Merc Magnums won't rev past 3500
 

Hi,

I'm new to boats but have lot's of car experience. I recently bought a 1989 34ft Scarab with twin 350 Magnum Mercruisers. They are replacement motors I'm told fitted about 5 years ago - they have Vortec heads and Holley carbs (6150). They only have 130 hours on them, and compression is in the range of 120-140 in all cylinders. They're fitted with stainless tube header style manifolds/risers - made by Hitek in Australia.

The engines won't rev past 3500 ( about 34 knots). The port engine has a bit of a hunting sound to it which I think is an overly rich mixture (plugs are wet and/or sooty). I haven't checked float levels yet - already replaced power valves.

Starboard engine runs very smooth, and picks up much quicker than port motor.

Each engine has a little facet solid state fuel pump.

I intend to change ignition leads, check float levels, and possibly re-jet carbs. I also have MSD boxes to potentially fit.
Don't think any of this would keep the engine revs down.

I'd appreciate some advice on what else to consider or look for that might be causing the engines to not rev and my max speed to be down at 34 knots. I haven't checked throttle lnkage yet to make sure the butterflies are wide open - I will do this too. I'm thinking fuel pumps might be inadequate - have bought a fuel pressure gauge and will fit it. I'm assuming if they are too small I should see pressure drop right? When the engines max at 3500 or so there is no misfiring that I can hear.

Thanks in advance, and thanks to all the posters here for providing inspiration and no doubt ensuring I'll be broke and addicted to boats for a while.. :-)

Cheers,

Craig

Do they rev up in neutral? My guess is they swapped these from Big Blocks to small and never changed props.

Need to know what props, what drives and ratio.

Need to know if they free rev

Hi,

The boat was delivered with small blocks. This is an Australian Made Fastlane Scarab - same hull as Wellcraft (made under licence) but some differences in the fitout.

It has Alpha legs however I don't know the ratio, nor how to find out?

I also don't know how to work out my prop pitch - is this marked on them?

The engines will free rev to 5000 or more. No rev limiter that I know of. Factory Mercruiser magnetic pickup ignition.

Thanks for your help!

Craig

Sounds like you are over propped, there should be numbers on the prop somewhere post them and someone can tell you the pitch and where to go from there

Sounds over propped to me also. There should be numbers on the props.

Thanks for the feedback guys.

However, doing some research on the fuel pumps that are installed (as mentioned above, facet solid state) at best they are 30gph free flow, which is probably around 12gph at 5psi.

To make 350hp (which my motors are apparently) I need 30+ gph at 5-7psi.

Hence I've ordered a couple of Mallory marine 70gph pumps, which will do 35gph at 7psi. The Mallorys are nice and quiet too from my previous experience.

I'll fit a fuel pressure guage to my boat soon and check the existing setup, but it seems like almost a must that it will run out of fuel at about 175hp...which is probably about 3500 revs..

Cheers,

Craig