Mercruiser 454 cutting out at wot

[mamboking]

I am hoping someone can help. I have an Extreme 24 fitted with a Mercruiser 454 Mag (carb) 365hp, Bravo 1 leg with 4 blade 24 pitch bravo 1 stainless prop.

I have had someone look over the motor as it has not been pulling enough revs, 4200-4300 and 58mph, he found; timing out 1 degree and plug gaps not set correct, the compression was tested and was all good, everything else seemed ok.

Launched a few weeks ago and the boat is pulling more revs but when it reached 4500, the motor cut out, the revs shot to zero for a second then back to 4500 the straight back down again to zero. It was not till I backed off wot the motor would run at lower revs.

In my four seasons of ownership it has never done this.

Please does anyone have any ideas as to what this could be?
Could this be an electrical fault? fuel pressure? where do I start?

[motorvator]

Sounds electrical. Not sure on your set up but I had similar problems which turned out to be a rotted blade fuse in the ignition cabinet that looked fine until pulled out and tugged around a bit.

You need to check the hook up plug to the harness and any wires round the ign circuit that would be prone to vibration.

[Jon Holmes]

Rev limiter?

[lain]

I had similar symptoms on my old welcraft both engines developed a similar fault.

once lost power instantly then all off a sudden power was back, then it was gone. it was all or nothing and intermittent.

The fault was the big multi wire adaptor (round rubber coated plug) with multiple round pins for connection. Now, i'd cleaned this previously and vaselined it and all looked fine, but pushed a screw driver in between the pins to widen them all about 1mm, problem went away.

So if the power problem is an all or nothing type thing check electrics.

The problem with the other engine was;

the engine seemed to slow up and lose power gradually, almost seemed as if I was easing back on the throttle.

Turned out the coil was bad, when it heated up it broke down.

hope that helps

[mamboking]

Cheers Mr Motorvator I will check the items you have mentioned.

turnip, thanks, I had a discussion with Gareth at Drivers about rev limiters and he said that when you hit it, the revs will not go higher than the amount set; not drop to zero like mine, its very unsettling when it happens, possibly even dangerous!

I am not even sure if my motor has a limiter?

Does anyone know what the wot is on a Magnum, standard 7.4 is 4600 in my book but it does not have spec for the mag.

[Ben]

Originally Posted by mamboking

Does anyone know what the wot is on a Magnum, standard 7.4 is 4600 in my book but it does not have spec for the mag.

5000

[mamboking]

Thanks for all the help guys.

Does anyone know if the Magnum timing is any different to the standard 7.4?

I have the motor timed at 8 degrees BTDC