I have been following this so far. You fixed the water flow issue. Good work. The surge and stumble issue would have me looking at air leaks and..Vacuum leaks. A simple test for a Vacuum advance leak: Pull the vacuum line off the carb. Apply a vacuum(yep, suck on the line) and block the line(yep with your tongue). If the Vacuum holds(you can tell) it is OK. If it does leak(and you can feel it leaking) it needs to be replaced or the fitting at the advance is leaking. When the carb is apart(Rochester) make sure the "power valve" piston is free and will move in the cylinder. A caution: The ball bearing in the top of the stand off( the one on top of the power valve must be one of the smaller ball bearings from the kit. The bigger ball will fit(sorta) but it will stick in the cover nut(not fun to remove) and the power valve will not work. I have seen some with the top of the piston rod deformed. Again not good(it can be fixed but is a pain) Setting the float is not too hard. Parallel to the cover. Drop set as instructed and then the close point. Again watch the floats move, they both must clear the gasket and...not rub on any thing. Not a lot of room but a bit of tweaking and it should be good to go. After all is done recheck the measurements A lot can move while you focus on each issue alone. Note: Some carbs require the gasket to be under the standoff(power valve support?) and some, do not. A gasket under when the carb does not need it will make the standoff tilt, or be too tall. No gasket under the support when it is needed will make the power valve non- working(only a small vacuum signal to move it. All of these things can cause the surging and stumble you have described.
Does the heat riser valve move? It is in the exhaust manifold and helps the intake get warm. If stuck open it takes forever to get the intake warm( driveability sucks) Cold intake allows fuel to puddle and (the mixture is unstable), can cause some Stumble and lumpy idle issues. If it is stuck closed, you will over heat the intake and most likely "vapor lock when the carb overheats. It must move and the "spring" should open the valve as it warms up. One more thing to look at: The base gasket and isolator(plastic thing between the carb and intake manifold). Is the isolator in good shape? Some get cracked(I have no idea how this happens) if it is cracked find or make(if you can) a new one. If it is chewed up on the mating surface(either side) you can clean it up with a plate of glass and some sandpaper. The base gasket, in most cases has a notch or 2, sometimes 4 on the inside of the big intake opening. This must line up with the hole in the base of the carb. Turn it around or flip it over until it matches. Then install so yo know where it fits best. Idle mix screw is set about 1 and 1/2 turns out to start, then adjusted for max vacuum( book says to turn in till RPM declines or it stumbles then back out 3/4 turn) Most of the time the setting with the gauge is very close to the book procedure. remember, the above long winded applies to the Rochester B only. With all of those things out of the way, I would look close at the exhaust leak. Not critical all by its' self, but it might be as good a time as any to replace the gasket. Watch for the alignment rings you will need them to reassemble. The last thing I would look at is the points. Spring tension good? Gap good? Is the rubbing block stable? Contacts clean? Simple stuff I know. But it all matters. Post any progress, and know these and other suggestions are to help as best we can.
