I own a stock 85 720 with a Hitachi carb which is expressing a confusing array of symptoms. Thanks to fastboatman for his deep insights, but the forum does not seem to have addressed the fully array of problems I am experiencing.
The truck will sometimes idle smoothly at 1000 rpm, which is the lowest I have achieved to maintain idle. Sometimes it will idle at 1750-2000 rpm, and other times it will simply drop out. All this on the same five mile trip.
Choke seems fine-snaps shut in the morning, and almost fully opens when warmed up. The anti-dieseling solenoid is energizing (yet sometimes the truck will diesel if it has experienced high idle).
Float bowl looks good. Timing is set at 3deg BTDC. No vacuum leaks. The idle set and the fuel mixture have extremely limited response. I can increase idle speed, but not decrease. Fuel mixture has no real effect. At the home, I achieved 800 rpm and smooth operation, but when I punched the throttle, it stalled immediately.
I really do not wish to re-build this unit. I have another I can replace from my former truck which was rebuilt. The issue here is this carb has been sitting in the garage for three years-not sure about the seals.
Any suggestions would be helpful.