350 mag EFI has no power over cruise

[northern baja]

OK heres the deal.
1995 21' Baja with a 350 MAG MPI.
My boats slow outta the hole. Appears to run smoothly up to cruising speed, then When I put the hammer down there is little more.
It used to cruise at 3500RPM and top out at 4500-4600 RPM(a little over proped). Now it cruises at 3200 and when I put the hammer down nothing happens(with the sounds of the butterflies opening and air going in) but over time it will get up to 3800-4000 is all its got. I don't have the power to use half the trim I did one year ago. :confused:
Been into the shop three times for computers checks, diagnostic, compression tests, tuneups, filters and they give it back saying everything is great.
It acts like I would expect if the fuel pressure were low. They claim to have cked the pressure in the shop and on the water at WOT and claimed the pressure was correct. I also wondered about the advance but they told me the advance was all controlled by the computer on my boat with no mechanicals to stick. ??
my topend has gone from 55-57 to 45-47 so everything is not all right.
Besides the expense these suckers damage(scratches, stains, holes) my boat everytime they touch it.
So I need an answer on this one. Last week they replaced the timing cover because of an oil leak. They claim they had to pull the motor for that. I certainly hope they looked at the timing chain while they were there. Another theory I had was it needed a timing chain.
In any case I've spent about a grand and the boat doesn't move. any ideas?
A point of clarification my 95 350 Mag EFI is not a TBI setup, It has port injection with a two piece intake that reminds me of a 502 MAG.
1/2 a tank of gas two people on a joy ride used to get the boat into the 4800-4900 range at about 60. But haven't seem it in the last 12 months.

[kevnmcd]

Been into the shop three times for computers checks, diagnostic, compression tests, tuneups, filters and they give it back saying everything is great.
How many hours on on the motor? If nothing has changed in the last 12 months, I would be suspect of the compression test being accurate. I have a 502 mag and have had the same thing happen over the last couple of trips. I used to see 70 mph and this last trip I couldn't get over 57 mph. Do you always take it to the same shop? Maybe you should try a different one and have them do a compression test? :idea:

[SmokinLowriderSS]

I have seen posts of the knock sensors going out and causing the ignition computer to interpret that as engine knock(detonation). Then the computer retards the timing to stop the knocking, and keeps retarding it as far as it can. I don't know how to check yours, I'd look there now. They COULD check the timing is advancing properly, I'll bet they aren't doing it. :hammerhea If you have an adjustable light, do it yourself. You should be running arround 10 deg at idle and up to a max arround 35 degrees by 3,000 RPM or so. I think I have read there is a way to set a bypass of the sensor but I have no certainty.

[northern baja]

Interesting responses. Thank you. Same shop. My house is across the street from the marina but it's only local place. If they could fix it it would have been nice I blow into town with friends just in time for happy evening. I'd like to go boating on Saturday. They mess with it during the week and I limp it along on weekends. The claim is compression was good on all cylinders.
The notion about the knock sensor is interesting. But they should have picked that right up with their scanner tool.
Maybe it time to head doen the road.

[Beer-30]

You still didn't mention how many hours?
And...you haven't moved recently have you? Almost acts like you went from sea-level to 4000'. Same lake as before?

[northern baja]

How True. Aproximately 375 hours. Been swimming in the same pond in the ol' Mississippi for most of its life, so no altitude change.

[bordsmnj]

maybe time for a new set of valve springs? did they do a leak down test or just compression?just curious. let us know what happens with it either way
-jason

[ECeptor]

Get a timing light and head to the lake. Check timing at idle and at 3500rpm (at the ramp, in neutral, boat on trailer, drive down in the water).
I had my Thunderbolt IV ignition module go out and it exhibited exactly the behavior you are describing. If you have no advance, let us know and we can make sure you get the right module to replace it with (there are several options).
Let us know how many degrees of advance you have at 3,500rpm. Don't trust the shop to do it...do it yourself (if you can).

[Beer-30]

Get a timing light and head to the lake. Check timing at idle and at 3500rpm (at the ramp, in neutral, boat on trailer, drive down in the water).
I had my Thunderbolt IV ignition module go out and it exhibited exactly the behavior you are describing. If you have no advance, let us know and we can make sure you get the right module to replace it with (there are several options).
Let us know how many degrees of advance you have at 3,500rpm. Don't trust the shop to do it...do it yourself (if you can).
Ooh. Good call! I was thinking coil earlier, but gave up because he would have a miss at times. Lack of timing control would do what he said. Hopefully that is all it is.

[ECeptor]

Ooh. Good call! I was thinking coil earlier, but gave up because he would have a miss at times. Lack of timing control would do what he said. Hopefully that is all it is.
My rig needed a new coil also. Accel 42,000V coil from Auto Zone for $30 cleaned up my high rpm miss. A couple guys from here helped me diagnose that. New plugs wouldn't miss, but then it would creep in after just an hour of run time. Old coil was the stock 1989 unit so it was time. If I had a miss with an old coil I'd change it before I sprung for new plugs.

[ECeptor]

Here are the specs for the Thunderbolt IV ignition modules:
V8-22-30 degrees @ 3,400 RPM
V8-22A-30 degrees @ 4,400 RPM
V8-24-32 degrees @ 3,700 RPM
V8-24S-32 degrees @ 4,600 RPM
If you need a Thunderbolt IV 22A, let me know.

[northern baja]

Get a timing light and head to the lake. Check timing at idle and at 3500rpm (at the ramp, in neutral, boat on trailer, drive down in the water).
I had my Thunderbolt IV ignition module go out and it exhibited exactly the behavior you are describing. If you have no advance, let us know and we can make sure you get the right module to replace it with (there are several options).
Let us know how many degrees of advance you have at 3,500rpm. Don't trust the shop to do it...do it yourself (if you can).
Have timing light will travel. Sounds like an intresting direction.

[northern baja]

My rig needed a new coil also. Accel 42,000V coil from Auto Zone for $30 cleaned up my high rpm miss. A couple guys from here helped me diagnose that. New plugs wouldn't miss, but then it would creep in after just an hour of run time. Old coil was the stock 1989 unit so it was time. If I had a miss with an old coil I'd change it before I sprung for new plugs.
It seems smooth enough, course with the motor covered and behind you I've wondered if I would pick up on a miss.
A friend was telling me his boat had been down on power all season but wasn't missing. On inspection a plug wire was off. No miss?!

[northern baja]

Here are the specs for the Thunderbolt IV ignition modules:
V8-22-30 degrees @ 3,400 RPM
V8-22A-30 degrees @ 4,400 RPM
V8-24-32 degrees @ 3,700 RPM
V8-24S-32 degrees @ 4,600 RPM
If you need a Thunderbolt IV 22A, let me know.
Obviously there are good reasons for all of these. But from a performance point for a small block in a 21'er wouldn't the 24 be the way to go? a touch more advance sooner. Compression is low, step up a fuel grade and go?
I'm posing a question here. I really don't know the pros/cons.

[Beer-30]

Obviously there are good reasons for all of these. But from a performance point for a small block in a 21'er wouldn't the 24 be the way to go? a touch more advance sooner. Compression is low, step up a fuel grade and go?
I'm posing a question here. I really don't know the pros/cons.
It would be interesting. If you needed to buy one, trying a 24 wouldn't hurt before you made a final decision.
A plug wire off/dead would not miss. It would just be down on power. A wire that was still hitting every now and then would be a miss you would feel underway. Nice new set of plugs, wires, cap, rotor would not be a bad thing at that age. I'm considering wires already on my '01.

[ECeptor]

The 22° modules let you run more inital timing so they should offer more performance. You set the timing at idle based on the desired timing when full in. Carnivalride recommended to me to set total timing based on water trials where I start with stock full in timing then increase in 2° increments until top speed drops. Then back off that setting to whatever provide the peak top speed or rpm (all indicatiors of power). On my 330hp 454 I ended up with 34° total and 12° initial.
A full blown tune up like Beer-30 mentioned wouldn't hurt a thing.