Volvo D1-30 Bogs Randomly

[murphness]

Volvo D1-30 with about 1200 hrs starts and runs great. At the end of last season it started to bog after operating for an hour or so. I would drop the throttle down close to idle, let it sit for a a few seconds, then throttle back up to cruising rpm (2200). Sometimes it'll bog a few times an hour, sometimes it'll go hours without bogging.

This year I started letting it go to see what happens. On one occasion it stalled, but for all other instances, it'll drop to about 900 rpm then climb back up to 2200 after a few seconds and run fine. The frequency is about the same as last season. Sometimes it'll be fine for hours of run time, sometimes it'll happen several times an hour.

I have changed fuel filters and racors with no difference in performance. The racor bowl shows clear fuel, no water, no debris. I've confirmed it's not pulling a vacuum on the tank (open fuel cap when it happens and no suction, doesn't fix the issue).

Anyone have any suggestions? I'm assuming it's growth in the tank at this point, but should I check anything else? Fuel pump maybe? How would I even test for the fuel pump?

Cheers,
Murph

[DougR]

Most likely it’s a restriction in the fuel pickup system, such as debris at the base of the tank suction tube, a plugged screen on the suction tube, debris or thread tape at the 90degree elbow on the top of the suction tube, restriction at the inlet to the rancor, etc. Any restriction such as these will be unaffected by opening the fill cap.....

Start at the lift pump and work toward the tank, checking every fitting, hose clamp, etc.