Ok guys I just bought a 99 skeeter 186sx 18.6 feet with a 2001 Yamaha fuel injected vmax 150. Boat runs strong but will only do 59 the guy I bought it from says it did 63 before he got a hot foot installed on it. My question is even if the throttle is only opening 90% shouldn't it still gain Rpms but slowly ? Also it has a 3 blade 25 pitch prop and it gets 5200rpms should it get more?

The answer to your question is no. An engine is nothing more than an air pump. The power it produces is limited only by the amount of air it can take in, or get forced in by superchargers or blowers.

Unless the butterflies are wide open, airflow is restricted. The math won't work out perfectly, but, if your air inlet is only 90 percent open, the motor will only generate 90 percent of its maximum power.

Thus, your 150 hp engine produces only 135 horsepower.

There has to be some adjustment which will allow the throttle to open fully.

Further, did the boat do 63 with as much weight as you have in it. Did it do 63 with a full tank of gas, or a nearly empty tank? Do you have the motor trimmed the same as when it did 63? There are a whole lot of factors which affect boat speed. Two boats can weigh the same, be the same hull and have the same power, yet one can run away from the other because of setup or weight distribution.

You titled this thread "Prop?". Did you change the prop?

The only way the hot foot can affect it, is not opening the throttle, it should be easy enough to pull the cowling and have someone fully press the hot foot while you check and see if the motor is going to throttle stop, or use a long stick, press it yourself and watch it.

No the motor will not make full rpm under a load if the throttle is only opening 90%, doesn't matter how you hold it.

As for your speed versus the prop pitch, it's going to depend on what gear ratio your motor is and how your boat is setup. If it's 1.82 then I would think you are is pretty close to being an obtainable speed of 59 mph, that's about 13% slip, a very common average for bass boats, depending on setup, it could be a little more or little less. To get 63 with that gear ratio and prop, you are at 7% slip, that's not very likely. Now if it was turning 5,500 rpm at 63 that's more realistic at 12% slip.

If it's 2.0 then the speed (59mph) is very generous, that's only 4% slip, this is down in ultra light tunnel hull efficiency, not many bass boats are going down the lake with only 4% slip. Even if it was turning 5,500 rpm at 63 mph, that's only 3% slip and I doubt that will ever happen.

From what I could find on the info you've provided it looks like your motor has a 2.0 gear ratio.

NOTE: I made some changes if you read this before this last edit.

Also note,it's going to run a few mph faster when the water gets cold.

59 or 63? Really, you should be happy with your set up as long as the hot foot is set up correctly. Those speeds seem very typical for a skeeter running a 150.

Chasing those extra couple of miles per hour in excess of that will cost you big $$ .

Just my .02

There's a bolt behind the pedal that's a little over a quarter inch in the air would it be safe to adjust it all the way down?

NO!!!!

DO NOT ADJUST that bolt until you check the motor and see if the throttle arm is making it to the stop. Then you only adjust that bolt enough to get throttle arm to the stop and no more. If you go beyound that point you WILL break something, the cable the, the hotfoot etc. Something is gonna give and you ain't gonna like it.