Old GY6 125cc to 160cc BBK timing issue

[Erik]

Hey everyone,

I am confident my gy6 is half a tooth off timing after a big bore kit install. Reason why I am confident is because the very same symptoms occured when I had it in stock setup a tooth off.

My only idea is to take off the camshaft or crankshaft sprocket and reinstall it so crank sits correctly - I have seen crank sprocket taken off with hand tools on youtube but it looks tough to get in the 1st try.

Lore:

I wanted to install a bbk over winter and it is almost summer.

I knew BBK would somehow turn out to be a headache but I did not expect this...

Got the 160cc bbk from Glixal store on aliexpress, biggest size which should fit without boring the engine block.

Turns out my old kymco 2007 125cc agility (engine says KN25) has the PAIR head and hence a 94 link chain, while the BBK is made for a 90 link chain. Also it had the steeper angled valves so I got a new rocker arm assembly as well as an a9 cam.

Timeline:

Installed just the cylinder and piston, kept the stock head. Everything runs fine.

Got all the other parts I needed to make it fit the shorter head, and decided to split the 90 link chain and replace the stock one. Since the head is shorther now, I had to cut up and weld a new pipe to the exhaust.

Everything looks fine, but I got timing off one tooth so I damaged the valves (tho I suspect they were bad quality because the ones on an extra head I got were even worse)

Got new valves and a new stock camshaft (both from a local shop) for the new type rocker assembly. Replaced the old piston rings that ran fine just in case.

Issue: engine feels low on power and does not rev out, instead bounces between mid rpm - just like when I had it one tooth off timing in stock setup. I have properly timed it now, verified TDC lines up on the flywheel mark, made sure even with the straw in spark plug method - and the camshaft lines up like apparent from the picture = slightly retarded, but that is the best it gets. One tooth either direction is way off.

Stock 24mm carb came with a #102 jet. I have all sizes and they do look different as well. Came with 102 stock. 115 smoked grey heavily, 110, 108, 105, 102 feel almost the same, surged after I left off the throttle and bounces mid rpm. 102, 105 I can hear ping.

Pics, I am sure stock cam markings were shoving it advanced quarter of a tooth, so currently it is 100% at least half a tooth off:
https://drive.google.com/drive/folde...yWCfZfBH0uewPd

Parts I have got:

GY6 160cc Chinese Scooter Engine 58.5mm Big Bore Cylinder kit with Piston Kit
https://a.aliexpress.com/_mP5AZY0

Glixal 58.5mm High Performance Cylinder Head Assy with Valves 4T
https://a.aliexpress.com/_mM3HyVI

Glixal 90 Links Timing Chain for GY6 125cc 150cc
https://a.aliexpress.com/_mLq53cs

Cylinder Head Valve Cover 157QMJ
https://a.aliexpress.com/_mKfa0fi

Glixal GY6 160cc 58.5mm Piston Kit Piston Ring 157QMJ
https://a.aliexpress.com/_mKc63U0

Glixal GY6 125cc 150cc High Angle Performance A9 Camshaft 152QMI 157QMJ
https://a.aliexpress.com/_ms3h1k0

Glixal Cam Chain Guide Tensioner for GY6 125cc 150cc 152QMI/157QMJ
https://a.aliexpress.com/_mO4t93m

Glixal 152QMI 157QMJ Rocker arm assy for GY6 Chinese Scooter Moped 125cc 150cc
https://a.aliexpress.com/_mszpkOs

Camshaft from local eshop, where I got 150cc valves as well:
https://www.pitbike.sk/nahradne-diel...del-gy6-125cc/

Thank you for reading, what do you think?

[sc00ter]

Not sure on timing being a tooth off. That sounds weird to me. My wife always sets the timing on engine builds. She loves puzzles and one day I put her to work.

Did you do a leak down/pressure check on the motor after putting it back together? The mid-range issue sounds like "Hang on" from a air leak, or the motor is extremely lean. Either from a air leak or lean jetting, or both.

You might have to do plug chops. I hate doing them but I do what's needed.

That's all that's popping in my head right now.

[sc00ter]

Alright, another thing(s) popped in my head.

You'll have to up-jet if you installed a bigger piston/cylinder kit. So if the OEM jet was a 105 don't reuse it. Go larger. There use to be a great website called Tuning Matters. His information on tuning is super helpful for inspiring tuners. I've used it several times to help dial in a tune.

After you get the motor sorted you'll next need to dial in the transmission. Your stock set-up will not like the bigger top end. The transmission will engage to early from the higher torque. But get the motor sorted first.

And you mentioned a ping noise. Did you do a compression test? If so, has your compression dropped any? Ping noise from the piston is bad! Usually a sign of an extremely lean motor. Way over-jet and go down till it runs. You'll also have to up-jet your idle as well.

We did a BBK kit on a Genuine Buddy 125. It went from a 125cc to a 161cc. Thing was death fast! I don't have the build sheet anymore and my old brain can't recall what jetting we ended up with. But anyone who did a BBK like we did on a Buddy will know what jetting they used, so that will give you a base line to work with. Elevation (like if someone from Colorado will obviously have different jetting than me on the east coast at sea level) will matter as well. A bunch of people have put BBK's on Buddy 125's. See in anyone has a build list on Modern Buddy for your base line jetting. The bonus is a Buddy uses a GY6 variant as well.

[Erik]

If you browse the Glixal store, you will see that I mismatched the rocker arms or rocker assembly with the valves. Thank you 49ccscoot video. If you do not hear back from me, that will have solved it ordering the correct arms now!!