Merc inline 6 no power

biffery

Recruit
Joined
Sep 5, 2002
Messages
2
Hi, I have a 79 model 115 merc inline 6, that I bought about 2 years ago. It is on a 17 foot Mustang, and has only ever revved to 4900, trimmed right out, with a 15 inch prop. I have had the carbs serviced, new fuel pump, distributor serviced, new plugs and leads, but no different. A friend has the identical boat and motor, and gets 5600 rpm with a 17 inch prop. He thought that maybe the motor was dropping cylinders up high, but, if you drop off plug leads, one at a time, while going along, the motor loses performance. It seems to idle really well, good compression, and will rev quite hard in neutral. Can anybody help? Thanks in advance<br />Biff
 

jamescody

Seaman Apprentice
Joined
Jul 15, 2002
Messages
34
Re: Merc inline 6 no power

so your friend has the identical boat?? I'm running an 82 Mariner 90 Hp in a fairly heavy 17 footer and I pull about 5000 rpm with a 15 pitch prop. If your sure you should be pulling more revs, check your timing, it should read 20 - 21 degrees on the starter, which would equate to about 18 degrees WOT on the water. These 90 HP motors are only reccomended to turn 5000 rpm max, as the ports are fairly small. Running them over 5200 will give cause the cylinders to lean out, bad news at high rpms. One good thing about these motors is they have nice low end torque, the hole shot will be as good if not better than the 115 - 155's, you just have to keep them in the power band. I would say that you might be slightly unerpowered with the 90 on a 17 footer. If you are worried about holeshot, try venting your prop, it really helps bring the revs up...kinda like slipping the clutch...this practice does not hurt the top end at all when it is done right.<br /><br />James
 

jamescody

Seaman Apprentice
Joined
Jul 15, 2002
Messages
34
Re: Merc inline 6 no power

is it possible your buddys engine is a badged 90 HP with a 115 replacement powerhead??<br /><br />Just a thought
 

biffery

Recruit
Joined
Sep 5, 2002
Messages
2
Re: Merc inline 6 no power

My engine is a 115, not a 90. The timing was set by the shop, and I checked it also. It was 4 degrees at idle, and 28 at WOT, which the sticker on the front of the motor said. Max rpm, in the manual is 5800<br />cheers Biff
 

jamescody

Seaman Apprentice
Joined
Jul 15, 2002
Messages
34
Re: Merc inline 6 no power

geez sorry mate, my apologies....yep that 115 should pull alot higher rpm with the 15 pitch prop....perhaps your prop has got a fairly good cup on it? A deep cup will act like a higher pitched prop....are the top end speeds similar between the boats? <br />If thats not it, it sounds like its not hitting on all six. I know you said everything was replaced or gone thru, how do the plugs look? is there one thats fairly clean?<br />Any chance of the flotation in your boat being waterlogged?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Merc inline 6 no power

See if it helps to spray a little fuel mix into the carbs at speed May need just a touch.If it helps, look for fuel problems.
 

aeronutt29

Petty Officer 2nd Class
Joined
May 28, 2002
Messages
112
Re: Merc inline 6 no power

Have you tried using your buddy's prop on your boat? That would eliminate any question about the prop being the wrong size. <br /><br />You mentioned that the compression was good but didn't say what the numbers were. I've seen a lot of people who assume that since the motor is hard to turn over by hand that the compression must be good. If you didn't measure it with a gauge, you didn't measure it at all! Post the numbers so we can make a more educated guess. <br /><br />Hold your hand 1 inch in front of each carburetor while the engine is idling. If one of them spits fuel onto your hand then you have a broken reed valve which will definately limit your RPMs. I had this problem on mine and had to rebuild the powerhead to fix it. Hopefully yours won't require such drastic measures.<br /><br />The first thing I do when trying to make ANY engine run better is pull the carburetor(s) off, open them up, and soak them overnight in a bucket of carburetor cleaner. The spray stuff is OK for quick cleanups, but the soaking bucket is the best method. I know you said they were serviced, but if they weren't soaked in cleaner overnight then they weren't serviced PROPERLY.<br /><br />Finally, DON'T REV THE MOTOR IN NEUTRAL! :mad: Bad things can happen!
 
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