Sputtering issue

[theguy2024]

Currently breaking in a 79cc 2 stroke china doll, running nice so far but it has this issue that i’ve noticed with my other builds, when cruising with throttle open, once I let off the throttle the bike sputters like crazy so i either have to keep the throttle open or pull the clutch in. Why does it do this? I feel maybe it’s part of the break in process or too lean/rich

 

[Greg58]

You can get surging or lurching may be a better description when you close the throttle completely, I have noticed this with both of my larger engines but not with my 48cc bike’s.

 

[Mossy]

Surging makes me think of a leaking seal ... A jet too rich to adjust for it... A crank that's out of true at hi rpm would leak... And be out of Ballance... If you're rich to lean with just the rpm change the seal could be leaking around the socket or not fully seated... Ultra grey around the seal and a clean surface will keep it from moving and leaking if this is what is going on... It also could be a tolerance issue if a batch of cranks we're slightly undersized or oversized seal... A double lip Vinton seal from 123 bearings is something...