Causes for less power from engine

[ni7irs]

I have a scr 2800 with the 7.4l mpi. When if first bought it, more than four people and it had a problem getting on plane. Last year I install a k&n spark arrester and the increase in power was noticeable as I was able to get on plane with eight people. Last week, I had to keep it at wot just to stay on plane and couldn't get on plane without three people on the bow with five on board.

What can be the cause? Short of a rebuild, what can I do to increase power?

[Huddo007]

Heaps of factors can lead to power reduction, remember the main points to running an engine, AIR,FUEL,FLAME,COMPRESSION! you need to look at these areas, having the same engines well two of them actually I have had a similar problem lately, I changed my fuel filters and spark plugs and that was it... for my problem, you need to look at your WOT, are you getting the proper WOT?, mine would fall short by only 400 RPM with the new plugs its back up, but more interesting is the ability and shorter time taken to get there, I would start at plugs and filters, if you have an engine compression gauge you could check that as well, also old fuel can be a problem, if you have not put fresh fuel in for a while (not normally a problem with how much the 7.4 drinks) you can have stale fuel this will cause a reduction in power, start small and cheap and work your way up to something as big as engine re-build, another thing but not as likely could be you plug leads, a trick an old mechanic showed me was checking the engine at night with no lights on, even at idle if your leads are leaking you can see the electric blue sparks jumping around your leads, this will cause a reduction in HP and performance. Remember these are large displacement engines that need lots of fuel , lots of air and a large spark to get going well, the engine is under constant load and needs everything to be running well to have the desired performance.

[pascavone]

i have the same boat, and i don't think its your boat.

I think its the conditions:

1. where the bodies in the middle of the boat with the driver, or the ass end, tabs down?
2. how much fuel was your tank?
3. high or low tie, slack time, or current?
4. windy day or chop?
5. if your rpm up to 4800, and the bow is up, your not going anywhere.
6. its time for a mobile marine tune-up.... let him plug it engine computer and dial the engine in.
7. usually, all the other items failures will give you other symptoms, not just when your loaded with guests.
8. In slack time you can run 32mph at 4800rpm.
9. in current 23 mph at 4800rpm, because the GPS dose not count the water moving the opposite direction under the boat.
10. I spoke to a dude with a 2002 with the twin 4.3- V6's and he gets the same performance.

[cunuck]

check the anti siphon valve it can starve the motor of fuel cheap to change and worth a shot.I just changed mine big change in wot good luck

[ni7irs]

Originally Posted by pascavone

i have the same boat, and i don't think its your boat.
See below
I think its the conditions:

1. where the bodies in the middle of the boat with the driver, or the ass end, tabs down?
One in back, one with driver and two on bow. Kids randomly scattered
2. how much fuel was your tank?
3/4
3. high or low tie, slack time, or current?
Lake michigan
4. windy day or chop?
No, except for wakes
5. if your rpm up to 4800, and the bow is up, your not going anywhere.
Rpm was at 4 and after plane crept up to 4200 wot
6. its time for a mobile marine tune-up.... let him plug it engine computer and dial the engine in.
7. usually, all the other items failures will give you other symptoms, not just when your loaded with guests.
8. In slack time you can run 32mph at 4800rpm.
I've never seen 4800
9. in current 23 mph at 4800rpm, because the GPS dose not count the water moving the opposite direction under the boat.
10. I spoke to a dude with a 2002 with the twin 4.3- V6's and he gets the same performance.

K

[ni7irs]

Where and what is the anti-siphon valve

[pascavone]

I believe you have the wrong props on your boat.

I can crank my 7.4 up 5500 rpm if i want, but your wasting horse power and fuel.

Some one tinkered with the prob, and the only way to figure that out is take them off, and get them measured and resurface at the prop shop.

Its maybe $370 bucks for bravo III.

[cj]

Hey there, I was down on power as well early this season. I checked/replaced all the obvious, replaced plugs, wires, cap, rotor...new manifolds/risers last year.....it ran better, but still felt there was more to be had. I did a compression check and #3 cylinder was "0".....****....pulled the head and found the exhaust valve cracked.....the shop vacuum tested the head and found a few more valves leaking.....with the cost of the parts/labour, it was only slightly more to buy two heads.....runs better than ever now.....only cost about 1K with the gaskets and a day of my time to install......Break Out Another Thousand.....lol.....good luck...

[cunuck]

Originally Posted by ni7irs

Where and what is the anti-siphon valve

the valve is in the gas tank the fitting the fuel line hooks on.It stops fuel from leaking should the line be cut.The ball and spring in the valve can get fouled and reduce the max flow of gas to the fuel pump.Cheap part $12-$14 dollars 5 min to change and it may do the trick worth a try.good luck