350 Mag RPM Problem

[popazog]

My new 350 mag horizon on a new 23'Chase (sport cuddy) is confusing me and the guys at the dealership... it won't turn the 19" Mirage Plus that came with it past 4400 RPM. I know several other boaters running very similar boats with the same power who are able to hit 5000 rpm even with a 21" pitch prop. We will be trying a 17" Mirage in an effort to increase RPM and hit W.O.T, but I feel that the problem may not be the prop. Shouldn't a 350 Mag be able to hit W.O.T with a 19" or 21" pitch screw? I know that I will loose a lot of top end the lower I go so if anyone can offer any suggestions as to what the problem might be please let me know.

 

[russthemuss]

Re: 350 Mag RPM Problem

Have you checked the gauge is correct?

 

[495v]

Re: 350 Mag RPM Problem

I have a 350 Magnum and just encountered the same problem. The prop is a 19" Mirage SS and my tach was only reading about 3800-3900 RPM. I hooked a dwell up to it this weekend and found the tach is going bad. First the tach should have three setting's; 4c, 6c, 8c. Make sure it's on 8 cylinder and try calibrating it to a master tech. I used a tech that attaches to the negative terminal on the ignition coil. My tach was off about500-600 RPM. I was able to calibrate it except it's still off by about 200 RPM. I will be replacing it. You said you are reaching 4400 RPM? I believe the recommended range is 4400-4800 with a rev limiter at 4900. You sound like your right there.

 

[popazog]

Re: 350 Mag RPM Problem

Thanks guys. I know 4400 is getting there, but the W.O.T should be 4800-5000 RPM for my 350 mag horizon model. 300 HP should turn a 19" no problem don't you think? The dealership wants to swap in a 17" to bring RPM up, but I think that is a lazy fix... I will have them check the tach, but the boat feels slow only getting to 50 when I should be able to hit closer to 60 so I don't think the tach is the issue. The engine just feels like it's only working 3/4. Maybe a compression problem or something. This is my first boat and I am not a marine mechanic so this is all new to me and to be honest I don't really trust the service guys at the dealership... they want to charge me $340 for the "20 hour service" which is really just a fluids and filters change and they try to make me feel like I am expecting too much in asking them to correct the RPM issue. I feel they should make every effort to deliver a fully functioning package with as little hassle to me as possible. Thanks for listening to me Vent.

 

[495v]

Re: 350 Mag RPM Problem

You state that WOT is between 4800-5000? I checked with Mercury and that range is 4600-5000. Your only 200 RPM off and by switching to a 17" will probably get you up to 4800-4900 which is at the top of your range. What kind of load do you have in the boat as far as weight??? If it's a brand new boat, it's probably setup for an average load. As far as speed, don't trust your speedo. Most of them can't read speeds over 50. Try your speed with a gps or radar gun.