50mm BBK advice

[jjohns2711]

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

[kz1000st]

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

[jjohns2711]

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!

[jjohns2711]

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.

[jjohns2711]

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.

[blueboy5000]

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.

[jjohns2711]

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!

[jjohns2711]

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...

[dwint51]

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

[jjohns2711]

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

[jjohns2711]

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?

[kz1000st]

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.

[jjohns2711]

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...

[kz1000st]

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.

[jjohns2711]

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...

[kz1000st]

I would first try spraying a little starter fluid into the carb and see if it starts. You may have crud in there. It would be unusual to have an engine running one minute and going to heck the next in this situation.

What do you mean "then backed the mixture screw back so the engine would shut off" Why would playing with the mixture make it shut off?

[jjohns2711]

Where do I spray the starter fluid? Like where the fuel line attaches to the carb?

I backed the mixture screw as far as it would go, so it cut off any fuel or air from getting into the carb. I read to do that somewhere online here!

[jjohns2711]

I honestly have no idea what's going on.

[kz1000st]

No spray it into the open end of the carburetor where the air filter attaches. If the carb is clogged it should only run for a few seconds.

[jjohns2711]

Gotcha. I drained the carb of any gas so I'll try that cleaner and see what happens!

[jjohns2711]

Damn, I feel like an idiot! I didn't end up using the starter fluid (although I still bought some just in case). I think the main issue was the battery! It was basically dead. Had enough juice to illuminate the dash, but not enough to turn the engine over. Plugged it up to me charger, installed in on the moped and voila!

I had a chance to mess with the carburetor... Is maximum idle when the back wheel starts the rotate? Somewhere online it said to match the idle and mixture screw to "maximum idle", then back the idle screw to a comfortable idle. Wasn't quite sure what it was getting at...

[blueboy5000]

1-Turn mixture screw CLOCKWISE until it STOPS TURNING.

then

2-Turn mixture screw COUNTERCLOCKWISE 2 complete turns.

3-Start bike. (It may be necessary to adjust IDLE SCREW. to obtain useable idle.) Allow TEN MINUTES of running

4- turn MIXTURE SCREW COUNTERCLOCKWISE in 1/8 turns incremets until your RPMS reach MAXIMUM. then turn MIXTURE SCREW 1/4 turn CLOCKWISE. This is proper mixture setting.

5-Adjust idle screw until IDLE RPM is at 1800-2000 rpms.

[jjohns2711]

I understand the above instructions, but my only question what IS maximum idle? This is all while the scooter is on the back stand, so the back wheel isn't touching the ground, correct...?

I read somewhere a solid idle setting is when the back tire barely starts moving, or enough to slightly engage the clutch... Thoughts? Is this maximum idle?

[dwint51]

yes otherwise your belt will grab and when its off the stand it will want to stall and you will wear your belt out

[jjohns2711]

Gotcha. Okay makes sense now. When I read "maximum idle" I imagined a scooter on a dyno about to fly off why they tune the carburetor haha! Let me tune the carb, ride around and check the spark plug and I'll let you know what I find (lean, rich, or optimal)

[jjohns2711]

I'm still running a little lean, with a #90 main jet... I'm in Hawaii with an aftermarket exhaust, K&N style air filter, with 93 premium gas. Surely a #90 is sufficient?

Also I couldn't get my scooter to start again earlier today. I'm plugging up my battery to the charger as we speak. Anyone have any links to aftermarket performance starters? This is getting frustrating!

[kz1000st]

Talk to the people at www.partsforscooters.com they build drag scooters and might be able to tell you about starters on their chat.

[dwint51]

a 90 should be plenty big.i would say keep adjusting your carb.my carb is richer to the left but guys on here will say different.lawn mowers,cars,motor cycles have always been like that but maybe that's just the usa

[dwint51]

try putting some tape around your filter.after market filters are a pain in the ass

[jjohns2711]

Okay so what EXACTLY does the mixture screw do? Literally, does turning it clockwise lean the mixture, and counterclockwise richens the mixture.

[jjohns2711]

Because my spark plug still reads my mixture is still lean, but I "tuned" the carburetor per the above instructions. Do I just need to turn the mixture counterclockwise an 1/8" of a turn, ride around a little bit, and check the plug to see where I'm at?

[dwint51]

its always good to start with a new plug but yes to your answer.once you get it in tune use your plugs u used in the past for testing.

[dwint51]

a lot of people on here don't use after market filters cause of problems you are having.read about how they put tape on them to make it richer.when you turn your screw you have to wait to see the engine rpms change.i always turn to the left and when rpms go highest I turn to the right same thing and put it in the middle

[blueboy5000]

turn the mixture screw CLOCKWISE an additional 1/8 turn, then run it for a few days and "plug chop".

You're looking for a nice brown coffee color on the WHITE CERAMIC INSULATOR directly under the spark points.

[blueboy5000]

Turning the MIXTURE SCREW COUNTERCLOCKWISE LEANS the MIXTURE.

[jjohns2711]

Gotcha. I'll try and retune my carb... And it could possibly be an issue with too much air either way because of the filter, so if I don't get results by tuning the carb I'll start removing surface area from the air filter.

This is frustrating business for sure!

Okay another question, specifically a CVT questions: I'm looking to squeeze another 10 MPH out of this thing (right now it cruises at 40-45). How might I go about doing this? Changing the roller weights? Clutch springs? Using a kevlar belt? I heard the A9 cam increases top speed. Thoughts?

[blueboy5000]

You're in break-in, do not try to ride faster than 40mph. ESPECIALLY with a Bbk. People argue w me about break in, but trust me I speak from experience, for the first 800 miles, vary your RPMs constantly and evenly. Only hit maximum for LESS than five second at a time.


Then, after break-in, get a KOSO-style variator w 6g flat-sliders and red-spring clutch. A Kevlar belt is a good plan as well.

[blueboy5000]

It is possible that you may need to restrict the filter. I use paper cone filters and electrical tape.

[jjohns2711]

Noted. I'm trying to vary the RPMs as much as I can, although I still drive it on the daily. So far so good.

I don't have an impact wrench to get the nut off that holds the variator and the clutch on. Any suggestions?

[blueboy5000]

Yes go get one of these, and pay the 6 bucks for the no-questions asked replacement plan.

http://www.harborfreight.com/12-in-electric-impact-wrench-68099.html

That's what I use.

[jjohns2711]

I'm going to check the plug tomorrow after a WOT run to see where my mixture is.

Current planned CVT upgrades: performance variator with 6.5g roller weights (not sliders, only because I already have the roller weights), kevlar belt, and performance clutch with red-springs.

My engine is 139QMA, so it's essentially the same as a QMB, but it fits 12" wheels, so it requires the longer belt. Anyone got a link to a good kevlar belt that'll fit a QMA?

[kz1000st]

http://www.scooterbelt.com/

http://www.partsforscooters.com/Street-Scooters-Belts

[jjohns2711]

Awesome, thanks for the link.

So I'm just a little confused. I do all these modifications, and my top speed is still 40-45 MPH. Am I missing something? My friend's stock 50cc GY6 scooter s just as fast as mine (although my acceleration is quite better). Is it the gearing? I want it to cruise at 50 MPH comfortably. That's my goal. And I'm not even that big either - 6'2" and 180 lbs.

Any ideas?