I have a bobcat 843, 1991 with an Isuzu diesel. It recently began surging, the RPMs are up at operating range and it falls back down to an idle with no throttle adjustment, eventually it idles back up to operating RPM and then falls again. It can go from 15 second cycles to 45 second cycles. I already checked the fuel pickup screen, it is clear. I also always notice the fuel filter bowl is full, it never looks empty. If the filter was shot clogged would the bowl empty, is the bowl filtered fuel? It wasn't that many hours ago I changed the filter but I have not changed it since the issue. Any ideas on where to begin troubleshooting?
It sure sounds like a fuel starvation problem.. You could jerry rig a can of fuel straight into the inlet to be certain.. Does yours have the squeeze bulb? Unhook it & see if you have fuel BEFORE the bulb.. Now hook it back up & see if you have fuel after.. Try loosening/removing the fuel cap too.. they have to be vented & they will get clogged sometimes.. I would unhook the bulb & blow back thru the line w/ compressed air.. Good luck.
I replaced the fuel filter and noticed the red wire coming from the lift pump broke from the 12 volt supply. I reconnected it and all is now well. By the way, there is no primer bulb on this unit. I'm surprised it ran at all with no lift pump.
My research showed, gravity fed w/ a primer bulb.?? Its not uncommon but rather the "norm" to install an electric lift pump & remove the primer bulb.. Glad you got it straightened out..