Marty,
Most likely it is an RK15.28 pump.
Does it look like this pump in the breakdown:
http://arnorthamerica.com/newbreakdowns/Pumps/rk_1450.pdf
The RK15.28 is used on both 4 GPM @ 4000 PSI units 1450 RPM,
as well as 4.8 GPM @ 3000 PSI 1750 RPM.....
Now might be a good time to consider upgrading pump,
either to a TS2021 General or an AR XWM2128.
You can upgrade engine at another time but already
have a higher volume pump. Use Hydro Tek pump rails,
Russ should be able to sell you any of this.
http://arnorthamerica.com/xwseriespumps.html
If it were me I would go with the
XWM2128 now
(
with Hydro Tek pump rails) and later a 21 HP Vanguard)
Then with the addition of a new unloader you have 5.5 GPM @ 4000 PSI !
I see you have Moore in your signature...... JHow is the recovery coming along?
Just thought I would mention that Hydro Tek and Alkota matched our $100 donation
for every machine sold between 5/24/2013 and July 1, 2013..
Here is a link :
http://www.powerwashnetwork.com/shoot-bull/12271-how-help-oklahoma-tornado-victims.html
http://www.easycleansystems.com/PDF_Folder/Buy%20Now%20Help%20Moore.pdf
Rest assured with Hydro Tek you have a great pressure washer, and although I have never personally met
Russ I can say he has an excellent reputation.