rpms just dropped outa nowhere

[coolkook]

1977 mercury 850...so was out on the bay today..was out a cpl hours no prob..had lunch then headed home...was running about 20mph "not WOT" cause it was kinda choppy...out of knowwhere the rpms just dropped and we limped home at 6mph...when i got home i put engine in gear out of the water. engine off..and prop wouldnt spin...i was told if i spun prop that it would spin freely in gear...so i took it to lake..at WOT it sounded like it was only at 1/4 throttle and again only went about 6mph...any suggestions ? ..pretty sure at idle i could get rpms up but i honestly didnt check...ty

 

[Chris1956]

Check compression on all 4 cylinders. If they are close, check spark on all cylinders. Post back results.

 

[coolkook]

I did check about 3 weeks ago and have only taken bout out 3 times they all read around 140psi...i did take cowl off when i got home...i notice some oil around plug area..one plug was loose...could this have been the problem ?

 

[Texasmark]

4 cyl engine, 1 plug loose, I doubt it. What were your rpms when running just 6 mph?

 

[coolkook]

The rpm were low...i dont have a tach...at full throttle in gear it sounded as though it was 1/4 throttle...i havnt checked idle throttle but i think idle throttle worked normal...ill test it later to be sure...ty for replies..very frustrated at this point

 

[coolkook]

Yes at idle throttle it will revv up...but in gear dosent seem to get rpms up...i got 8mph tops...i looked at prop i dont think its a spun prop

 

[Texasmark]

If you have a spun prop 2 things happen: When you firewall it the engine will scream and you will go nowhere.....essentially. If you run a scribe like from the spline of the prop straight out across the middle.....to where the exhaust comes out and take the boat out and hammer the throttle a couple of times and remove the prop and see that the line is no longer straight, meeting both requirements, yes you do. If you miss either, no you don't.

 

[jimmbo]

Have you had the prop off recently? Check the oil in the gearcase? With the engine off and in neutral, does the prop spin freely?

 

[coolkook]

Ty guys...Prop spins freely in neutral..no resistance...i took it off it looked fine..pretty sure its not prop..im honna check comp and try to clean carbs as best i can as well as check plugs...its not water in fuel ..i did run seafoam in the mix but i also doubt that had anything to do...im leaning towards fuel, electrical, or compression ...my mech is 3 weeks out at this point..so ill just do what i can until then...the lower unit was installed 3 trips ago...and the gearoil was changed then

 

[coolkook]

So to refresh in nuetral i can get rpms up..in gear not so much...i looked at the carbs with engine off full throttle it looks to me like butterflys are wide open

 

[Chris1956]

Compression and spark test are necessary to diagnose. It is no unusual for rings to break or pistons to seize while running. When that occurs, RPMs drop significantly. sound familiar?

 

[racerone]

Stop running this motor untill you confirm that carburetors are clean with no blockage !-----No fuel going in means no oil !---A quick way to damage a fine motor.

 

[coolkook]

Rpms dropping significantly does sound familiar..8(..thats exactly what happened when it occured it sounded as if throttle was dropped to idle in 1 second..i was only half throttle going about 20mph max...did i do something wrong running the seafoam in gas...would that ruin rings or piston ?....i was running 93octain gas with royal purple 2 stroke oil at 50:1...and put 1/2 of reccomended ounces of seafoam.....i will stop running it ty

 

[jwdavis28]

My experience is more with jetski two strokes but this sounds a lot like when I would stick a piston. Usually meant I had to tear it apart and rebuild again.

 

[coolkook]

What does "stick a piston" mean ? Please excuse my ignorance i havnt heard that term b4

 

[Chris1956]

I guess "stick a piston" is the same as seizing an engine.

 

[coolkook]

And it would still run ? Can a piston be unstuck ?

 

[Texasmark]

What does "stick a piston" mean ? Please excuse my ignorance i havnt heard that term b4

piston stuck to the cylinder wall due to friction and heat.

While I'm answering this your question about Sea Foam causing you problems.........forget that. It's been effectively used as a fuel supplement in millions of engines for the last 50+ years.

 

[coolkook]

Ok well thats good i didnt do the wrong thing running seafoam....i guess in wondering if engine would still run if piston was stuck?...im thinking this may not be problem..i wasnt pushing motor and it was running good , mix was correct...ill check comp on mon and report my findings...ty all

 

[jwdavis28]

The piston many times will unstick but performance sucks. On a 2 cylinder jet ski you would notice it more than your motor but same principle.