opinions please Horn on 91 Merc 150 sounding intermittant

Hello All
First I am new to the forum and haven't been around outboards for years. We purchased a used 20' Wellcraft with a '91 Mercuery Black Max 150. We've used it almost every weekend since we got it in the beginning of July. Last weekend we got a continuois(sp) horn from the control box while at full throttle which I assumed was heat. Since we were near the beach and I could see water coming out the telltale I throttled down and stopped at the beach. After spending a cople hours there I ran the motor for a bit at idle and it seemed fine. At the end of the day we heading back to the trailer ~20 minutes various speed without incident. This Friday We took the boat out and ran it at various speeds for about 45 minutes stopped for a picnic dinnner and then another 1 hour home. No incident. Yesterday we put in early and ran the boat for about 30 minutes at mid throttle through some swells, stopped to pick up some people and then headed back out into the swells at mid throttle. In about ten minutes the horn came on. It was not a safe place to stop but I throttled down to idle and in ~30 seconds the horn stopped. Again you could see the telltale. As we were with another boat I decided to start again and the boat ran without incident at about 3/4 throttle for ~45 minutes. at the end of the day we had better wave conditions so I was able to run for another 45 minutes at 3/4 throttle, drop the same peole off and then run another 30 minutes at 3/4 throttle back to the trailer all with no horn at all.
This was all in saltwater if that makes a differrence...
Anybody have any idea where to start?
Thanks in Advance!!!!!

Re: opinions please Horn on 91 Merc 150 sounding intermittant

Do you have a water pressure gauge on it?

John

Re: opinions please Horn on 91 Merc 150 sounding intermittant

I'm afraid I have no working gauges other than Tachometer and even that is questionable

Re: opinions please Horn on 91 Merc 150 sounding intermittant

Ran motor on "muffs" tonight for ~20 minutes at idle with no horn. Motor was warm but touchable...

Re: opinions please Horn on 91 Merc 150 sounding intermittant

You need a water pressure gauge. You're probably trimming it out till it starts to suck air into the cooling system.

hope it helps
John.

Re: opinions please Horn on 91 Merc 150 sounding intermittant

Hmm
I have to admit I can't quite get the hang of the trim thing. The only other two boats I've owned had no trim. The trim gauge doesn't work and so I have no reference and just try to tap it up till it seems to ride better. Any tips on how to get the "feel' for correct trim? Also it seems like trim the motor up would push the nose up but it does the opposite, correct?

Think I would have hurt the motor?

Sorry for my lack of knowledge... and THANKS for the help and advice!!

Brian

Re: opinions please Horn on 91 Merc 150 sounding intermittant

Any other thoughts?
As I'm not sure how to install a water pressure gauge I'm going to try the pond this weekend and leave the trim alone to see how it goes...

Re: opinions please Horn on 91 Merc 150 sounding intermittant

Without gauges you have no idea where your engine trim is, so you can't even learn effectively how to trim the engine. You also have no idea if you've trimmed the engine to the point the water pickup is sucking air. You have no idea what your engine RPM's are. You don't even know if you have enough gas to get home.

You can continue to be willfully ignorant, or you can fix the damned thing and learn how to drive it.

Meanwhile, you have probably taken the goodie out of the water pump. Again, you can remain willfully ignorant and drive it till it freezes up, or you can fix it.

hope it helps
John

Re: opinions please Horn on 91 Merc 150 sounding intermittant

Wow... that's a little harsh.
I wish I could have bought a turnkey boat but that's not the case. I'm doing my best to get this thing fixed up and figured out as time and budget allow. I will be having it serviced this fall but am hoping to scrape out a few weekends before then. I've been trying to get the many different things done as best I can.
I'm not so sure most older boats would come with trim or water pressure gauges. I know this one had trim only and didn't think it was "that" necessary to run the boat with as it is would be for reference only anyways.