1978ish Thnderbolt 850 wont idle

Hi guys, My 850 has been a great runner for the last few years. Last fall i was busy, so the Merc Mech installed a new waterpump, filters, plugs etc befor fogging her for the winter. I took her out yeasterday expecting her usual robust spring preformance but she wouldn't idle on the hose or in the water. In the water I tried to use the fast idle lever while shifting her fwd and after getting into gear she would die/stall out after 1/3 throttle. Even the fast idle wouldn't run her rpm's up as fast as she did when healthy.
So back in the driveway; I ran her on a vented spare tank with a bulb, more fresh 50:1 gas with seafoam in it(to rule out my supply causing the problems)she run the same as before, I used seafoam spray and tried to clean her out with that for a while but haven't noticed any improvment. No unusual smells or noises.
I'm thinking it could be a deteriorated cable-stretched maybe, or is she out of adjustment, maybe the jets are gunked up too bad. I was hopeing to get some words from our wise ones on here, any help appreciated

Re: 1978ish Thnderbolt 850 wont idle

Check the throttle cable to see if the carbs open all the way at full throttle. I doub't that is the issue, but check it out. Take a compression test and a spark test as well to rule them out. It is likely the idle passages. If you have siebowl carbs you may be able to blow the gunk out of them with some spray carb cleaner. Clean the main jet, the idle passages in the throat, the idle restrictor tube and the main nozzle. Maybe you will get lucky...A better idea is to remove and clean the carbs.