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Have 1989 Yamaha 225 HP V6. Starts & idles great. Will run 2800 RPM @ about 20 MPH with no issues, if given more throttle it runs for a few seconds and motor dies off as if being de-throttled or running out of fuel. If pump primer bulb it helps a couple hundred RPM but only if constantly pumped. Have installed new fuel lines from pickup in tank, new back flow, new primer, filter, water-fuel separator, new filter on motor end, new fuel pumps w/gaskets, new lines from pumps to carbs. New plugs.
Checked piston to match TDC mark on fly wheel & used timing light to ensure firing 4-8 degrees ATDC.
Checked cylinder compression
1-92
2-105
3-110
4-110
5-95
6-95
Placed pressure gauge between pumps & carbs, at idle is 2-2 1/2 PSI. As engine RPM is accelerated with just throttle (not in gear) the pressure drops on pumps to near 0.
Installed electric fuel pump 4-7 PSI before primer bulb, this lets engine run around 3400 RPM @ about 29 MPH, but if try to accelerate faster the engines dies off again.
Have carefully checked the entire fuel line for possibilities of sucking air, it doesn't.
 
Those compression numbers are a concern.------I would be taking the head off to look at # 1 cylinder.----Does spark jump a gap of 3/8" on all leads??----Installed a new water pump impeller ??
 
The #1 does cause some concern, but don't think that would cause die off of motor. Now the water pump impeller is a very possible suspect as the engine is controlled by YMIS, and if at high RPM not getting enough water could trigger that sensor. Thank you very much for input, I will look at impeller first.
 
Racerone, I appreciate your help, I just bought this boat & motor. The motor seemed to run and sound fine. Tell tale was spewing water and seemed to shift in out of gear ok, even took gas fine while on a water hose. It was only after getting it into the lake this acceleration issue popped up. So I am ordering parts to replace water pump. Will let you know the outcome. Thanks again.
 
About 50 years ago , I heard it many times working at a boat shop,-----" i just bought it " -----Seems it is still true today !!-----And they all sound good on a hose.---Willl bark and roar running on 4 of 6 cylinders.
 
It is running on all six, just seems to run out of fuel or maybe the YMIS shuts down some of cylinders after it reaches temperature.
 
But you are right, should have insisted on a test run in water, but boat is a little different than test run in a car. Buyers beware got me😞😞
 
I ran into a similar issue with my sons boat. He's a pretty sharp mechanic, but his 90 Johnson was kicking his butt. I was helping him one day and while solving the world's problems over a couple of beers and leaning against the transom of his boat, I kicked the propeller on his motor just to make it spin. It didn't! I could barely move it by putting my body weight on it. The foot was all but completely locked up from seals leaking and watered down oil in foot. It was so locked up that it would actually shut the motor down.
 
I would remove the cylinder head to inspect #1 cylinder walls.-----There is no amount of fiddling that will compensate for a motor in poor condition ! have a shop inspect cylinder walls.-----And oops it looks like head gasket is not available.----Shame on Yamaha !!
 
racerone said:
I would remove the cylinder head to inspect #1 cylinder walls.-----There is no amount of fiddling that will compensate for a motor in poor condition ! have a shop inspect cylinder walls.-----And oops it looks like head gasket is not available.----Shame on Yamaha !!
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I found the problem. Previous owner had been into carbs, all new stuff inside them & were really clean. But on inspection of float levels that appeared to be correct I noticed metal flange the actual float mounts to seemed bent. They were! Instead of bending the little flange tab that makes contact with the needle valve to adjust float, they had bent that part of float. Ordered new gaskets and readjusted floats. Runs 5000 RPM WOT. lots' of power. I am some concerned with the #1 cylinder compression, but as you indicated the head gasket is an issue. Have to do some research on that. Thanks for the input and advice.
 
Also the throttle positioning sensor is no longer available. Typical of manufactures, Had some top officials in Harley Davidson company state concerning old parts for their bikes "We are in the business of making and selling new Harleys, not keeping antiques on the road. Find parts aftermarket if you can"!
 
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