Mercruiser 7.4 Magnum down on revs

Hello everyone, straight in needing some help!

My boat is not on song this season, i have a Mercruiser 7.4 Magnum (carb) in a 24 ft sportsboat; 24 pitch four blade prop; cant seem to pull any more than 4200 at wot. This is lower than previous seasons, where do i start?
Fuel? Electical?

Hoping somebody can point me in the right direction.

Re: Mercruiser 7.4 Magnum down on revs

Have you changed anything or done anything to the powertrain since it worked last?

Re: Mercruiser 7.4 Magnum down on revs

Thanks for your response; The motor was rebuilt two seasons ago to stock spec using original parts. Boat went well 1st season after this and was layed up over winter. Been out 3 times this year and boat is not as quick, no changes have taken place.

Re: Mercruiser 7.4 Magnum down on revs

Have you checked and replaced your fuel/water seperating fuel filter?
Have you done a tuneup this year? Compression check, timing checks, anything?
You aren't down a lot, are you sure the prop doesn't have any tweeks in it?
Here is a list of possible causes of low WOT rpm. Time to start checking.


Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation
• Remote control cables and linkage for proper travel to open throttle plates fully.

Re: Mercruiser 7.4 Magnum down on revs

Cheers Don great list! Top site!

its not the following;

The fuel is fresh but using 95 pump gas not 97 might give this a go

Prop is right type (boat manufacturer tested using my boat) not damaged

Oil level is correct

no marine growth as boat is dry stacked and original gell coat

gear ratio is right, not been messed with since new.

Flame arrester not clogged

exhaust is through prop, not sure how to test this.

not overloaded or wrong trim

not overheating

Carb is the right type but may have an issue.

This still leaves a lot to work through and eliminate, thanks this is exactly what i needed.

As you say i am not down a lot but boat was running late 60s its now at 60 dead and 4200rpm. It was reputed to do 70mph, would love to hit that.

I will check some of this myself and work with the engineer who rebuilt the motor with the rest of the list.

Will keep you posted with the results.

Best Regards

Re: Mercruiser 7.4 Magnum down on revs

The weather here has been terrible and i have not done any testing but just had a thought i would like to explore.

When i bought the boat, 3 years ago i fitted a Navman plotter and fuel transducer. The fuel thing stopped working early part of the second season and i was meaning to purchase another one but not got around to it. The `broken` transducer is still plummed into the fuel line; question......

what is in the transducer, what could break and could this restrict my fuel flow, hence my top end loss.

Without pulling it apart i have no idea what is inside.

Thankyou in advance.

Re: Mercruiser 7.4 Magnum down on revs

could this restrict my fuel flow, hence my top end loss.

Click to expand...

Ayuh,....

Anything is Possible.....

It could be as simple as New fuel filters,+ a Tune Up....

Re: Mercruiser 7.4 Magnum down on revs

Been out for a run this week and made a significant discovery;

At the end of last season i did a Poker Run boat was fine; the day after took some friends for a run out and boat was running rough and called off the run.

I got a friend of mine (the one who rebuilt the motor) to have a look, the first thing he did was to put another set of plugs in, no different so he looked a little further and found i had snapped the end of a push rod, (end still attached) He put a new set of push rods in and i was running on 8 again.

I have not used the boat much since then and have noticed the performance loss; thanks Don for your list, i have worked through a lot of it. While away i removed the spark plugs to clean and check them and found that two of them had cracks in the ceramic so went for replacements.

The guy at the parts place while looking at the computer discovered i had totaly the wrong plugs! i had the ones that fit a 350 Mag which my said friend runs x2 in his. The wrong plugs are aprox 1cm too long in the thread that screws into the combustion chamber.....the old plugs looked ok but have i done any damage?

With the new (right) plugs back in i had a run back, 20 mins at high speed, the boat seems to be almost back to normal, more responsive but still only pulling 4200 rpm. I was running against the tide and a very stiff breeze with a lot of kit on board (been away for 5 days) and gps was showing 62.

Does anyone know what using the wrong plugs may have done?

Re: Mercruiser 7.4 Magnum down on revs

Was there any visible damage to the electrode side of the plugs you took out? If not, I would say you are ok.

-Eli

Re: Mercruiser 7.4 Magnum down on revs

Cheers, good point no damage to the electrodes. Was worried if i might have done some other obscure damage.

Also wondering what using the wrong plugs would have done to the combustion and would this have accounted for my performance loss.

Re: Mercruiser 7.4 Magnum down on revs

I hope people read this.... Did your loss in power coincide with the use of gas that had Ethanol in it???? Note that since Ethanol was mandated, the gas stations/marinas no longer have to post that it in the gas unless it exceeds 10%. I found this out the hard way. I monitor the gas mileage on my suburban pretty closely (6.0l V 8) and suddenly I was getting 1 to 2 miles less per gallon, every thing else the same. I was B tching about it when I was informed about the reduced power/energy content of ethanol blends. I stated rather stongly, that I lived in geogia and there has been no posting about the use of ethnaol blends. Much to my surprise I found out that sure enough, the gas I was buying had ethanol in it (per gov mandate and no notice had to be given). I think (I'm not positive) that ALL gas produced in the US now "can" and most likely does have ethanol in it. And yes at WOT, it can result in up to a 10% drop in RPM's. Note cars never operate in this range so they don't notice the RPM drop, just the drop in Mileage.

Re: Mercruiser 7.4 Magnum down on revs

By the way I live in the UK and am not sure of the Ethanol situation over here, i have lost roughly 10% at WOT. I will do some digging, thanks for your contribution.

Re: Mercruiser 7.4 Magnum down on revs

You did check to see if you have water in the hull?

Could be dirty filters, tune up is more than just changing plugs.

You check if the fuel pump is giving you 4-6 pounds of pressure.

Compression is good?

Re: Mercruiser 7.4 Magnum down on revs

Cheers

No water in hull, the boat is dry berthed and has been since new.

I did not get a chance to change the fuel and water separating filters before going away, so they are on the next to do list.

Not checked the compression as the motor was rebuilt and new rings went in; i have a gauge so will do this as well to eliminate the possibility.

May not get another run this season, looking for a niceish weekend that i am free.

Fustrating!