MEP-803A surges

Hello everyone,
New owner and to site. I have a MEP803A given to me to fix up and maintain for emergencies, if needed.
The problem I am having currently is it surges up and down after 12-15 minutes of operation. I haven't done much to the unit but filters, fuel and fuel return line. Still needs the small stuff(nuts, bolts and straightening) but concentrating on functionality of it.
The only load that it has seen was a 5000 watts garage heater hooked to it, currently have a HDT 3 ton 36k BTU unit I'm trying to get running for a load for the gen.
Checked fuel lines and flow, checks good but not with a gauge. Any suggestions?

Welcome to SS. When you say surges... are you talking about the RPM of the engine, or the gauges? Does it surge when hooked to a static load (like the 5kw heater) or when hooked to a load that is able to be changed? If you aren't sure... hook it to the static 5kw load and see if it repeats.

Have you inspected the fuel tank for any debris, which could be causing any fuel flow interruptions? Is the fuel pump warm/hot under use?

How old is the unit, and how many hours are on it? What condition are the fuel lines going into the cylinders? Sometimes they swell... which causes intermittent fuel starving too.

Also post a video of what its doing if you can. Give us a tour of the unit in part of the video so we can see general condition.

Please post a film, if you can.