HP 500 "Missing and Sputturing" Please Help

[checkrocket]

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. The missing goes on for a good one or two minutes. Once it goes away the boat runs great. Need Help or advice

[Jordy]

In your batch of the usual things did you replace the fuel filter(s)? :D

[checkrocket]

Fuel Filters were replaced at the begining of the season. Would they cause this missfireing between those loads only? Wouldn't it do it at idle and 4000 +RPMS?

[rude235]

i run an '01 500efi. i had a similar problem at the beginning of the season but once i burned the old fuel out of it the problem seems to have gone away. you could try running some octane boost. i was also going to suggest maybe some injector cleaner, but you've got the carb version right??

[powerplay230]

Had a similiar problem...-but mine was just under load at same place and then only at full throttle at 5K+. Ended up being exhaust manifold/riser leaking water.

[Dave C]

I had some issues on my old one.
try taking off the air cleaner for a quick run or cleaning it.

[checkrocket]

My HP500 is the Carb version. The flame arrestor looks good and clean. Someone suggested a rebuild on the carb, but it wouldn't just go quickly like it did. Would it??? 130hours doesn't seem like a lot for the carb to need a rebuild. Also why does it run great 3000rpm-5200rpms once it catches and kicks in. Need Help

[checkrocket]

Does anyone think the Carb needs a rebuild?

[checkrocket]

I am looking for more responses, I am counting on the experts

[celeb1]

I would first check your fuel to make sure there is no water contamination, check your fuel water seperator. It could also be ignition related, might want to go over your ignition components.

[checkrocket]

Fuel is good. I checked the water seperator and I pumped off some gas into a conatiner and checked it. All looked good. Like I said, the boat starts and idles beautifully. I don't think it could be ignition related. New pulgs, wires, cap, rotor. Does anyone know anything about timing advance getting stuck??

[redi4fun]

If ignition and fuel systems are OK it may be a problem with the choke. :idea: Good Luck

[spectras only]

I've had surge problems ,and thought it was the carb. I have the MSD pro billet distributor and I've found both advance springs broken under the plate. The springs looked like new [ springs rust out by moisture sometimes]though. Replaced the springs ,no more surges. You may check into the carb though ,the booster jets get fouled by old gas .

[shirkey4750]

Had the same problem in my 26' Rinker, and found out that the coil was getting hot and causing the voltage to drop from 8.9V to 5.6V on the downstream side of the resistor thus causing a low discharge to the plugs and not burning the fuel properly and causing the cylinders to load up. Would idle fine till you put it under load. Solved it today.

[vandeanoo]

I had same problem. It was caused from a water leak where the manifold bolts to the riser. Was getting the plugs wet.

[checkrocket]

All of this and the new set of wires that I got from Mercury one of them had a break in it and was causing the problem. They sent me a new set. I changed it and all is perfect again.