Question on 1975 KA Carburetor Proper Linkage Adjustment

[bpadowicz]

I have a 1975 50HP Mercury 500 with 2 type KA carburetors.The motor was running fine and quit when tanks switched on a 20 mile trip. Couldn't get restarted. Rebuilt the 2 carburetors and fuel pump. Motor ran about 10 minutes on a hose, quit, and wouldn't restart. Water pump good, perfect compression, and strong spark.

In checking carburetor butterflies I found that when fast idle lever moved from minimum to maximum there was ONLY ignition advance and carburetor butterflies were still closed.It appears that this might be correct since there were small holes in the butterflies to allow some airflow when closed. Is this correct.

Wanted to resolve this question before removing the carburetors and disassembling them again. These particular carburetors have "idle tubes" with very small passages and holes; that could possibly still be clogged.

[boats.net]

Carb butterflies should open as throttle is advanced, recheck throttle linkage where it fits against bell crank on top carb.

[bpadowicz]

Per boats.net response.

The carb butterflies do open as the control lever advances the throttle.

My question is "should the carb butterflies open as only the FAST IDLE LEVER is moved"?

Previously I carefully adjusted the cables and the the motor was starting and running well. I would have to advance the fast idle lever fully for the motor to start; but after the motor had warmed up it would idle in when first shifted into gear without any further movement of the operating lever (the fast idle lever has to brought back to minimum to allow shifting). Since the motor was operating properly I didn't look to see when the carburetors started to open with throttle advance.

I know I need to remove the carburetors again and re-clean the jets and passages. But I was trying to find out if these particular carburetors were designed to provide an idle fuel mixture to the engine with the butterflies closed? This is the first time I have seen carburetor butterflies with holes. Perhaps the holes in the carb butterflies and the idle tubes are supposed to provide an idle and fast idle fuel mixture?

[boats.net]

Carb butterfly do normally open a small amount using fast idle lever, that would be an cable adjustment. Yes carb will supply fuel/air mixture to engine with butterflies closed.