I have an 1999 HP500 (129Hours). It has run great all summer up until this point. Here is the problem, boat starts great idles perfect. Push down the throttle 1500RPM - 3000 RPM the boat is "Missing and sputturing" can't get on plane. temperature is where it should be. 140 degrees. Eventually it will just kick in and run perfectly 3000 un up to 5200RPM. NO Problems. Bring it back down or shut the boat off for awhile. Same thing happens missing and sputturing.
I have done the usual stuff that are obvious, maybe someone has a suggestion. I have replaced the plugs, wires, cap and rotor. There is no moisture in the cap.

If anyone has a good idea please post. Can't waterski, tube or race when I can't get out of the hole, plus I don't want to damage the motor.

sounds like a fuel problem on the primary side of your carburetor (lean misfire)(i'm assuming it's carbed?)RXR your carb, clean and rebuild it and check for restrictions in feed circuits,primary power valve etc. Ive heard ALOT about valve spring problems and exhaust riser problems on these motors but i picture it breaking up at high rpm's if it was broken valve springs and having water contamination in the oil if it is the typical riser leakage problem,Smitty

Would the carb just suddenly fall out of adjustment and need to be rebuilt. 130 hours on the motor, doesn't seem like the carb would need a rebuild with such few hours or am I wrong. Some resonses that I have gotten say it could be a fuel filter problem. I took off the fuel and water seperator. It was fine and it is new 20hours ago. Would the fuel filter at the carb casue this problem that I have described?????

The reason i think it could be your carb(and just like anyone else i'm just making a guess from previous experiences) is that it is in the bottom of your power band and what your describing sounds like a lean missfire caused by not getting enough fuel in the first 1/3rd of your throttle. I wasn't looking at the 130hours being a factor as much as the fact if your motor(i'm assuming from your info it is the original that came in your boat)was a model year 99 it was new in summer of 98 and may have 5-6 years worth of sediment,corrosion,crud etc built up in the carb. To have that low of hours on it there may have been seasons where it just sat or was used very little. I'm hoping that one of the people out there that have same exact motor as you can jump in with experience with same exact problem. Smitty

Could it be the filter near the carb? Is there a way to check the carb without pulling it to see if it needs a rebuild? I just don't have a ton of money to do things that may not solve the problem. Let me know

Sounds like timing or timing advance.

I can't believe with all of the subscribers to this website and HP500 owners that nobody has had this experience happen to them and knows the answer

This is one of those problems where a definitive answer is a little difficult to come up with. There are just going to be educated guesses. Hopefully, one of them will be your problem.
Does this problem happen when the motor is revved up in neutral, or is it just when put under a load?

Just when under load. the Hp500 doesn't have an EGR valve does it? Guess I am going to have to get a mechanic to diagnos the problem. Hopefully I can find someone near the lake so they can go for a ride. can't simulate this in a tank. Needs to be under load.

Exhaust leak or reversion?? Easy to check for just pull a riser and look for water track mark. Water weeping in could foul a cylinder and at higher rpm enough back pressure and juice to clear it up (or not noticeable then) Just a repeaters 1.5 cents...
Good Luck