1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to speed

[Tommyj16]

1999 Mercruiser 7.4L 454 MPI in a 25' Baja.

When I take off the motor just doesn't push the boat. It used to jump out of the water. It sounds like it is bogged down. It makes a "muffled backfire sound". Its not a true backfire like when a golf cart or something backfires. Once it gets going it hesitates as it is accelerating. Once I get it up over 4000 RPMs it is very responsive and runs great.

I have check the fuel in the fuel filter/water seperator. There did not seem to be any water in the fuel. I use enthanol treatment very frequently. I also replaced the fuel filter.

After replacing the fuel filter, the motor showed no improvement.

I went and bought new spark plugs today and will install this weekend. I ordered new wires but they won't be in until next week.

Any suggestions? Any help will be greatly appreciated as I need to try and fix this myself if possible.

 

[dingdongs]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

have you checked the compression lately and if you have growth on the hull there's more friction for the engine to overcome.

 

[Tommyj16]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

have you checked the compression lately and if you have growth on the hull there's more friction for the engine to overcome.

Never checked the compression, and don't know how.
The hull is spotless, I trailer the boat and wash it after every use

 

[dingdongs]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

it may be electrical and plug leads breaking down.these can be checked by running boat up in the complete darkness and see if you have any arcing going to earth which will sap the power from the coil.cost is nothing to look.

 

[Tommyj16]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

I will be changing the plugs out today, however I had to order the plug wires, they won't be in until monday.

Any ideas, on what else I can try?

Some more background:
boat is in great condition and boat/motor has only 360 hours on her.
I also installed a stereo system with 2 amplifiers, but I doubt this could have anything to do with engine performance

 

[Tommyj16]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

Any help guys/gals?

 

[OldNBold51]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

check your ignition timing as well.

 

[Tommyj16]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

I am trying to find a timing light now. Also, I sucked up alot of sand few weeks back. After I flushed all the lines, I washed the motor area off. I wonder if water could have messed up the distributor?

 

[rschap1]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

The back fire is going to sound muffled and duller due to the spark arrestor containing it.
I read often that the big "cannon" plug wiring connector gets loose and causes issues.
Make sure that is plugged in good and clamped.
You may still have a fuel delivery issue.
I also read alot of clogged and plugged check valves in the fuel lines choking off flow.

 

[tswiczko]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

I am trying to find a timing light now. Also, I sucked up alot of sand few weeks back. After I flushed all the lines, I washed the motor area off. I wonder if water could have messed up the distributor?

Yup, that would cause the distributor cap to crack if you did it when it was hot or at the least caused moisture infiltration into the cap,

pull the cap and see if there is any condensation under it or if you can see a crack.

 

[Tommyj16]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

Yup, that would cause the distributor cap to crack if you did it when it was hot or at the least caused moisture infiltration into the cap,

pull the cap and see if there is any condensation under it or if you can see a crack.

I did not wash it while it was hot. But I will check the cap today.

 

[Tommyj16]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

I took apart the dist cap and it had white growth looking corrosion on the all the terminals. I scraped them with screwdriver and a steel brush. (waiting on a new one to be shipped to me). Also, changed all the spark plugs out. The wrong size plugs were in the motor. I have had the boat for 3 years and they have been fine until now. Now I have the manufacture suggested plugs in.

After all this, the boat is running great. It jumps out of the water better than it ever has. Only issue is that is slightly hesitated a few times when at top speed (4600 rpms). It didn't do it but a few times. Hopefully when I put a new dist cap, rotor, and wires in, everything will be perfect.

Thanks for your help and suggestions everyone.

 

[Tommyj16]

Re: 1999 7.4L mercruiser MPI, struggles to accelerate, takes long time to get up to s

Ran boat all weekend. Only one issue. The warning alarm sounded a few times. Only would do it after I started the engine after the boat sat for a while(not running). Did it about every other start up. Gear oil was fine in resevor, temperature was fine, engine oil was slightly low, but still between the lines on the dip stick. I added a 1/2 quart of oil, but it the alarm sounded twice after oil was added.

From all my research, there are only 3 things that can sound the alarm... and all three seem to be fine to be in normal levels. What should I be looking for?