5.7L Loss of Power MPH Help!

Talari201

Cadet
Joined
Jul 11, 2010
Messages
6
Hi guys. New to the forum. I have an issue with my boat, and I hope someone can help. I have noticed my boat losing power at WOT, and my MPH is down from about 58 mph max to about 50 mph max. It idles a little rough, but doesn't miss. Once on plane, it seems fine through the middle of the power band, but when I put the hammer down, it sort of falls flat, doesn't like to rev up, and sort of has a "growling" or "bogging" sound from the motor. It just isn't as crisp sounding anymore. I am pretty convinced it is a carb issue, or fuel issue. Any help is appreciated. Here is what I have, and what I have done:
1996 5.7 2bbl Mercarb,thunderbolt V,Alph one. Mariah 20 ft talari. Prop is a SS 21 Pitch, in good shape. I have replaced plugs, wires, cap, rotor, fuel filter at carb, and seperator. Cleaned pickup in tank,and checked for water in fuel (none). ran 2 tanks of gas, no change. Checked Dist pickup (ok) checked choke for opening when hot (ok). Have not checked base timing. or fuel pressure. Does this sound like a carb issue? Thanks in advance.
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: 5.7L Loss of Power MPH Help!

Take a look at this diagnostic list: http://forums.iboats.com/showthread.php?t=295070

Also, we're going to ask lots of basic questions like prop condition, bottom condition, have you or your crew or your boat gained any weight? Could very easily be carb stuff, but you really need to go through all of this stuff to be sure.
 

Talari201

Cadet
Joined
Jul 11, 2010
Messages
6
Re: 5.7L Loss of Power MPH Help!

Thanks...the thread is closed though, so here are my answers:

Prop is a new SS 21 pitch also tried a new 23 pitch Aluminum....same
Hull is clean, Trailered out of water
New Gas 91 octane. 2nd tank
Cleaned all fuel filters, and pickup screen
No weight gain....all tests with me in alone

It is a sudden power loss, after storage last year. Completely new ignition, except module, and coil. (wires, plugs, cap,rotor)
Checked all engine grounds...ok.

Hard starts.....pretty much always have to choke it to start first time, and sometimes after sitting at beach for an hour or so.

I will check base timing, and comprwssion.
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: 5.7L Loss of Power MPH Help!

Thanks...the thread is closed though,
It is really supposed to be as it is an FAQ type deal. So keep posting info and we'll help out as we can.

It is a sudden power loss, after storage last year
Just to clarify . . . the ignition components were replaced before or after the power loss?
 

Talari201

Cadet
Joined
Jul 11, 2010
Messages
6
Re: 5.7L Loss of Power MPH Help!

>>>yes, the power loss was before the Plugs, wires,cap, rotor, distributor check (for slop, and sensor buildup/corrosion). It had a slight miss, and the ignition cleared that up.

the easiest way to explain it, is at about 3/4 throttle, up on plane....about 30 mph, when I excellerate, it gives out sort of a "growling" deep throated "bogging" sound as it ecellerates up to about 45 mph...and then I can get the trim just right, and get 48-49 mph out of it. It used to be able to excellerate from 49-50 on up to about 56-58, and it sounded crisp, with snappy throttle response...just feels like its laboring...

I also want to check the anti-siphon valve.....as it had the fuel stoled again this year in storage. Ive worked on cars all my life, so I have a pretty good grasp of things (except the Marine additions)....my gut is telling me carb/fuel delivery....Thanks guys!
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: 5.7L Loss of Power MPH Help!

Can you give us RPM at those points? How are you getting the speed numbers?
 

Talari201

Cadet
Joined
Jul 11, 2010
Messages
6
Re: 5.7L Loss of Power MPH Help!

I am unable to give you accurate RPM at this time, as my dash mounted TACH, just peggs itself at WOT(always has)....So, I will have to get a tach hooked up. The MPH is through my Garmin GPS. The dash mounted spedo is pretty messed up too....but I have owned the boat for 10 years, and have a good reference point as to what she's doing (or not doing)...but yes, those numbers are GPS
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: 5.7L Loss of Power MPH Help!

Seems like you are on the right track. GPS gives me confidence that the performance numbers are good and the 10 years history doubles that confidence. So I'd just start working away at the diagnostics listed.
 

Talari201

Cadet
Joined
Jul 11, 2010
Messages
6
Re: 5.7L Loss of Power MPH Help!

..ok, I checked the anti-siphon valve. At first it didn't "shake" so I cleaned it with cleaner, and also checked the screen, and fuel filter at carb.....all good, and siphon valve rattles, and seems to be performing well. I also checked choke operation, and throttle cable...all seemed fine...

I am down to checking fuel pressure and timing? UGHHHH!!

I feel confident the fuel delivery system is clean.....so Carb or fuel pump are my last hopes! I just know that mechanical pumps on a small block chevy are pretty reliable. Maybe that is different in the marine engine.....not sure.
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: 5.7L Loss of Power MPH Help!

Compression?
 
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