How to increase max extruder temp in repetier host?

I would agree with Carl maybe you could benefit from doing a PID autotune.
http://www.soliwiki.com/PID_tuning

I have the setting at 280 but 240 is as high as it will let me go? not sure why?

If you are having to print at that high of a temp then your real temp is not being reported correctly. You should never have to go above 220 or 230 at max for ABS. If you go above 235 it will cook the filament in the heat zone of the head and cause it to clog and it is a royal ( as in disassemble) pain in the A$$ to clean up.

I would get a thermometer that uses contact and can measure that high to confirm your head is reaching the actual set temp. The IR non contact thermometers don't measure the temp correctly because the source is so small.

If you must go higher then in RH Click on printer settings on far right top. And then click the Extruder Tab. The max settings are there. you can also edit them in the EEPROM data.

Sorry, thought I saw it there but your right. You can still set it in the extruder tab, although it is ill advised.

Hansen- I do not know which printer you have, but the max temp is hard set in your board's firmware. Simply flashing the firmware to a higher setting will not help since a stock PEEK hotend will become damaged as wardjr stated. Upgrading to an all metal hotend like an E3D v6 will require some simple assembly and updating firmware (3 lines to be specific...one of which is max temp up to 285c).

I have been using E3D exclusively for over a year now. I have only had one clog...but that was sleep deprived user error

On a sidenote, what material is it clogged with? ABS can be dissolved in acetone, PLA will require a torch without the ability to heat more than your current setup.

I am not a chemist. But your concept of chemistry, safety, and practical thinking are not as firm than mine.
http://hackaday.com/2013/06/07/smoothin … ted-parts/
After reading the link, read the comments. Chemists chime in.

Tetrahydrofuran has, in studies, been linked to lung cancer, brain cancer, skin cancer, and all the happy go lucky cancers we all enjoy.
Even from a consumer's point of view, a less than comparable vessel of THF costs $20. 600 grit sand paper and a hot water rinsing are far cheaper and safer than boiling a toxic chemical. Whereas certain sates (NH, NY, GA) state there is no link between THF and cancer causing carcinogens, CA (no surprise there), WI, AZ, NM, MA have conducted studies with correlations between cancer and tetrahydrofuran showing a substantial link.

Napalm can dissolve them both. Why not recommend that?
Remember...machines like Makerbot, Solidoodle, Da Vinci, Prusa are not being marketed to manufacturers, but home hobbyists. Please bare this in mind before recommending harsh chemicals. At no point in this forum is acetone described as safe, so don't compare in a pleasant approach. Why not post safety links or studies on worst cases scenarios?