1978 150 horse mercury black max on 1978 sea star 18ft tri hull

m.qualls89

Petty Officer 2nd Class
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Jun 4, 2012
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This is my first boat so im new to all of this. My question is how fast should my rpms be turning at full throttle down the river im only getting about 4000-4500 rms on a 21 pitch prop top speed about 35 loaded with 5 people. Is this right? Also it idles fine on the hose and when i first start it in the water but after heading to a cove and shutting it off to swim it doesnt want to idle and slow soeed trying to get it on the trailer it dies also but will run fine full throttle. Any help is greatly appreciated.
 

sutor623

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

I believe that motor should be turning at 5500RPMS. BUT your rpms should pick up when you have less weight on the boat. How often do you take 5 people with you? I try and get my motor spinning at the top end of the spectrum when it is just me on the boat. That way when there is an extra 3-400lbs. the rpms will stay in the recommended band. It sounds like the motor is stalling because you need to do a link an sync. THere is a great deal of info on that subject for the merc inline 6's in the FAQs of this site.
 

m.qualls89

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

With just me and my wife it was at 4500 rpms ill look at the faq section thanks
 

westexasrepublic

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

assuming your motor is running properly. I run a 24 pitch on my 18 foot V hull and doing around 65 mph @ 5500 rpm
 

m.qualls89

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

I looked but didnt see anything on my motor or a link and sync... What is a link and sync?
 
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Chris1956

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

That motor is a V6 right? If it is an inline 6 it is either not 150HP or it is an older Merc 1500. Link and synch is when you set the idle pickup timing to match the carb opening, and set the max timing, throttle stop and idle stop. A service manual will explain it all.
 

m.qualls89

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

Yes its a v6 ill look up a manual in the mean time i think ill clean the plugs... The previous owner put a new fuel pump in, could it be starving for fuel?
 

m.qualls89

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

I checked the cylinder compression when i got off work it seems ok i checked it cold the highest was 120 and the lowest 110 psi
 

sutor623

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

I checked the cylinder compression when i got off work it seems ok i checked it cold the highest was 120 and the lowest 110 psi

Well hell thats definitely in spec. Do you know the gear ratio of your L/U?
 

Faztbullet

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

The rpm for that engine is is 5000-5500 and it only puts out about 135hp at the prop.
m only getting about 4000-4500 rms on a 21 pitch prop top speed about 35 loaded with 5 people. Is this right?
With that much load thats about right.. whats the rpm/speed with 2 people??
 

m.qualls89

Petty Officer 2nd Class
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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

The most i got out of it with just me and my wife was 4500 rpms the speedo said 35 but i think its off because the next time i went i had a speedometer app on my phone and according to it the speedo is about 10 mph off... I cleaned the plugs and sprayed carb cleaner in it last night so ill take it out this weekend and see how it does... Can anyone tell me how to check/set the timing? Never tryed it on a 2 stroke before
 

Chris1956

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

Checking the timing is real easy. Remove spark plugs 2-6. Install them into their leads and ground their bases. Disconnect idle stabilizer. Adjust throttle until carbs just start to open. Crank motor with timing light on #1 spark plug and read Idle pickup timing value. Now advance throttle to full and read max timing.

You can use the inline synch procedure, if you can figure out which adjustment screw on your motor does what. Idle pickup timing s/b 4* ATDC on your ADI ignition. Max aprak advance is 21* BTDC. If you cannot figure out which screws to adjust for what feature,(it is not easy) then you will need to get a service manual.
 

m.qualls89

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

Ok update, i checked the fuel filter and it was clean but when u went to put the cover back on it i noticed it leaking fuel around the seal it took forever to get seated properly but when i got it it fired up and went to almost 6000 rpms in neutral full throttle but still in the water im only getting about 4000 rpm no matter how i trim pulling a tube or not 2 people or 6 people... Any suggestions?
 

Chris1956

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

Well, if you rev it up to 6K with no load, you will soon have no more questions on that motor, as it will throw a rod.

The proper way to choose the correct propeller is to have an accurate tach and a GPS. Run the motor up to full throttle, and maximize speed with trim. Note reading on tach. If the tach reads low, 2 Inches less pitch will bring RPMs up abput 400. The inverse is true for increasing pitch.
 

m.qualls89

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

I only ran it up there for just a second didnt wanna mess it up. I have a lower pitch prop the guy said was for pulling skiers, ill try that and post my results.
 

sutor623

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

I only ran it up there for just a second didnt wanna mess it up.

This is a contradictory statement. Have you ever heard of 2-stroke runaway phenomenon?
 

m.qualls89

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Jun 4, 2012
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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

No im new to 2 strokes... Im guessing i shouldnt have done that? Whats got me wondering is why it will run those rpms with no load but wont do the 5500 it should in the water
 

m.qualls89

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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

Ok so i changed the prop to the smaller one that the guy i bought it from said is for skiing. It biosts my acceleration tremendously but lost top speed. Also it is running to many rpms now at WOT so im gonna have to get a different proo. My only problem is figuring out the pitch of the smaller prop it isnt stamped on it it just says michigan powersports. But either way the 24 pitch seems to be to large causing my problem. Thanks to everyone for all your help.
 

Tregaskis

Seaman Apprentice
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Aug 23, 2005
Messages
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Re: 1978 150 horse mercury black max on 1978 sea star 18ft tri hull

That 24" is way to tall for that combo. Your 150 probably has the 2:1 lower unit and a 14 1/2 x 19" is going to be much closer to your perfect prop. IMHO
 
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