YBR 125 (carb version) bogs at full rpm

[dluberk1973]

Greetings.

After a DIY rebuild of my YBR125 (changing the piston along with rings and gaskets only), the following issues crop up. AT full throttle, bike previously can hit 110km/h. It now does only about 95km/h. At that speed and RPM, the engine bogs down but resolves when I ease off the throttle. Can anybody advise what is wrong?

[WD Forte]

So many variables but will assume same carb
and same ignition timing because that's set at the crank/stator
This leaves stuff like valve timing and clearances
and the fit of the new piston and rings.

The new piston and rings may be tight and need running in or
poorly fitting and not developing the same pressure in the cylinder
so ultimately less powah.
Did you bone the hore?
____________________
bikers smell of wee

[jeffyjeff]

My 2015 YBR could not routinely hit 110 kph until it had around 12000 km on the clock. Assuming you fit the piston/rings properly, your bike's top speed may be pretty much within expectations. (by the way, did you hone the cylinder bore?) The bogging down until you back off the throttle is odd, though. If carbureted, you probably have a CV carb, and it would be pretty unlikely (impossible?) to under-fuel the engine at WOT if the diaphragm and slide are working properly. Yours is probably fuel injected. It would not hurt to recalibrate the throttle position sensor, or at least check it with an ohm meter for abnormality at WOT.
____________________
History shows again and again how nature points out the folly of men - BOC

[jeffyjeff]

By the way, if the bike was performing so well...why did you swap out the top end? 110 kph is about all anyone could reasonably expect from a YBR. Did you change out any other parts (exhaust, airbox mods, camshaft, or the like)? If all stock, you might have got the timing off by one tooth, although were that the case, I would expect poor performance all up and down the rev range.

You can always reinstall the old top end and get your 15 kph back.
____________________
History shows again and again how nature points out the folly of men - BOC

[Robby]

Most likely reason is that you fucked something up during the rebuild. Bits to consider, in no particular order:

Camshaft timing is wrong. You can check this would taking everything apart.

New piston is the wrong size.

You didn't scrape off the old gaskets well enough, so it isn't sealing well.

You didn't refit the intake or exhaust properly, so you've got an intake or exhaust leak.