lost rpms

I have a 1966 merc 650 65 horsepower outboard motor.I have been getting about 27 mph at 4800 rpm.The last couple times I had my boat out it barely would plane and at wot I only got 3150. I have a new fuel line from tank and new primer that pumps up and stays firm,the motor fires right up never dies till I kill it.I was looking at the tank tonight and the vent screw on my gas tank was screwed shut and I am not sure if when it was screwed shut that was where my problem came from,does this sound possible? would you lose all that power from that? thank you all in advance for your time and information.

Re: lost rpms

Hi mercuryblues<br /><br />That vent cap can have a dramatic effect if you don't open it. At first glance I would say that you may have solved your own problem. Check that first before you start to "adjust" things on your motor.<br /><br />Greg

Re: lost rpms

I am going to take it out tomorrow after work just to see how it does,I will post results,and thank you for your reply.

Re: lost rpms

Air needs to get into the tank to displace the fuel used and if the valve is shut, a vacuum will be present restricting the flow.You would get limited supply to the motor. Agree with Greg

Re: lost rpms

If opening the vent cap gets you back to where you used to be you might want to look at another prop for that engine. I believe 4800 rpm is low and you should be closer to 5500 rpm. Your engine is old and I don't know how good its compression is but you want the engine to be able to reach max rpm at max throttle.

Re: lost rpms

Ok guys,I took it out and it ran just great with the vent open of course,I will never do that again.I played with the trim/tilt of the motor while I was out I have to pull the pin and move it up or down that way,but I pulled it up all the way and if from a dead stop I went full blast like I was pulling a skier or something the motor would"cavitate" I guess you would call it the motor would rev up but not go anywhere,but if I slowly accelerated to about half way then go all the way down it would just fly,my speedometer is not working so I do not know how fast it was going but I am sure I got another 5-6 mph like that,also my rpms were up to 5000 at one point and I have never seen it get that high,so I think I am back in business,just out of curiousity can I hurt anything by having my motor set like that? It seemed to lift the whole boat out of the water and really move,it was smoother and everything.

Re: lost rpms

Put the trim pin on the motor bracket assembly to where it looks straight or a little bit nose up along the waterline. Much more of nose up and you lose performance. Watch out for shallow water. A handheld GPS is a great personal life vest. It also makes a straight line between 2 points. Some fuel tanks dont have vents. The screw top has to open a turn. Catch a fish and Keep us posted.<br />RE<br /><br />Cavitaton is where the prop bites air from the hull. The prop is eating bubbles over revving the power head.

Re: lost rpms

Thanks again guys,you all are great