Marine Frequently Asked Questions
Courtesy of A J's Marine
. I just purchased a boat with a Mercruiser GM 5.7 and Mercruiser stern drive. The boat runs well, but it is several years old and I am wondering what should be inspected or changed so that I can expect to run all season.

A. That is a good question and shows that you realize that boats need a lot of maintenance on a regular basis to deliver reliable service. You didn't say whether the boat has been used in fresh or salt water so I will cover both situations.

Assuming the engine starts easily and runs smoothly at all throttle settings, you probably don't need to worry about a major tuneup. What you do need to worry about is the condition of the elbows or risers that sit on top of the manifolds. Even on fresh water cooled engines -- with antifreeze in the engine -- the risers deal with hot raw water all the time the engine is running. Once the engine is shutdown, the risers at least partially drain and let in air. Heat, air, and moisture are eventual death on all cast iron. When raw salt water is added to the equation, things get worse quickly.

Pull the risers and carefully inspect them. If they look rusty or are flaking off inside, replace them. Once they fail and begin leaking water into the exhaust mainfold, you are risking a suddenly ruined engine. If you pull the dipstick and warm oil looks like a milkshake, you already have water in the engine and have to act quickly to change the oil and filter and determine where the water is coming from. As with any engine, there are several possible causes of water in the oil -- blown head gaskets, cracked block or head, bad manifold -- but the most likely culprit is the riser and that should be checked first.

Okay, the risers look good and just show a layer of carbon inside. Great! Replace the gasket -- never reuse a manifold or riser gasket -- and move on.

Next remove the outdrive. This allows you to inspect the gimbal bearing located in the transom assembly. The bearing should turn smoothly with no rough spots. If it is rough or frozen, replace it. The area inside the "boot" or driveline bellows should be dry. If there is any water there, the boot is leaking somewhere and must be replaced. Also inspect the shift bellows. This little rubber boot can sink a boat in a hurry if it leaks and the bilge pump quits.

If either of these boots shows signs of weathering or cracking, replace them before they bite you. Boats kept in salt water usually grow barnacles on these boots and barnacles can cut a hole in the flexible rubber without warning.

Next inspect the u-joints on the outdrive. If these are rusty or muddy, you are getting water on them and they are probably ruined and require replacement.

While water usually enters from the bellows, it can also come in from the outdrive itself. When the seals fail on the outdrive, you get an oily mess inside the boot. If lower seals fail, the goop is laden with water.

If the driveline area is clean, you are getting there. Next drain the oil from the outdrive. There should be little or no water mixed with the gear oil. If there is a substantial amount of water, say more than a half ounce, you probaly have a lower seal that has failed and the outdrive will need to be disassembled and seals replaced. If the oil is clean and no chunks of metal sticking to the magnet on the plug, go on to the next step.

Most outdrives have the raw water pump inside the outdrive wrapped around the vertical shaft. This pump should be replaced each season and more often if you operate in sand, mud, or grass. If this pump fails, you will overheat the engine quickly and have some really expensive problems. Replace it!

Remove and inspect the prop and look for fishing line wrapped around the shaft. This stuff can eat seals like crazy. While you are at it remove any small dings on the edges of the prop with a file. If there are serious dents or curls, replace or rebuild the prop.

When all of this has been done, reassemble the outdrive and pressure test it before refilling it with gear oil. If it holds 10 psi for a few minutes, release the pressure and refill with marine grade gear oil.

Inspect and replace any zincs that are corroded away, get a fresh gasket set for the outdrive, lubricate the u-joints if they have fittings and put the whole thing back together. You should be set for a season.

Once everything is back together, put water to the engine and start it up. While it is warming up, check the ignition timing and reset it if needed. Once the engine has warmed up and the oil is hot, change the oil, oil filter, and the inline fuel filter. Restart and look for leaks. If the antifreeze in fresh water cooled engines is more than a year or two old you might want to flush it out and add fresh water and antifreeze. The mixture should be at least 50/50 but you can add up to about 75% antifreeze for extreme conditions. Be sure and dispose of the old antifreeze where dogs can't get to it. Antifreeze tastes sweet to dogs and will kill them if they drink it!

That's it! Go have fun!

Q. My engine is hard to start and has hesitation when I give it throttle under load. Sometimes, it backfires through the carburator. Is my timing off or what?

A. The timing could be off, but likely your engine is starving for fuel. When under no load, an engine requires far less fuel than when it is pulling hard. It sounds as if the accellerator pump has quit and maybe some stuff has grown in your carb, restricting or disrupting fuel flow. All fuel contains some moisture and little hard deposits can form inside the carb itself. Usually the solution is as simple as a carb rebuild. In a few cases, however, your only fix is to actually replace the carb. I have seen carbs that simply would not take a rebuild -- even by a remanufacturing plant.

Besides poor performance, you run the risk of buring valves or pistons by running too lean. Fix it and you will be glad you did.

Q. What can I do to make my marine batteries last longer. It seems like I am always replacing one of them. I keep my battery switch in the "BOTH" position so they should both be charging, Right?

A. Wrong! Batteries may be discharged in parallel but should never be charged in parallel. Each battery has a slightly different internal resistance. Thus one battery in parallel with another will be overcharged consistently and drastically shorten its life.

Run your battery switch on either "1" or "2" but not "BOTH" when you are running your engine. If you want to start on one battery and switch to the other while running, that is okay as the switches make before break. Just never turn the battery switch to "OFF" while the engine is running. The excessive voltage can damage the alternator, instruments, lights, and expensive electronics.

If you must charge more than one battery from a single source, add an isolator block which feeds both batteries through diodes and protects the batteries. This way your alternator "sees" only one battery -- the one it is running off of -- and you avoid overcharging. I really don't know why this information is not better known and disseminated, but now YOU know the secret. Most batteries used in Marine applications will require replacement every two or three years for reliable starting and adequate supply for the electrical and electronics systems on board.


Q. Hello, I just purchased my first inboard 5.0 [302 ford] I knew it was overheating at time of purchase. If you open the drain in the port side manifold while it is running it does not overheat. Engine runs like a top and no water in the oil. Appears to be pumping sufficient water. Do I have a bad manifold or riser? Thanks

A. I would remove both risers. Inspect risers for clogged water passages. These can be cleaned out. If they break while cleaning they needed to be replaced anyway.

Q. Thanks for the info. I should have done this first instead being the last thing . One had a lot of rust falling out when removed from manifold and the bellows looked a little burned and flaking off. Is it worth taking a chance trying to clean them out some or just replace?

A. I would recommend replacing the risers. If they fail you run the risk of getting water in your engine. If you run in salt water check risers every year.

Grinding when shifting:

Q. I have recently started pleasure boating. I bought a 1988 boat that was repowered in 1997. I believe that the outdrive is the original Alpha I. Sometimes it grinds while I am shifting. Is this enough information for you to troubleshoot? Thanks

A. Lower shift cable might need adjusting if it grinds more in forward or reverse than the other.

Outdrive Oil:

Q. Hi, I was wondering if you had directions on how to change the outdrive oil on a Alpha 1 outdrive. I have a 1995 Searay Sundancer 250 with a Mercruiser 5.7L V8.

A. Remove upper and lower vent/drain screws. Drain old oil. Fill outdrive from bottom - usually 1 quart. Inexpensive pump kits are available where you buy Mercruiser lower unit oil. Unit is full when oil comes out top vent. Hint: Replace top vent screw first. This will prevent oil loss when bottom screw is installed. AJR

Q.Thanks for the quick reply. I have two more questions though about changing the outdrive oil. 1.)Should I lower the unit all the way down when draining the the outdrive oil? 2.)When draining the oil, will the oil in the resouivour in the engine compartment drain too? You don't need to change that also, do you?

A. Place outdrive in full down position to change oil. If you let the outdrive drain all night, yes the reservoir will empty. To stop this, remove reservoir from its bracket. Pinch hose very gently to block the oil flow. AJR

Replacement Carb:

Q.I have a 95 Malibu Response ski boat with a 265hp 5.7 liter chevy. It has never started very well cold and not much better after being shutdown over 6-7 minutes hot. It has a Carter AFB 625cfm carb. It seems that most ski boats are running Holley. Here's the question A. do you agree that the Holley is a better carb for a boat...B. 600 or 650 cfm.....C. Vacuum or mechanical secondaries....I'm thinking 600 with vacuum?

A. First, be certain you use a marine carb NOT an automotive carb. Your choice is a good one. Holley, 600, vacuum advance. Holley is more adjustable than others. Good Luck AJ

Volvo 280 Aquamatic Outdrive Leg:

Q. I need to understand in simple terms the gear ratio in my volvo outdrive. This model comes with overall gear ratio of either: Type 280 B 1.61: 1

Type 280 C 1.89: 1

Type 280 D 2.15: 1

Assuming my engine RPM was operating at 2000 rpm, what would the amount of turns of the prop be for

Type B ______

Type C _______

Type D _______

I need to understand whether the gear ratio of the leg is a reduction of the RPM or an increase. My way of thinking is that at 2000 rpm and a gear ratio of 1.61: 1 that my prop is turning 1242 rpm. Or is it actually 3,220 rpm.

A.You are correct. Gear ratio reduces the rotation of the prop.

Rebuilt outdrive:

Q. I recently went thru the pain and expense of having my alpha one outdrive being rebuilt. I was wondering if there is any break-in procedure. The shop that did the work told me that there is not. I would like to be reassured that I was given the right information before I run the risk of future repair.

A. To my knowledge, there is not. The outdrive is only gears and roller bearings similar to the differential on a car. AJ

Automotive carb on boat:

Q. I have a 351 windsor with an automotive carb.It has a heat rise choke.How do i install an electric choke? Also do i install a switch on the console?

A. I'm sorry I can't help you. Automotive carbs are dangerous on boats! AJ


Q. Hello, I just purchased my first inboard 5.0 [302 ford] I knew it was overheating at time of purchase. If you open the drain in the port side manifold while it is running it does not overheat. Engine runs like a top and no water in the oil. Appears to be pumping sufficient water. Do I have a bad manifold or riser?

A. It is very possible that the riser is clogged . Remove both risers and inspect the water passages for restrictions. One is likely corroded closed. Clean or replace as necessary. Note: drain manifold before removing riser. I`m guessing you have a Shamrock boat with a PCM engine .AJ

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