Won't run at low rpm

About 4 years ago I bought a used boat with a 2000 Johnson 50hp with VRO. Seller said it sat for about 2 years and it ran rough. I pumped out tank put in fresh fuel and changed plugs and it ran well enough to use for about 18 months. Had to run at about 1500 to 2000 rpms until it warmed up then was ok. Then one day it just plain quit on the water. I went ahead and rebuilt the carbs and purchased 6gal tank and new hose, fittings and bulb. After the cb rebuild it ran pretty good on flush device. Next day in yard it seemed to gradually go back to the same routine of stalling at low rpm. Checked and replaced all fuel lines still no good. Replaced plugs too. Threw hands up and took it to mechanic. He rebuilt carbs and drained main tank and hooked up to that. Said it ran great. He put in non ethanol fuel which I have done as well. I got home and it ran pretty good, but stalled several times at 800 rpm idle. Next day started and it progressively got worse. I went back to small tank as I can assure fuel quality. I let the boat sit for about 8 months due to surgery, but drained carbs. I have spark across half inch on spark tester, replaced a fuel connector but it still does same thing. Compression is good in both cyl. Starts, will run at higher rpm, lower throttle it sputters, giving more throttle usually prevents stall. Any thoughts?

Re: Won't run at low rpm

Sounds like you have something working its way into the carburetors..... you or the mechanic works on the carbs, then the engine runs okay, for awhile anyway. If you don't have a good quality water/fuel separator between the fuel supply and the engine, I'd suggest you have one installed.

Re: Won't run at low rpm

I am using a new 6 gal tank with new line, bulb and connectors. I put in a new separator when I rebuilt carb and mechanic did the same when he rebuilt and cleaned out under deck tank. I may tear off carbs again, but I don't believe they are the problem. Every time I drain them the fuel is perfectly clean and it is fresh.

Re: Won't run at low rpm

I had a motor recently that acted the same way. It turned out to be a needle sticking in the seat in the carb and it was only running on one cylinder. Did you try taking the plug wires off one by one to see if the engine rpm's progressively drop. That is how I figured out I was not running on one cylinder.

Re: Won't run at low rpm

I didn't do that for fear of damaging electrical components. I did, however, see if it would fire when testing for spark with one plugwire disconnected but it never fired for either cyl doing that? From what I read the wire needs to be grounded.