7.4L mag MPI tops out at 3900 - Troubleshooting?

[dcg9381]

1998 Glastron CSX-23
109 hours on the meter.
New boat (to me).
385hp Magnum MPI 7.4L

She's topping out at about 50mph, previous owner said speedo was off, but I confirmed it via GPS. The factory tach is off, which I think is a little suspect... Running a 23 pitch propeller.

31mph 2600rpm
43mph 3460rpm actual, indicated 4600rpm
50mph 3900rpm actual, indicated 5200rpm (top end on this boat)
At 50, I've got prop slip of 11% - which is well within range.

This boat tested out at around 68mph in 1998 via a magazine, so something is up with it.

Any suggestions on troubleshooting?

The only issue I know of with it is that one of the "quick and quiet" exhaust baffles is clicking quite a bit - I assume that it's internal "flapper" or whatever routes exhaust is having problems, but I don't think this should cost me 18 mph.

Actual RPM comes from an inductive tach that I bought today.

I've confirmed that I get induction on all 8 plug wires.. I guess I can pull plugs and test compression... Any other suggestions?

[dcg9381]

Compression numbers

Front of boat
158 157
162 155
150 150
150 156

That means it's not a major mechanical issue (at least to me) - Still need troubleshooting advice on this motor.

Plugs look a little old, nothing spectacular.. Will replace and try again.

[Pat McPherson]

You need to check the timing.
She could be going into knock retard.

[dcg9381]

Can you guys give me the low down on checking the codes on the boat.. I know that I can jumper the boat to base timing, but i need to know what plug and which terminals of that plug.


I believe the codes can be read with a 12V LED and jumpering something - but I'm unable to find the details. Someone will have to point me to the right diagnostic plug also.

I've done EFI tuning before - nothing big block or marine, so I'm familiar with the concepts.

Help appreciated.. I don't think the local marine dealer does EFI yet.. :-)

[Pat McPherson]

I bought a code tool for $25 from Reta.
There is a way to jumper the plug to set base timing. Someone will chime in or you can do a search in this forum; it has been asked a number of times.

[dcg9381]

Can someone give me Reta's email (I assume this is a user name).

Interesting - I got under the dash tonight and found that the tach was set to a 6 cylinder calibration.
I wonder if someone monkey'd with this thing so the boat would sell to some poor sucker (me) - or if it's been off from the factory...

[US1 Fountain]

The OSO resident EFI geru.......Dave N

All your answers:

http://www.offshoreonly.com/forums/s...ighlight=codes

You are trimming up, right? Both drives and tabs.

[dcg9381]

Thanks.. That's exactly what i was looking for and couldn't find! I'll run the codes tomorrow.

I am trimming the out drive up, just shy of half way on the gauge. I know what it feels like to have the boat free up on the water with correct trim.

No outboard tabs (yet) - we'll solve the motor issue first.

[the duke]

23p could be the wrong prop for your boat. a 21p is going to put you up 4-500 RPM, so 4400, which is a lot closer to where you want to be. (5000)

scan tool is the best way to go. Most of the sensors would have given you a problem even running it to 3900. its an impossible guess to tell you which one "could" be wrong. the code reader is a great tool. check your base timing it could be an issue. check your plugs for the correct gap, and your wires for signs of breaking down. If they are original, they have probably seen better days. Also check cap for cracks, and cap/rotor for carbon tracks.

[dcg9381]

Originally Posted by the duke

23p could be the wrong prop for your boat. a 21p is going to put you up 4-500 RPM, so 4400, which is a lot closer to where you want to be. (5000)

scan tool is the best way to go. Most of the sensors would have given you a problem even running it to 3900. its an impossible guess to tell you which one "could" be wrong. the code reader is a great tool. check your base timing it could be an issue. check your plugs for the correct gap, and your wires for signs of breaking down. If they are original, they have probably seen better days. Also check cap for cracks, and cap/rotor for carbon tracks.

Originally Posted by captJason

play with the prop selector some more.... it could be as simple as being over propped

I've played with the prop selector.
I've got test data from one of the boating magazines that tested this boat in 1998 with a top speed of 68.5 mph. That could have been in cold weather, flat water, and minimum fuel - I don't know... So I'm assuming that the boat should be a 65mph boat, give or take. If it'll do over 60 and over 4700 RPM, I'll consider that close enough. Right now I'm not near that... :-)

A 23P puts me to 63-65mph with a little less than 10% slip, so I think that's where it should be at. I do have an aluminum 21 pitch prop, slightly smaller diameter, that came with the boat so I could try that also - just to see if I can get more RPM out of it.

New plugs tonight. I hate replacing the darn GM cap/rotor as it's always wedged in the back of the motor against something. I'll check DTC codes also.

You guys have been great...