dowhat

I have an HP500 carb engine and through the season I have been steadily looseing RPM at WOT from 5000 down to 4700. What are somethings that could cause this.

blown formula

could it possibly be that the hotter the air gets as summer progressed the A/F ratio changes & leans the engine out, reducing the rpm??? I have also noticed the temperature of the water has an effect on rpm/speed also due to temp/density. Also, a ding/bent prop? Boat bottom crud? not trimmed the same way? a million reasons???? Just possibilities, no single answer....

Mr Gadgets

dowhat,
There are number of things to look at. First is the motor still healthy? Is there any added drag to the hull? Is there any added drag to the drive line.. outdrive? Did you add weight to the boat at all? Has it sat in the water all summer? Take it one step at a time. To check the health of the motor, do a leakdown test and a compression test. Thats a start.. Let us know what you find.. This is a learning process. The more you look at it the more you will learn about how it all works.

Good luck

Dick

blown formula

& oh yah.... pull the anchor in!!

NEVRENUFF

Lots of the guys I boat with (fresh water) complain about the accumulation of junk on the bottom of the hull throughout the season. I've heard them say they lose 3-5 MPH over the summer due to it. I trailer mine in and out so I don't see the same problem.

I'd say the grunge build-up along with water temps mentioned could be the contributing factors. Especially if you leave it in the water over the season.

my .02

HyperBaja

Ask just once about when we did that at the Alum Creek shootout...

could it be?
-plugs check gap, it will widen
-plug wires, will break down
-coil
-carb settings
-air temp

just guesses, fill us in on what it is.

dowhat

Thanks everyone, I am going to take it in to get it checked out. The only difference that I can detect is that it is starting to run rough when idleing out, and when I first get on it, it starts to cough and choke abit, and then it smooths out,,

Is that carburation you think? HP500 Carb..

GETTINBYE

Dowhat,

Early HP 500's (pre sometime in 1999) had a valve spring issue. The inner springs were allegedly breaking by around 200 or so hours. I have a pair of late HP 500 carbed with approx. 300 trouble free hours. My fingers stay crossed. Merc. made a valve spring change with the introduction of the HP 500 EFI that was carried over to the remaining carb models. At the same time they went from a low rise riser on the exhaust manifold to one of a higher rise to help with water reversion. If yours is low rise you may have an early model with a spring issue. This would cause the RPM that you speak of.

Using the parts look up feature on Bam Marines Mercruiser Parts section and you engine serial #'s you should be able to get an idea of when yours was made if you do not already know. You did not state what year they / it is.

My .02, good luck.

Be Safe
Mark

GETTINBYE

Dowhat,

You posted more symptoms while I was typing my post. They indicate a valve train problem. If it was mine at this point I would check the valve springs BEFORE running the boat again. May save you lots of money on the repair.

Be Safe.

Mark

paradigm shift

dowhat I would check valve springs. Carburation could be part of the cause but like gettin'bye said 500hp carbs motor are known for breaking inner springs. Check them now.

Gettin'Bye you have good advice you shold follow it. Be worth the piece of mind I would think. Springs are cheap compared to the alternative!