If all that was changed was the clutch and fly wheel then I suspect the clutch is slipping. What happens with manual transmission and bogging down is the clutch is not fully engaged to keep the RPM in its power range so while you may not see the engine RPM drop, the power is not actually be transferred to through the transmission to the drive-train and then the wheels.
Did they replace the slave cylinder or any other components along with these? Also, did you happen to have a performance clutch in it prior and have you switch brand or used a different clutch material?
SPONSORED LINKS
Sunday, May 24th, 2020 AT 5:19 PM