycles and hours have always worked for me. Ride it like you will ride it, with short bursts of speed, and several oil changes along the way should have it happy and purring in no time.

Thanks for the reply. I'll get to replacing the roller weights sometime this weekend. I hope it's not too much rev's for the engine. If it is, I'll go higher in weight 'til I find a weight I feel both the engine and I are comfortable with.

I have, however, noticed, it much more difficult to start than before. I know the new engine has more compression than the old, so it makes sense, but it's still somewhat frustrating. Any ideas on a performance starter? Can I replace the current starter with a 150cc GY6 starter? I'm assuming they aren't interchangeable but still an idea...

I would go to www.partsforscooters.com and chat with someone about the starter. They built a drag scooter recently and should know about a heavy duty starter for that engine if anyone would. They might also be able to give you solid advice about the roller weights.

I'll get into contact with them and see what they say.

Bad news. So I rode my scooter a little bit this morning to warm up the engine, then came back to tune the carburetor. I played with the idle a little bit, then backed the mixture screw back so the engine would shut off. I turned it 1.5 turns, and tried to start the engine again. Nothing. That was 2 hours ago and I still can't get it running. I checked the plug, and I was running a little bit lean. Sometimes a little puff of smoke comes from somewhere below the engine (not the starter, something along the new piston/ camshaft). Any ideas? It doesn't respond to anything, and I'm afraid to ruin my starter.

I have the battery on a charger (the battery is brand new, and haven't had any issues with it). I'm lost here. It worked great, I played with the carb a little, now it won't start. At all...