Yamaha 9.9 4-stroke stalls

[hcccs 0]

I rebuilt the carburetor of my Yamaha F9.9BM (6G9 S 008982 Z 1994). It starts really easy but sounds like it's running on one cylinder and when I throttle up the slighest it stalls. I ran it again, wihout hood and when I put my thumb close to the carb intake it reved up. It seems like my thumb acted like a choke and lowered the intake pressure (increased the suction).  Any ideas?

[Mercury_44XS 162]

It seems lika you still having fuel starvation problems.

 

This engine has a "twin ignition coil" and fires (spark) on both cylinders at the same time, but they are in different cycle. Eg. cyl #1 is in compression cycle and ready to fire, but cyl. #2 is in the end of exhaust cycle. Therefore we can exclude ignition problems, except spark plugs, which one of then can be defective.

 

Did you check the "choke plunger" on top of carburetor with two wires?

It is electrically and automatic operated.

This "choke plunger" or enrichment valve decreases AFR at cold start to appr. below 10:1 and the gradually increases to normal levels at 14.7:1.

 

Have you checked compression?

 

If there is low compression on one of the two cylinder, it might increase temporarily when you put your thumb in the carb air intake and choked the engine (decreasing AFR, more fuel, less air).

 

If you don´t have a compression tester, you can dismantle one spark plug at a time and crank enginge by hand on the flywheel and check and compare the resistance caused by compression. Crank flywheel clockwise (RHR).

 

//Pär

 

 

Redigerad 14 Juli av Mercury_44XS