1991 Yamaha 5.0 YEMS carburetor

[xanadu3000]

Hello all; Just bought a 19' Dynasty open bow with a Yamaha 5.0 V8 Yems inboard motor. I was told that the boat sat for almost 2 years (with a full tank of gas) and the gas may be bad. I took it out anyway for the first time, and the motor started fine, but would not take any throttle; it would just bog down and die. So, I removed all of the old gas and filled the tank with fresh gas, and it runs a little better. However, when I give throttle anything above about 50%, it begins to bog again, and I have to throttle down. Therefore, I suspect it is a carburetor issue. I removed the flame arrestor to inspect the carburetor, (please bare with me as I am an extreme novice with engines!) and found that the front valve (I believe it is the choke valve) was not tightly closed, and that the spring that is suppose to hold it closed seems to maybe be worn, or lost its ability to fully close the valve. I am wondering if this could be the problem, and if anyone has insight into this sort of issue?! Thanks fellow boaters!

 

[Maclin]

Re: 1991 Yamaha 5.0 YEMS carburetor

I think you have a Rochester 4bbl, so if not the rest of this post is irrelevant.

Looking down at the top of the carb there sill be the small primary barrels at the front and the much larger secondary barrels at the rear. The Secondary barrels are covered by a spring loaded plate call the air valve plate, this controls how the secondaries are brought into play based on airflow demand across the plate, you will notice that the hinge is off-center. The trottle movment opens the butterflies at the bas of the carb but the airvalve opnes only when enough the engine demands more airflow.

The smaller Primary barrels are what the engine starts with and runs on at idle and then obviously all the way thru the RPM range, but that is where the choke plate is. The choke plate should be closed when the engine is cold, then it gradually opens as the engine warms, is it not a spring loaded mechanism like the other larger Secondary airvalve plate.

If you are decribing the action of the SECONDARY air valve then that could be the source of a bog. Too much air too soon and the engine's demand cannot draw enough fuel in to cover the too-lean condition of the open secondaries. They need to open gradually as engine demand increases.

If you are describing the much smaller choke plate on the primary then that HAS to be fully OPEN/VERTICAL when the engine is warm. If the choke plate is not fully OPNE or VERTICAL then the engine will not run very well at speed.

 

[Maclin]

Re: 1991 Yamaha 5.0 YEMS carburetor

The terms "front" and "rear" refer to the carb itself.

 

[Capt Ken]

Re: 1991 Yamaha 5.0 YEMS carburetor

The choke isn't suppose to be closed after warmup. Remove the carb and have it rebuilt.