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454 Mercruiser performance problem

5 posts in this topic

I wasn't sure where best to post this - here or in the boat or automotive forums. I'm hoping that someone can provide some good advice on how to diagnose this problem.

I recently purchased a 1987 cruiser with a 454 V8 Mercruiser I/O in it that runs well (below 3K RPM) in gear, or smoothly to above 4500 RPM in neutral. The boat lacks power and won't plane unless empty or climb above 3000 RPM at WOT. Even after getting on a plane, the RPMs won't climb above about 3100 (WOT should be 4600). Otherwise, the engine runs smoothly for hours with only the infrequent "cough" while trolling or momentarily after dropping back to lower RPMs after running at WOT. It also seems like it is consuming more fuel than it should, but this is my first experience with a big boat (26' 5500#).

I've progressed through:

1) Changing the water seperating fuel filter

2) Changing spark plugs, distributor cap and rotor

3) Verifying good strong spark at low and high RPMs (only checked one cylinder)

4) Verified the timing is correct and advancing appropriately across the RPM range (so ignition module must be good)

5) Cleaned the carbon out with "engine tuner" a couple of times

6) Had the carburetor cleaned and rebuilt (should I recheck?)

7) Compression check had four cylinders at 120, two at 110, and two at 130 PSI (forgot to WOT)

8) Fuel tank vent is unobstructed

9) Tried changing the fuel/air mixture at WOT with no difference in performance by experimenting with removing the flame arrester, closing off the intake, and by spraying engine tuner down the throat.

10) Propeller has some dings (not bad) and is the appropriate size and pitch (15x16 3 blade), consistent with the boat manufacturer's recommendations. I may try a new four blade replacement/spare prop, but don't think thats the answer.

I'm wondering about the mechanical fuel pump output or possibly an obstructed fuel line "sock" in the fuel tank, but that doesn't seem likely given how smoothly it runs otherwise at WOT or at high RPMs in neutral (with no miss or sputtering).

Any thoughts or recommendations?

Thanks in advance for your help!


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You did a good job with the information. I'm going to move this to the Boat & Motor forum.

I was going to ask, "are you trimmed down all the way. Gauge might say so but verify this while on land.

Do you have trim tabs and are you using them." but don't think thats you problem.

Pouring engine cleaner down the carb while running won't increase your RPM's if you have a fuel shortage problem.

Did you replace the fuel filter? Did you inspect the fuel line from the pump to tank? Inside the tank is a pickup line, on the end is a valve. That valve will only open to let fuel flow by suction from the fuel pump. So an air leak or weak pump and the symptom will be magnified. Before you tear things apart I'd use a portable tank and test run or better yet test the fuel pump pressure.

Is the choke butterfly open?

How old is the gas?

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Has it done this since the day you bought it, or did it just start recently?

Does it;

1. Bog

2. Miss or sputter

3. Does the engine pitch change or does it stay the same, but rpm's just dont increase.

Verify throttle plates are opening.

Spraying engine tuner into the throat of the carburator will bogg the engine down. If you want to check to see if it is running too lean, you will have to spray something like carburator cleaner instead of engine tuner.

Check fuel pump pressure. Using a portable tank as frank suggested is a very good way to eliminate the tank, pick up and tank to pump line.

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Thanks for your help, Frank and Bruce!

I've been using the trim tabs (bow down) and the outdrive is trimmed all the way in when I'm trying to get on a plane. I changed the water seperating gas filter (old one looked good with clear gas in it) and the tank gas was fresh. The carb screen should have been cleaned when the carb was rebuilt. All four barrels were opening at WOT - I'll double check that they are wide open and not still choked. The boat has done this since I bought it. The engine doesn't bog down or sputter - just the occaisional "cough" as noted earlier. The engine pitch climbs to 3000 RPMs and then stays there at WOT, without the RPMs climbing any any higher.

All I had was engine tuner when we were testing the fuel/air mixture - I had thought that would add some BTUs. I'll have to try it again with carb cleaner. I wanted to test the fuel pump output pressure, but I don't have the fittings or gauge yet. The gas lines "look" good, but I haven't inspected the pickup line inside the tank yet - it is welded to the top of the gas tank. So, there's a check valve on it and not a "sock" that could be plugged? I was thinking I may be able to see the pickup end by looking throught the fuel gauge sender port, but that sounds like a waste of time.

I think the next step is to try carb cleaner at WOT to rule out fuel/air mix or starvation, and then an external fuel tank to rule out problems with the fuel lines/pump. Can I just slide the rubber tank fuel line over the carb's metal gas line with a hose clamp and let gravity feed the carb?

Is there anything else I should do next?



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When you check the throttle plates, make sure you are using the control handle and not doing it at the carb. And just to make sure we are talking about the same things, look past the choke plates for the throttle plates.

Using the remote tank with out the fuel pump (gravity fed) may not work. That baby is capable of using more gas than gravity may supply. It would be better to use a remote tank, and to check fuel pump pressure and vacuum.

Verify that you are getting spark on all 8 cylinders through the entire range using a timming light if you havn't already.

What did the spark plugs look like when you pulled them.?

It also sounds like there could be an exhaust restriction somewhere.

Something else to keep in mind about your particular set up. Mercruiser used an Alpha I stern drive in 87 and 88. The alpha is a smaller drive and really isn't built heavy enough for a big block. They hadn't come out with the Bravo yet. If you are going to experiment with props, be carefull. Too much pitch is going to cause much more heat and stress to the gears in the upper gear case (which should be 1.32:1 and be heavy duty, it is stamped on the side of the upper gearcase). Always use High Performance gear lube.

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