What causes a loss in rpms

[swafia]

Ok I have a 2000 xl800 rated for 7000 rpms I'm getting 5000 so I rebuilt the carbs I haven't tested it yet but I'm not getting 7000 when I rev it on land.and yes the power valves are moving last time I took it out I watched the pv open up

Question 1: should it rev to 7000 on land

Question 2: the thremo sensor if that is bad will the motor go into limp mode

Question 3: if all the above is not my problem what elts could it be

[izz0]

q1 it should rev harder then 7k without load on the prop

q2 i dont know

q3 it couldbe many things

whats the compression come in at?

[redneck71]

both mine will easily hit 7K on the trailer xl and gp800 from the same year,you can test the thermo switch it shouldn't have continuity untill about 169-183 degF,i would due a compression check

[swafia]

Compression 125 on both

[salty]

As Izzo explained, the engine is not working againts any thing, therefore the engine should spin freely and zing right out of sight when it is out of the water. Fortunately, Yamaha installs a Rev limiter to keep this very thing from happening. The engine was not designed to operate at RPM's much higher than what we normally operate at.... i.e. it would destroy itself at high rpms. The 1200 engine has a rev limiter of about 7400 rpms, I do not know if it is the same for the 800 engines.

Common things that would effect your rpms would be the carbs and the power valves. Impeller size, general engine health (hours on your boat) and electronics are other things.

Typically, if you have a carb problem you would first get a base line of what your rpms are so that you would know the minimum rpms that you should have once the carbs were repared/cleaned. If your boat is stock, then you can ask around to find what the typical rpms for a stock 800 would be. Always take into consideration your impeller size when you try to figure out what rpms you should have.

You said that your power valves opened, but are you sure that they opened all the way? The power valves need to be serviced (cleaned) from time to time depending on the number of hours you put on it and the type of 2 stroke oil you use. I once broke a power valve and lost 300rpms. The valve broke on the shaft where the valve and shaft connect. You could not tell that there was a problem until the valve was removed. My point: you must eliminate power valves from the equation when trouble shooting. You do this by making sure that they are functioning properly.

It is hard to diagnose a problem over the net. There are times when we can only point you in the right direction and say "check this and that". There is a method to the madness. Go to the frequently asked questions part of this forum and read up on power valves and carbs. You problem more than likely will be there some where.

Reguardless of where the problem is though, you must have an accurate after market tachometer to help you in your troubleshooting.