My first BBK install

[Fox]

I just installed the BBK I got for $60 off CL.

It's exactly the same as this one:
cgi.ebay.com/ebaymotors/100cc-Big-Bore-Kit-139QMB-GY6-50cc-scooter-parts-gasket-_W0QQcmdZViewItemQQhashZitem4cf0eafaacQQitemZ330459445932QQptZOtherQ5fVehicleQ5fParts

They are calling it a 100cc but I'm not convinced after reading a few posts from Brent. It's probably an 80cc. Thing started first try! Idles great and seems to run well. I installed a 20mm carb I got with the 49ccscoot contest winnings. Installed a 32 idle and a 105 main I got from an eBay dealer. Stock air box. It smokes some when I revved it up. Maybe too much jet... I wasn't sure which ring went on the top but Brents BBK guide answered that so it went smoothly.

I didn't ride it yet because the front of the scooter is apart so no headlight and it's dark out. I figure there's no reason to put the shrouds on until I see how it's gonna run up and down the street a couple times. It has 4 gram weights in it now but I'm gonna probably have to install some heavier ones. The kit came with a set of what looks like standard weights. I can't wait to ride it tomorrow!

[90GTVert]

Good to see you got the time to get this done. You work on scoots too much not to have a modded one. That's where a lot of my enjoyment of scootering comes from, so it's good to see you've came over to the dark side with the rest of us.

It definitely is not 100cc. They show right in the pics that the bore is 49.69mm, which most would call 50mm. With a 41.4mm stroke, that's just over 80cc. The 105 main does sound pretty hefty. I used a main that big in my 80cc at one point, but that was with a 24mm carb, 24mm intake, and worked over head or big valve head. With port matched stock 50cc stuff and modded stock carb I was closer to 95-98 IIRC. ...And all of that was with a UNI, not an airbox. Definitely leave those panels off till you're sure the carb is straight.

[Fox]

Yeah I should have put the fan shrouds on the engine but I was afraid I would end up having to remove 'em again due to some unforeseen issue but I guess I got lucky this time. No biggie. Just have to remove the intake and exhaust real quick. I'll do a plug read after I ride it a little bit and see where the jetting stands. I have a few #95's but I'll prolly end up ordering a #100. We'll see...The air box is opened up so I'm pretty sure it's getting plenty of air. I filled the tank with 91 octane. Is that better for break in or should I go with some lower octane stuff?

Dark side huh?

[90GTVert]

Stick with higher octane for now. It looks like it will probably accept lower octane, but get it tuned and put it under some loads with that octane at first. If you don't hear any spark knock, then maybe try lower octane stuff. Just better to play it safe at first IMO.

[2strokd]

Cool :devil:

[Fox]

Okay so I just rode it for a bit without the fan shrouds. It's a little bit slow on take-off but after you hit 10-15 mph it starts pulling hard. It always acted like that though so I'm thinking the rollers are a little light but my street is a fairly steep hill and as some of you already know I'm over 200 lbs. and it's pulling me up the hill way better than before. It used to take me about 7 minutes to get from the little market down the hill to my house but today it only took about 4 1/2 minutes. The plug looks good. It's light gray with a just hint of brown and has a dark ring around the bottom of the threads. I have a fan blowing on the engine right now to cool it off faster so I can get to installing the fan shrouds on the engine. Then I can take it for a longer ride with no fear of burning it up. I'll play with the rollers and see what happens. I hope it lasts for a long time. I've seen plenty of BBK threads over the years that ended in disaster after only a short time which is the main reason I have been loath to do one myself til now.

[shakerdriver]

Glad to hear you put that certificate to good use Fox. And :thumb: two thumbs up for getting that bbk done right.

[90GTVert]

Obviously clutch springs come to mind with initial takeoff suffering, but I always found my GY6 50 to not like the stiffer clutch springs. The 4T makes too much torque to go and bypass it all with stiff springs IMO. I assume it's revving high and getting nowhere by your interest in heavier rollers. Ideally, you're gonna want it to stay somewhere around 7,500-8,500RPM I would imagine. Itwill probably push higher when you stretch it's legs, but that's where the peak power should be for accelerating. I used to tune mine for closer to 9,000RPM, but I had a cam and porting etc... and I just wanted max performance. Another thing to check when they're slow on initial takeoff is the belt travel. If the belt doesn't get far down in the front pulley, you lose your "1st gear" so to speak.

[Fox]

So I took it for another ride for about ten minutes with the same setup. It's definitely faster on the flats and hills but top speed hasn't changed. Checked the plug again and it looked a little darker this time so I tried a #95 jet thinking leaner might = faster but it made the damn thing slow as hell. Checked the valves and they are both dead on .003in. I'm gonna go back to the #105 jet and try it without the air filter. If it improves then maybe I need a UNI pod and possibly even a #110 jet.

Hell I don't know...

So you say this is the enjoyable part huh Brent? ;D

[90GTVert]

I'd say if it works better with no filter than either a UNI and that jet or very close or maybe move down to 100-102.

I more or less like the engine building part and tinkering with that stuff. Learning what does what. Carbs can be a pain sometimes. Depends what setup you have. The 4Ts in general aren't as bad as the 2Ts, so if you aren't liking this... think hard before messing with the QJ.

[Fox]

Okay so here's the verdict:

#105 jet w/air filter attached I got an indicated speed of around 27-28 up the hill.

#105 jet with no air filter I got an indicated 30 up the hill and a slightly faster take-off. The plug color is still a tad on the darker side of good but still acceptable so I'm gonna put a UNI pod on it and play with the rollers a little to see if I can increase that top speed a little. Maybe grind down the bushing a little if I have any more room for belt travel.

I'm never modding that QJ.

[90GTVert]

Even with the UNI, pick up a 100-102 if you can.

If you grind down the drive boss, you might really make takeoff suffer. It's gonna push the belt higher up in the front at rest. If you were to file the vari face just a little, it would do two things. It would create additional clearance to let the belt sit low at rest ("1st gear") and it would allow the pulley to get that much closer together and push the belt out farther. Before modding anything though, mark the vari face with a sharpie and see how much room is left on the vari.

[Fox]

Okay I'll try that.

[Fox]

Man I'm stumped! This BBK ran great the first day and then I let it sit for a week or so and fired it up and it immediately started stuttering really bad at WOT. I figured there was some debris in the carburetor left over from the factory and a cleaning would solve the issue. Today I cleaned the carb and found some fine particles in there but nothing major. Put it back together and now it revs up high to about 2/3 throttle and after that it has a pulsating bog. WeeeAhhh Weeahhh WeeeAhhh and gas sprays out of the back of the carb on the "Ahhh" part of the bog. I tried it with and without the air filter and there was no change. I raised the needle clip a couple clicks and the same thing happens.

I'll check the valves tomorrow and try a different CDI and coil.

Anybody have any other ideas as to why it would start acting like this?

80cc, stock head and cam, 20mm carb with a 32 idle jet and a 105 main.

[90GTVert]

Air leak? Fuel delivery issue?