1985 Mercruiser, Won't make RPM, Mystery

PWJax

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Nov 24, 2007
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I have looked though as many as the threads on this as possible and can not find anything I have not already tried. Ok here is the situation. I have twin Mercruiser 1985 350's inboards in a Wellcraft. Both rebuilt at the same time, Recently switched from 3 inch exhaust to 4 inch exhaust. Starboard engine does not come up to RPM. When throttles advanced, goes to about 2500 with out issue then as the other engine take the load off will advance to 3200. Sometimes will stay there, Sometimes will just pause there for a few mins, then will go on up to about 3600, Port engine is in the target range of 4400 RPM. OK now for the things that I have tried:

Rebuilt carb. no effect
Swaped carb, to other engine, No effect
Swaped coil, no effect
Replaced Plugs and wires, no effect
Pulled props and checked pitch, was good,
Cutlass bearing and gear box check out good, (will turn freely by hand)
Replaced lifters, on starboard engine No effect
Replaced complete Disturbutor assembly, No effect.
Swapped electronic ignition, No effect
changed fuel filter Nope
Ran engine straight from 6 gallon fuel tank, elimating all fuel lines before the fuel filter, Nope.
Convinced it is not a tach problem, you can hear that the engine is not running up. And at a lower RPM, you can hear them when they synk...

Engine ran good after rebuild, and after exhaust upgrade, the problem has slowly developed, or it has taken me a long time to notice it. I normally cruise at 3300, to 3400 RPM, Never run at WOT for long

Any Help our suggestions would be greatly apprciated!
 

Don S

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62,321
Re: 1985 Mercruiser, Won't make RPM, Mystery

Here is a list of possible causes of low WOT. Mark them off as they are actually checked and actually do the tests and you will find your problem.

Engine Won't Reach Operating RPM. Check

  • Fuel condition. Type and Octane possibly old fuel
  • Propeller pitch or diameter, damaged blades
  • Restricted fuel pickup tube or anti siphon valve Fuel System Test
  • Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
  • Marine growth on hull and outdrive
  • Wrong gear ratio in outdrive
  • Restricted carburetor air intake (clogged flame arrestor)
  • Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
  • Poor cylinder compression Compression Test
  • Carburetor defective, or wrong type.
  • Fuel pump pressure and vacuum
  • Boat overloaded, improperly loaded, or improperly trimmed.
  • Engine Overheating
  • Engine timing and ignition system operation
  • Remote control cables and linkage for proper travel to open throttle plates fully.
 

PWJax

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Nov 24, 2007
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Re: 1985 Mercruiser, Won't make RPM, Mystery

Don, Thank you for the quick response. I will be working your list today.
 

steelespike

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Apr 26, 2002
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19,069
Re: 1985 Mercruiser, Won't make RPM, Mystery

Maybe something loose in the exhaust.Try to rev in neutral see if it acts like exhaust becomes restricted.Had a car that would sort of shut off as it reved.
It was a loose baffle in the muffler.Maybe the water some how backing up.
Perhaps a hose or flash supresser on the intake or an exhaust hose collapsing.Try installing a vacume gauge these can give you clues about all kinds of engine problems.
 

PWJax

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Re: 1985 Mercruiser, Won't make RPM, Mystery

Thanks for the input Steele; I checked the exhaust system today, Nothing blocking it,

Went though most of the list that Don put up earlier, working on the fuel pump theory at the moment, Will be double checking compression tomorrow, I believe I have eliminated ever thing else off the list.

Had an old timer at the dock today tell me that I should lower my spark plugs two heat levels, currently using AP26's. His suggestion didn't make a lot of sense to me, but I thought I would throw it out there and see if it made sense to any one else.
 

Bondo

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Apr 17, 2002
Messages
70,464
Re: 1985 Mercruiser, Won't make RPM, Mystery

Mercruiser 1985 350's

AC MR43T is the right plugs for those motors......
 

sippi

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May 20, 2007
Messages
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Re: 1985 Mercruiser, Won't make RPM, Mystery

Somthing you can try also is to check the exhaust temps while running on both motors. Like your compression they should all be fairly close, if one side or the other is considerably higher or lower it will let you know thier is somthing going on with that side of the motor.
 

JustJason

Vice Admiral
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Aug 27, 2007
Messages
5,319
Re: 1985 Mercruiser, Won't make RPM, Mystery

using a kv meter will save a bit of time and indicate a dropped cylinder... wether its a spark or compression related drop is a different' story, but its a quick easy way to get you in the right direction. How well does the bad motor idle? is it lumpy, high, low?
 

PWJax

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Nov 24, 2007
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Re: 1985 Mercruiser, Won't make RPM, Mystery

Update, Seems that the mystery is solved. A Recheck of the compresion today showed #6 Cyl, with low compression, Will be breaking it down tuesday to check for blown head gasket, Will let you know the outcome, Thank you all for good suggestions.
 
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