[Jeff Barnett]

Hi folks, for some time now, well since I've owned it, my 96 Z255 with 7.4 MPI and Bravo 3 would not go faster than 42 mph, and will not rev past 4200 rpm.
I pulled off the flame arrestor, and set the throttle to wide open to make sure the butterfly valve in the throttle body was fully open, it was, however what I did find was odd, and I am sure the cause of the problem.. at the bottom half of the throttle body, behind the butterfly, I found a gasket blocking almost half the lower portion of the throttle body. See the attached drawing, the red indicates the gasket. I used and exacto knife and trimmed it away and I am hoping this solves the problem. there was all kinds of carbon and crud in front of this restriction, which I also cleaned out. Any opinions out there? did someone use the wrong gasket? did I cut off something that is supposed to be there? Thanks
Jeff

[ShabahZ280]

Not quite sure, but I found a similar thing on my Jeep when took apart the airbox. I removed the plastic plate and never had any problems, or never experienced any gains. For some reason, on the 4cylinder engine there was a restrictor that wasn't there on the 6cylinder models! Strange! Could be an interesting thing to research, especially with a marine engine. Worst case scenario, you might have to replace the gasket. Let us know how it turns out!

[milemarker60]

This is a very common "find" in the boating world on this engine. It has often been thought to cause the high pitched whine coming from the intake. However, if you do the research on the larger boat forums you'll find fairly consistent results after trimming: no noticeable difference in sound and probably no difference in performance. I trimmed mine in hopes to quiet it down but it made no difference. Also, I could not quantify a diff in RPM.

I think I read it was designed to alter the airflow coming in to protect one of the sensors in the throttle body (IAC I think). I've not read any detrimental effects on making the cut.

Dave

[mikeyt]

yeah.....what Dave said.

[Jeff Barnett]

Ok I took it out today, I may be crazy, but it seemed to idle smoother, and slightly higher, maybe 100 rpm more than normal, the water was a bit choppy with about 12 mph winds, I could not get it past 43 mph @ 4400 rpm, it just stops there, and if you pull the throttle back off pinned full, it almost has no effect until it's about mid position, mid throttle seems to get me too 39 mph, when I floor it from mid throttle to full, it's gets up to 43 mph . I've played with the trim to try to get it as smooth as possible, and it trims out nice, at 43 mph I WANT MORE!! it's a 454 with a bravo 3, I should be able to at least hit 50 mph I would think. I am having a mechanic go through the boat this coming monday, perhaps a weak injector or fuel pump or something, as it just stops going faster and wont run past 4400 rpm

[ShabahZ280]

When's the last time the engine was "tuned up"? New plugs, fuel injectors cleaned? Have you ever done the diagnostic trick with the LED and the paper clip in the DLC to try and read if your engine has any codes?

How do I get codes from a 7.4 MPI?

4400 seems pretty low, let us know what your mechanic finds out!

[milemarker60]

I have the 7.4 MPI / Bravo III in my Z302 and it turns 4600-4700rpm max w/ 28p props and shows 48-49mph on the GPS. What props are on yours (26p or 28p)? While I too would like to see higher top speeds, the important part of the performance is how she does at cruise RPM (75% - approx 3400RPM). With the tabs and lower unit trimmed to optimum, mine cruises at 36 mph which I'm quite content with on this size of boat.

The 496 Mag and 502 engines were designed with higher rev capabilites thus giving the boat a higher top end. I'm pretty sure you should be able to squeeze a few more r's out of yours but keep in mind the prop will make a diff on speed. My last boat had the same engine w/ a 26p prop set and would only do 45-46mph at the same 4600-4700rpm (cruised at 32-33mph at 3400rpm) and it was 3' shorter and 400lbs lighter.

Dave

[97 Z222]

According to what my mechanic told me, was that the pitch of the prop should be determined by the RPM of the engine. I have the 7.4 engine and he said that a 21 P (pitch) will allow the engine to turn at full throttle, between 4400-4800 RPM, depending on the load in the boat, and that the RPM should not exceed 4800 RPM. This is a number put out by Mercrusier on that specific motor. My boat is also smaller at 22 ft and lighter, so there are a number of variables to look at. You dont want to small a pitch to over rev the motor, or to large a pitch to put to much tourque on the motor. With this pitch and motor combo I can do 60 MPH with 2 adults and 3/4 tank of fuel. This kind of speed is not very comfortable, as I am new to boating, so I don't see myself doing that speed very often.

[Jeff Barnett]

Hi there, it has a 24 pitch, and a bravo 3. My Mechanic is coming out tomorrow. he says it could be many things, computer, fuel pump, injectors, we'll see. I'll post his findings if any..

[stalysr]

my boat grids into forward drive but not reverse does any one have an idea on my problem?