HP500 CARB TIME CLOCK
Question: I have a pair of 1999 HP500s with carburetors. They have about 150 hours on them, a few hard, but most of them easy. What is your experience with rebuilding the top end on these motors? I have heard of spring and lifter issues and would like to do a pre-emptive strike before the life expectancy expires. I am sure there are more than a few of your readers in this position. What repairs would you perform and at what hour reading?

Shawn Smith
Baltimore, Md.

Answer: The HP500 carbureted engine was provided with a valve spring that later proved to have some breakage problems. It is likely that you currently have some inner springs broken. Moisture in your crankcase from exhaust-water reversion accelerated the deterioration of the valve springs. If you have been running for a while with an inner spring broken, it is possible that the pieces have damaged the top of the valve guide. Let's hope that is not the case.

With 150 hours on your engines, I would immediately have the valve springs changed. Change to the current spring that is used on the HP500EFI. It is a Competition Cams 929-16. The retainers also will have to be changed. The correct retainer for the new setup is a Crane 99961. The valve lock is the same, which is a Crane 99098. The valve seals are a PC. Mercury requires that the tensioning spring on the lip of the seal be removed on the exhaust valves. The best installed height for the valve springs is 1.880". The lifters should be OK for a while, unless you find other damage that could accelerate their wear.

On some installations, the HP500 with the GIL exhaust system has the tendency to revert water, which is one of the most common causes of damage to these motors. If you see signs of moisture in your motor when you have the valve covers off, you should take steps to prevent the water reversion.

Later model HP500 carbureted engines used a higher-rise exhaust elbow to help minimize the problem. On some twin installations in smaller boats, the exhaust risers are too close to the water line, which makes it easier for the water to revert up the exhaust. Mercury published a service bulletin that required that all HP500 carbureted engines be adjusted to not idle below 750 rpm in gear. When the engines are idling slower than that, the reversion is more likely. If you cannot idle at wakeless speeds at 750 rpm, leave one engine in neutral so that both engines can stay at the elevated rpm during long idle periods.

CMI offers an Elbow Top exhaust conversion for the HP500 carb engine similar to the exhaust used on the HP500EFI. Ultimately, if you cannot solve the water-reversion problem, installing the CMI's will do it