1978 Johnson 75HP Stinger stalling

Just rebuilt the carburetors because it was running rough at idle. Now it idles great. Motor starts up nicely and idles nicely. When I rev it in neutral it purrs like a kitten. However, when I put it in gear and use the controller it runs terrible. It doesn’t get above 5mph, when I go anywhere near halfway with the controller it bogs down, and it will just die. Starts up just fine again, but in gear it just bogs down. Any ideas? (New gas lines, pump, gas tanks have worked fine… but since rebuilding the carbs I have this new problem)

 

Hi Zingaro, sounds like starving for fuel under load. Check the main jets; have you changed their settings; if vacuum diaphragms used, are they in good condition and well sealed / seated, and vacuum lines good; check your fuel filter for restriction too. If it runs sweet on idle and low revs, does it have butterfly valves out of their settings, or your enrichener or air bleed needle valves not set to the right number of turns open. Could be something simple you haven't done right. At least it starts so can't be too bad.

 

Are the tanks portable or built in?
If portable:
When you have the engines and increasing rpm/load, have someone squeeze the priming bulbs to see if you can supply extra fuel. This will confirm fuel supply and overcome any air leaks
I am not sure if there are crankcase pressure driven fuel pumps on the Johnsons. Older Mercs had them, and the diaphragms used to perforate and starve the carbs
Borrow someone elses newer tank to eliminate some of the following fuel line possibilities:
Some of the older portables had a prefilter that was attached to the pickup inside the tank. You may be able to remove the pickup assembly, usually 4 screws and a gasket on some. You may be able to see the filter through the fill cap. Use a sealed flashlight on an empty tank and do not turn it on close to the cap, the spark at the switch might cause an explosion. Best to do any
inspection of the tank hose and fittings away from the boat and off the water.
Check the fuel hose for leaks, at the connection at the portable as well as at the engine inlet for leaks. The small o-ring inside the quick disconnect can be damaged and
allow air in when drawing fuel.
On the primer bulb, the check valves can accumulate crud and but you can remove and clean.
If there is an inline fuel filter, change it
If there is a bulb, when running, check to see if the bulb collapses as if it is sensing a vacuum. If so then the obstruction/resistance to flow is between the inlet check valve into the primer bulb and the raw fuel in the fuel tank
Link to show the oring replacement on the quick disconnects.

I am assuming that the vent on the fuel fill is open?

If the tanks are built in and the fuel line is "hard wired" ie no disconnects:
By ABYC and Coast Guard rules. Built in gas tanks had a suction operated fitting/valve at the top of the built in tanks so that in the event of a fire, or a broken line, the fuel would not siphon
into the bilge and increase the fuel for a fire. If the tank is steel, (more than likely) ( and galvanized by regulation) the inside can rust and the debris can plug up this very small
spring loaded check valve and limit fuel flow. When the engine fuel pump creates a suction, the valve will open the check valve to allow fuel in. The fittings are hardly noticeable as they are in line. Ensure that the tanks vent line is clear of fuel.

Link to the types of anti-siphon valves
Anti-Siphon Valves & Fittings - Marine Parts Source https://marinepartssource.com/marine-parts/inboard-engine-parts/fuel-systems/anti-siphon-fittings-valves

Ouch for the size of the pickup image!!
Long shot: Back in the day the actual fuel line was made with a neoprene liner, some "rope braid" with an ABS sheath. Occassionaly, the joint between the neo and ABS would separate and plug the fuel flow. Perhaps the ethanol fuel has caused this issue?

 

I appreciate your attention! Just to be clear.. when I use the ‘warm up lever’.. I can rev the motor to the max and it runs beautifully… the second I put it in gear it changes its mood and gets really boggy. So it’s running great on ‘high’ revs, just not in gear. Because of this I ruled out fuel supply issues… but I’m an idiot when it comes to outboard repairs. My friend knows more and we’ll read your thoughts together and see if there’s something obvious. Greatly appreciate your help. The boat and motor are super sweet and I want to show it off! Haha. Z