While turning engine over gas flows from overflow pipe

If your Utica is a high efficiency 90+ condensing furnace, I suspect not too much is wrong. A condensing furnace removes so much latent heat from the burned fuel that it condenses the moisture out of the flu-gases. The result is a very low flu temperature, AND water.

The water must be flowing correctly down the proper drain though, or you could suffer damage to the furnace itself. If there is an 'overflow' pipe and it is running over though, the drain downstream must have an obstruction. Clean the drain out and the overflow situation should go away.

Black smoke indicates that your mixture is way too rich. Now, you didn't say what carb you're running, stock or aftermarket. But, what it sounds like to me is the needle and seat in the carb is leaking fuel and overfilling the float bowl. When you're running down the road, you'll using enough fuel so that the float bowl does not overfill. When idling, you don't use as much fuel, the bowl overfills, and fuel flows into the carb creating a very rich mixture. Is you overflow hose that connects to the bottom of the float bowl connected, clear and open? If so, turn the petcock on without the engine running. If the overflow is clear and the needle valve is leaking, you should see fuel start to flow out of the overflow hose. This is very important that it do this. If the overflow hose is blocked and you forget to turn your petcock off, you could wind up with a couple of gallons of fuel in your engine. It will leak out of the float bowl, into the carb past the intake valves, past the rings and into the engine. I've also seen it get into the primary drive case as well. I'd put a new needle in the carb and check the float setting according to the book. it's sets with the carb on an angle if the carb is the original Keihin CV.

Good Luck
Steve

exhaust gas coming thru the resiovoir, sounds like a head gasket

Check temperature off the water for proper functioning thermostat. Look at set points on heater's thermostat. It's made to let water out when water get's to a certain high temp. More than likely it will be the pressure relief valve going bad and not holding.

T&P releif valve can be replaced on heater. Turn water off and remove with a pipe wrench. It will come out and any hardware store will carry it. T & P pressure relief valve is 3/4 threads.

When installing new one use teflon tape on threads.

The most common overheating problem with the Northstar engine is a blocked purge line. It's job is to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank(not the cap),this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.The hose goes in to the engine to a crossover and comes out the other side to near the thermostat housing,the blockage may be there.

There is most likely corrosion on the aluminum or steel connector pipe for the hose, you must drain the coolant and remove the pull start housing to gain access to the clamps for the hose, unscrew and remove the hose and clean the pipe or pipes connections with a wire brush and sand paper strip to remove any corrosion on the aluminum or steel tubing, also make sure the inside of the rubber tubing is clean I would just replace the hose while I was in there. Replace hose, tighten clamps, and add coolant to proper level. Start and check for leaks.

engine cooling should be a close circuit. there should have not over flow. however, the is a over flow container in the from of the car to catch any overflow.
the little tube may be the overflow tube that suppose to go to the overflow container.

not sure of the exact height, but if you hold the carb level with fuel hose attached and pushing float up with finger, turn on petcok and slowly lower the float until it is just above horizontal. at this point, gas should start to flow out of the needle & seat.

if float turns off gas too soon, then engine will starve.

if float doesn't turn off gas, then it will flow out of the carb overflow tube.