Date: 201912
Flight conditions: VMC
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Aircraft component: RPM/N1/N2/Etc Indication
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Private; Flight Crew Multiengine; Flight Crew Commercial; Flight Crew Instrument
Human factors: Workload; Other / Unknown; Time Pressure; Situational Awareness
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Rejected Takeoff; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Gate; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: The Captain that brought the airplane into ZZZ briefed my Captain on why they wrote the autothrottles up. Supposedly; the autothrottle system had kicked off during their takeoff roll as well as in their climb because of a somewhat large split between the N1s. Or at least that was the presumption. Maintenance in ZZZ deferred the autothrottle system.During my Captain's brief; he mentioned that if he could not manually marry up the N1s to an acceptable level on the takeoff roll; he would reject because he felt that could possibly be an unsafe situation.During the takeoff roll; I manually set the thrust levers to achieve 40% N1 and waited for engine stabilization. I then advanced the thrust levers to about 80% N1 and hit the TOGA button. I continued advancing the thrust levers to somewhere around 85-90%N1 and called 'check thrust' while removing my hands from the thrust levers. The Captain then put his hands on the thrust lever and began fine tuning the N1s to meet target N1 of 95.5%.As we advanced down Runway XXL; I did not get the timely response of 'thrust set 95.5%' like I was expecting. I could tell the Captain was still fooling with the thrust levers out of the corner of my eye. I glanced at the N1s and saw about 91% on the left engine and 100% on the right. I took a look at the airspeed indicator and noticed about 105-110 kts. I made the call of '100 kts.' because Captain had missed it. I heard Captain say something to the affect of 'I can't get it to go past 91%'. It was not long after that when Captain called the reject. The last airspeed I recall seeing was 135-140 kts. I called the Tower and told them that we were rejecting the takeoff. The Tower copied and asked if we could exit left at [taxiway] after issuing a go around to aircraft behind us on final approach to Runway XXL. We complied and exited [taxiway]. I gave the 'Flight attendants remain seated' call. Captain called for the rejected takeoff QRC upon exit of runway; which we completed as well as the back page of the QRH.I then called for CFR (Crash Fire Rescue) and requested they take a look at our brakes as well as tires. CFR took a look and told us they read 100 degrees on whatever tool they use for inspection. We did not suspect tire damage after hearing back from CFR and did an after landing flow. We taxied back to the gate and Captain wrote up our rejected takeoff with speed of occurrence as well as why the reject occurred. We then handed aircraft over to Maintenance.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 pilot crew reported that inability to match N1 power settings resulted in a rejected takeoff.