Merc 350 MAG MPI Bravo WOT Question

[Flynny]

My boat has a 2007 Mercruiser 350 MAG MPI Bravo with a Bravo 3 outdrive (Serial 0W670262). From what I've read, the MAG is supposed to run faster than a typical Merc 350 at WOT, between 5000 and 5500 RPM. However, I'm only seeing a max of 4100 on the tachometer. This is after coming on to plane, trimming the tabs up, and trimming the outdrive up a bit for max speed.

I'm thinking there are two possibilities: 1. I'm actually running at 5Kish RPM and the tachometer somehow fell out of calibration. 2. I have a fuel or prop issue.
My gut tells me it's probably #2, but I wanted to check with you guys and see if its even possible for the tachometer to be inaccurate.

This is my first year with this boat, but I think I have a fuel issue, causing it to make less power, because reviews of similar boats with the same engine show top speeds in the 35MPH range and I'm only seeing 26MPH. I'm hesitant to think its a prop issue because it has quicksilver props on it, and nothing had lead me to believe the props were ever changed from the factory. Plus, its just not running as fast as it should.

Engine serial tag is attached for reference.

 

[harleyman1975]

Well I guess you need to start at the beginning. You have no info regarding prior repairs maint. so I would check the firing order. it is very easy to have 2 wires backwards but it will still run "ok". next I would make sure it is firing on all 8 by pulling one wire at a time while running and look for a drop in RPM's if pulling a certain wire does not cost you like 200 rpms then you are getting somewhere. From there I would do a compression test and look at the plugs while you are at it. plugs should be tan in color light to white is lean and dark to black is fouled (gas or oil). is it missing, shaking, surging or bogging? all these things will give us more to go on. as for the tach you can double check with a hand held unit while running. Test before you start throwing parts at it. Much cheaper.

 

[JASinIL2006]

Easy to check if your tach is off... either get a shop tach or you can get a TinyTach (https://www.tinytach.com/). You need a reliable tach reading anyway to see if you're propped correctly, so confirming your dashboard tach reading is a good thing to do.

 

[alldodge]

Your motor should make about 300hp
Plugs AC Platium 41-932 Gap at .060
Fuel Pressure 43 psi @ 1800 rpm

Timing is not adjustable, but may need to be touched just a tad. There is a Merc service bulletin but currently cannot find it. It deals with performance issues, but yours is far enough off I'm not thinking of it currently.

What prop pitch are you running?

 

[Flynny]

Great idea with tiny tach. I actually have one, so I'll bring it down to the boat this week. I'll just have to figure out how to set it it to 8 cylinder mode..

As far as bogging and any of that, I haven't noticed anything that would point twoards bad ignition. It got new plugs at the start of the season (I will check that they're the right ones), and she starts right up, idles smoothly, and never has an issue other than this 4K RPM limit.

So I'll confirm my tach first, and then I will test each plug like you mentioned, harleyman. If none of that works, I have a spare fuel filter I'll try.

If not any of those, the boat needs to be pulled soon for some fiberglass repair work on the swim platform (don't back into your dock), so I can check the prop pitch then.

 

[JASinIL2006]

Great idea with tiny tach. I actually have one, so I'll bring it down to the boat this week. I'll just have to figure out how to set it it to 8 cylinder mode..

If you don't have the instructions that came with it, I might be able to find mine. I recall having the same question, but it wasn't hard to set to 8 cylinder mode. Let me know if you can't find yours and I'll look for mine. (I didn't see any posted on their web site.)

 

[Bondo]

I've read, the MAG is supposed to run faster than a typical Merc 350 at WOT, between 5000 and 5500 RPM.

Ayuh,..... I think ya read Wrong,..... None of Merc motors are rated at 5500 rpms, unless from the Racin' Division,.....

Some are allowed to run 5000 rpms, some use a rev-limiter at 5200 rpms,.....

'bout All are cammed to pull the Best power at 4800 rpms,....

'n I wouldn't count on the props bein' Right either,..... What pitch are they,..??

Is the bottom of the boat Clean, absolutely No sealife,..??

I'd probably start with a compression test, 'n fresh fuel filters, checkin' the contents of the filters ya take off,.....