7.4L (454) Mag MPI - tops out at 3900 RPM

[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]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

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.

 

[180shabah]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

Start with basic maintenance. Cap, rotor, plugs,wires, fuel filters, timing etc.

Is the prop in good condition?
what does the boat weigh? what was the test weight in the magazine?

 

[dcg9381]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

I'll do the plugs tomorrow as I've already got them out.
I need a diagnostic connector to set the base timing, that's on order.
Doesn't sound like a fuel issue - no missing, stalling, backfiring - I don't think it's going lean.

It could be a single cylinder that's dropping out or dropped out - I don't have enough experience with Marine V8s to really know for sure if I lost a cylinder.

Note, I did find that someone left the stock tach on a 6 cylinder setting... Hmmm.. I wonder if that was intentional or not..

Boat was tested by a boating mag in 1998.. I don't know the tested weight, I just know it was equipped with the same motor that I've got (there are a ton of motor options for this boat). Note, I'm not expecting to hit magazine tested speed, but I expect to be within 5-6mph. I believe the boat weighs between 3800-4200 depending on fuel and motor options. I've got a heavy motor.
The doc I've got which lists the tested speed:
http://lakebox.dnsalias.com/photos/1998csx23/boat-tests-csx-performance.pdf



Prop looks to be in good shape, 23 pitch stainless Mirage Plus.

 

[180shabah]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

Switch the tach back to the 8cyl position.

Merc FSM's are available here

 

[MikDee]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

I see the motor has good compression, and if it runs good, & smooth, first I'd physically check to see if the throttle butterfly opens completely with the controls at the helm, next I'd be checking the timing to see if it's set right at idle, and getting full advance at WOT.

 

[dcg9381]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

Switch the tach back to the 8cyl position.

Merc FSM's are available here

I did that.. I think it's a pot (potienometer) so I should actually calibrate it.

Thanks for the manual link.. That'll come in handy.

Someone else posted a link on how to pull the MPI trouble codes:
http://www.offshoreonly.com/forums/showthread.php?t=103342&highlight=codes

 

[QC]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

Is 7.4L actually printed anywhere on the engine? If so it's a 310 . . . The 385 is badged 454 Mag. Any history on the boat? Can you find out what the original owner saw speed wise?

 

[Tail_Gunner]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

Is 7.4L actually printed anywhere on the engine? If so it's a 310 . . . The 385 is badged 454 Mag. Any history on the boat? Can you find out what the original owner saw speed wise?

Damm intresting boat is it not........ and i bet he stoled it........ 50 mph @ 3900 what a tourqe monster..... she's a Mag..

 

[wire2]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

It could be a single cylinder that's dropping out or dropped out - I don't have enough experience with Marine V8s to really know for sure if I lost a cylinder.

You would feel a single cylinder missing. Theoretically that's 12?% of your total power missing, but with that cylinder still compressing air it's closer to a 15% loss.

I experienced it with a bad (but brand new) plug. It would idle and cruise fine but misfire on acceleration or top end.

 

[Tail_Gunner]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

1. Timing issue or ecm or elec short
2.fuel pressue unlikely
3. bad cam or head's

I wish i had his problem, man i have a itch for that boat..

 

[QC]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

Damm intresting boat is it not........ and i bet he stoled it........ 50 mph @ 3900 what a tourqe monster..... she's a Mag..

Huh? Pass me what you is smokin' there TG A 310 could possibly pull 3900 with a 23 blade . . . My 320 6.2 would and has done better than that with a 24. Uh, like, yeah I know different ratios and different boats etc., but I have seen more than one 7.4L advertised as a 454 Mag MPI.

 

[HT32BSX115]

Re: 7.4L (454) Mag MPI - tops out at 3900 RPM

I had a similar problem with my Liberator when the OMC 460 was installed.

It would only turn 4000-4200 (50 GPS mph with a 20p SS) at WOT when my brother told me that he used to get 5100(& 60+) or so.

Compression was 160+ all cyls.

I tried everything. When I tore it down I discovered several cam lobes were flat.

Now I turn 4400rpm @ 56mph with 26p props



Cheers,

Rick