350 Mag Bravo 1 WOT RPMs Low.. Newby needs help

[S8861]

Hello All,

I have a 350 Mag MPI.. Manufactured Jan 2006:

Motor: W617548 Drive: W905294 Gimble: W491858

This is mated to a Bravo One outdrive.

My question is as follows: My motor has started to top out at 4200 RPMs at WOT. I can hit the same number at 3/4 throttle. When I push it down to WOT or 100% throttle, the engine gets louder, but the RPMs do not increase. Prior to the last time I went out for a run, I was able to get closer to 5100 RPMs and faster on the GPS.

Other Info. When turning the key, there is only one audible beep, so I know the 'trouble chime' is working. All other gauges read within spec and there's no additional beeping when underway, or at cruising or WOT. (Temp is 160 to 170, Water/Oil/Voltage, etc. read normal)

As I'm totally new to Marine/Mercruiser engines any suggestions common culprits and on the sequence that should I start troubleshooting?

Change Fuel Filter?
Check Fuel Pump?
Check Distributor/Plugs/Wires?
Prop Rotation?

3/4 throttle gets me to 42mph on the GPS, and WOT only increases the noise, but not the speed. (I was getting just under 50 prior to this)

Let me know what other info you might need to know.

Thanks,

Sam

 

[S8861]

Re: 350 Mag Bravo 1 WOT RPMs Low.. Newby needs help

PS found this list in the stickies...

Below is more info for the things I did check

1. Fuel condition. Type and Octane possibly old fuel === Just filled up last week
2. Propeller pitch or diameter, damaged blades === prop looks fine
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse === Oil volume is at right level on dipstick
5. Marine growth on hull and outdrive === No growth here
6. Wrong gear ratio in outdrive === No changes made to outdrive
7. Restricted carburetor air intake (clogged flame arrestor) == Assuming this doesn't apply to MPI Motors? I'll take the cover off and check the intake though.
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test === Tested between 156 and 170 for all cyls when performed before putting into the water.
10. Carburetor defective, or wrong type. === Assuming not a problem for MPI
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.=== It has only been me onboard
13. Engine Overheating === Temp reads normal, just changed thermostat and sender.. IR gun pointed to thermostat housing correlates with guage reading.
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully. === This seems to be OK.

 

[Bondo]

Re: 350 Mag Bravo 1 WOT RPMs Low.. Newby needs help

PS found this list in the stickies...

Below is more info for the things I did check

1. Fuel condition. Type and Octane possibly old fuel === Just filled up last week
2. Propeller pitch or diameter, damaged blades === prop looks fine
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse === Oil volume is at right level on dipstick
5. Marine growth on hull and outdrive === No growth here
6. Wrong gear ratio in outdrive === No changes made to outdrive
7. Restricted carburetor air intake (clogged flame arrestor) == Assuming this doesn't apply to MPI Motors? I'll take the cover off and check the intake though.
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test === Tested between 156 and 170 for all cyls when performed before putting into the water.
10. Carburetor defective, or wrong type. === Assuming not a problem for MPI
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.=== It has only been me onboard
13. Engine Overheating === Temp reads normal, just changed thermostat and sender.. IR gun pointed to thermostat housing correlates with guage reading.
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully. === This seems to be OK.

Ayuh,... Check the contents of the fuel filter, for water, 'n crud,....

You added fresh gas, but what was left in the tank, from before,..??

 

[S8861]

Re: 350 Mag Bravo 1 WOT RPMs Low.. Newby needs help

Thanks Bond-o, will check that first. The tank was filled before winterizing and i bought the boat after that, so maybe some moisture got in?

And thanks to Don S. for the list.

S

 

[S8861]

Re: 350 Mag Bravo 1 WOT RPMs Low.. Newby needs help

Changed fuel filter and added some sea foam to the fuel per the suggestion on another forum.

No wake on the lake, so couldn't go wot. If that's not it, cap/ rotor next?

 

[CharlieB]

Re: 350 Mag Bravo 1 WOT RPMs Low.. Newby needs help

EFI - Check fuel pressure at the fuel rail feeding the injectors, screw the gauge on and watch the pressure when you go to WOT. A weaking pump may make sufficient pressure at idle and light load then drop pressure at WOT.

TEST before you start tossing any parts at the motor.

 

[S8861]

Re: 350 Mag Bravo 1 WOT RPMs Low.. Newby needs help

Thanks Charlie B. I did swap out the cap/rotor as they looked pretty corroded. Hopefully the lake wake restriction will be lifted to run it WOT to see if that helped. There was some shaking at 1200-1500 rpm, that seems to have smoothed out.

I got 45 psi at idle on the valve I tested under the spark arrestor. There are 2 Schrader valves under there, however, do I need to test both?