Briggs 16HP Vanguard carb / fuel problem

That carb has had water inside it. See that green corrosion on the needle? Looks to me that needle might not travel up and down proper. You need to clean all those parts in CLR or similar. Remove all that corrosion.

Outside of the carb is filthy too. You said you boiled it clean? It's filthy. Might be an air opening of some kind caked up with oil and grass not allowing the carb to breathe.

You've missed something I feel. We are at 10 pages now and the thing still has issues.

That carb will need a CLR bath if it will even come clean. Carbs like those (corroded) are tough to get going again.
 
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As slomo says:

That green corrosion not coming clean from your boiling and cleaning is also a clue that the passages inside are not getting clean plus the outside of the carb is not even cleaning itself.
That green gunk has most likely hardened inside a small orifice and restricting a passage.

If the CLR don't get it loosened up eventually try a 50/50 mix of vinegar and warm water, soak the carb for 30 min's, send some air through the passages and the outside surfaces, brush the carb with a stiff paint brush or toothbrush, then back into the warm mix for about 45 min's, rinse again, blow dry the passages, and as a final add a spoonful of baking soda to water and brush and rinse the carb good. You can gently clean the needle with the tooth brush and a cleaner. do not get aggressive with the vitron tip.
I suspect ethanol gas has went bad inside that carb, seeing the blue corrosion everywhere on the internals.

Vinegar is a acid and if not neutralized with baking soda it will keep chewing on the magnesium/aluminum and the carb will feel greasy and turn black. No major harm but just do the neutralize thing.

The 50/50 mix of white vinegar and water will not harm the carb, I use it in Ultrasonic tanks for carbs and sometimes have to make several runs in heated vinegar/water mix when I get into one like yours and I mainly use the baking soda rinse to eventually stop the acid reaction on the external body of the carb causing it to change color and remove the greasy feel.
What it really needs is a good UltraSonic cleaning.
The linkages on them Amazon clone carbs is going to be wrong for your throttle and choke. Only thing a cheap one would be good for is to just bolt on and operate the throttle manually so as to see if you get a good run on the engine manually operating the choke and throttle. You need to check your bolt on spacing width and the size of the carb throat opening. These dimensions are sometimes listed when looking at pictures of carb part numbers on fleece bay then go elsewhere in your vicinity to maybe find that same carb.
 
Yes I should mention external photos were taken pre-boil. Its cleaned up the outside well enough but will need the acid / salt or acid / baking soda mix to sort the green, CLR eats brass from what I see.

I've found one Chinese one with horizontal throttle and choke plates and what looks to be the right bolt pattern, but it'll take a few weeks to get here and is still a gamble, also no mixture adjustment or anything on it.

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It would be far better to get this one working - the fault has been completely consistent all the way through, even when it was on one cylinder, so I feel sure the fault is closely related or in something already covered.
 
Maybe try what is it citric acid??? Mercedes Benz people use it to flush out radiators and such. Works but is gentle. Maybe Bert can chime in here with some help. Do some research on it.
 
Well more cleaning with particular attention on the main jet, reaming through with some old wound guitar strings and I think while it looked clear before, there may have still been a slight restriction, is certainly revving better now. Also put more high pressure air through everything again which may have moved something. It's still hunting a bit at high revs, but way better than before.

Thanks again for everybody's help, have learnt a lot and delighted to get it back into usable order.
 
I noticed in your last pictures that it appears that the Low idle jet still has the plastic limiter adjust cap on it even though you got it removed.

If it's not adjusting for a good smooth low idle you might have to remove the plastic limiter to get a full adjust. Do not try to break the plastic off or you might break the jet. Take a Dremel tool and grind the plastic off. Sometimes when the plastic limiter comes off you will have to use the dremel tool and slot the metal top of the jet for a small screwdriver.
I've seen some carbs that just a 1/4 inch turn of the l jet would greatly improve/reduce high rpm hunt or slight roughness but the 1/4 turn would still give a good low smooth idle. (this tweaking is done with a warmed up engine and the breather installed) Do not tweak/fine adjust when cold engine)
Also sometimes a high speed slight hunt will quit when the mower deck is en-gauged and a load is placed on the engine.

I'm assuming that the governor is adjusted properly and their is no slop/slack in the throttle linkages.
 
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