To 50mm or not to 50mm

[dumkid]

I've been toying with the idea of buying the glixal 50mm big bore kit for my 50cc taotao. The scooter currently runs, but it's bad at sand, hills, and getting to 30mph in a reasonable amount of time. Would a big bore kit make the scooter more unreliable? I also assume I'll need to get a bigger jet for the carb. What size jet should I start with? 95? 90? It has an 85 in it right now and runs slightly lean but that's how I got the scooter so I haven't messed with the carb yet. I have the skills to do this (I've done it on a few cars) I just wonder if there's anything scooter specific that I should know, and is the bottom end strong enough to take the abuse

[aeroxbud]

Quite a few people have had problems running the 50mm bore with the standard head. It can cause the compression to be very high. The big piston also can cause a lot more issues with reliability.
What about the 72cc kits? A happy medium?
At the end of the day you pay your money and take a chance. I have read that the Naraku 47mm kit makes more power. It's a lot better build quality.
But then again you could buy a couple of the Glixal kits for the same money. 🤔

[dumkid]

If the 50mm kit causes problems (pinging?) Then I'll probably take your advice and get the 47mm kit. I just want a bit more torque than stock. This scooter was free, and I put less than $10 into making it run. If I buy a bbk it'll be the cheapest one with good reviews.

[GrumpyUnk]

Personally I would go with the 47mm sized cylinder. I have done over a half dozen and they all ran well. I checked the ring end gap, and had to file a few to get minimal clearance, so check if you do an update.
The 50 seems a bit much. Draw out a circle 39mm and then 50mm... Compare. BIG difference. The 47 seems to be a good compromise. Memory says a 105-110 main jet worked well, but it does depend a lot on altitude. Higher requires less increase in main jet. Pilots in the mid-30's seem about right. Using the plastic, factory air cleaner and ducting.
I kept the factory heads, and used fiber-style gaskets. A fiber gasket is thicker and cuts down a smidge on effecitve CR. I think a 50mm would increase the CR a bit past where it would remain reliable, especially if you kept the factory head.
One thing of note is the piston/cylinder clearance. I have had some that seem to fit the piston too tightly. I used sandpaper to reduce the diameter to where the piston seemed to slide through the piston without binding.
Check the piston-cylinder wall clearance. Oiled cylinder walls, the piston should slide through without binding. If you watch YT videos where 139 engines are THE engine of choice for 99% of the drivers, removing and honing the cylinder, and then fitting new pistons and rings, perhap .05mm oversize, seems common. There are roadside shops that re-size cylinders that are brought in. Watch the videos where the piston is placed in a jig, and a spinning cylinder with abrasive paper is moved back and forth for final piston fit. When done, and cleaned, a slightly oiled piston will slide back and forth, top to bottom, as the cylinder is tilted in different directions. The piston does not FALL out of the cylinder, but slides gently. If your piston is hanging and won't do that, I suggest that the pro doing the job daily knows more than I do, and would adjust the cylinder/piston fit to match somewhere close to what he shows. YMMV.
tom

[dumkid]

Ok, thanks for the advice on piston fit.

[dumkid]

While I have the engine torn apart should I get an a9 camshaft? Do the hotter cams work with a stock 50cc head?

Edit: after some research I see no reason to not get the glixal a9 camshaft

[GrumpyUnk]

I have a supposed A9 in my box of parts. I installed it on one machine, and it ran poopy. It was not the cams fault, but I did not put it back on. Given that, I cannot say if they are worth the time & cost of replacing a stock cam with an A9. I do not know if they do anything noticeable.
I do not think anyone has published the A9 specs for lift/overlap/timing... and likely no one ever will.
For a few bucks, it may be worthwhile to see if it makes a difference. Personally, I would like to see someone post before and after numbers. Will it happen? dunno.
Can't hurt...
An additional thought is to check on the results of ovaling the mount holes for the ignition pickup. You can advance the spark timing a few degrees with little effort of time & materials. On many China engines, the timing is not as advanced as it could be, and there is a bit of performance gain, and less engine heat, when the timing is advanced. See GT90's video on same.
tom

[dumkid]

I mostly built the engine and then it wouldn't turn over. The exhaust valve spring is too long, so I'll sand it down a few mm.  Can't wait to ride this thing again! 

[GrumpyUnk]

Could an incorrect spring be installed? There are two valve stem lengths, and maybe two spring lengths sized appropriately. If you have more than one set of springs floating around I am suggesting that you may have gotten the longer size instead of the shorter. It is very unusual for springs to bind unless the valve lift has changed, the rocker arm ration changed, or a different cam with higher lift..
If any of those, sanding the spring seems an unusual fix. To me.
tom

added... OH.  A9.  Did not note that until after posting.  Did you check the washers under the springs to be sure they are correct.  I dunno if the intake and exhaust valve spring washers are the same thickness.  If one is thinner... swap?
 Surprising that an A9 would have a large enough lift difference to cause binding.  surprise!
tom

[dumkid]

There's washers under the valve springs? I used the springs that came off the scooter, it has 69mm valves. Sanding the spring worked, I only took 2mm off. And the new cam (glixal a9) definitely has a taller exhaust lobe than the stock cam, on the a9 the lobe is taller than the outside of the bearing while on the old cam the lobe was shorter than the bearing. The difference in lift was only ~1mm but it was enough to get spring bind

[pili]

I’m all about 2t so this is outside my knowledge through experience, but dealing with car engines my whole life it seems high lift camshafts usually produce more high RPM power while sacrificing low end power. That seems to be a tall ask for a scooter. Of course it depends on what you’ll use it for.

[GrumpyUnk]

I thought the A9 cams were ground to a smaller heel of the lobes. That would allow a higher lift on the other side from the followers at rest on the heel. On some designs the lobes have to be able to fit through the bearing hole machined to hold the outer race in place. That kind of limits lobes being taller... so they cut the other end. I think.
I have a reputed A9 somewhere, and am thinking about making a 72cc with an A9 and a 19mm CVK. I get around to procrastinating better'n ever...
tom

[br4inl3ss]

i'm one of the people that had trouble running 50mm 81cc bbk.
but i have the first version of the NCY bbk. domed piston aka very high compression.
with stock head it was ok when cold but halfway to operating temp it would ping like no tomorrow at WOT.
had to let it cool down for 30-40 second or only apply very light throttle if i want it to move.
pinging and no more power to the point it was slowing down.

NCY then changed the piston in the kit and now its flat. so MAYBE its fine now...

my engine is on the bench becausde of many problems and had to put back this 50mm bbk ( 47mm bbk broke ), but i since have the 47mm head that fit the 47mm cylinder. much lower compression. so i hope it work better.

else we can buy the 50mm head from glixal. or just the 50mm piston in my case.

but i also suggest the 47mm. the crank will thank you for that. and in a normally sized scotoer its very powerfull and fast.
on mine i wanted the 50mm only because its a HUGE model ( between a 2 seater and a ugly burgman ).
its actually a 150cc model sold with a 50cc ( i even saw a 200cc scooter of the same size as mine ).
with the 47mm i was barely following my brother 4t STOCK 50cc yamaha vino ( faster top speed but slower acceleration, because of massive weight ). with the 50mm i was way faster even with vino modded cvt, and it wasnt even working right.