1986 25 mercury Proble with knocking

[Gafisherman]

Hey guys,
I have a 1986 mecury 25 hp tiller motor. It has been running great earlier this year, but now i am gettin a knocking sound in my motor and the power has gone down. When i first start the motor it will plane me and my buddy out with no problem but after running problly 5-10 mins it will start getttin the knock in it and will not get all of its power. It runs fine on the low rpm side but when i turn to WOT it just revs up and starts knocking with no power in the motor barely runs faster then trolling. I have checked the reed valves to make sure they are not stuck which they are not. My comp in both cylinders are 125 #1 123 # 2. It also now has developed getting stuck in a high rpm state, like the throttle is stuck open.
What would cause that cause the Butterfly will be complete closed but still at a high idle and i have to pl;ace my hand over the barrel to choke it off. thanks

 

[Maxz695]

Re: 1986 25 mercury Proble with knocking

Other than through the carburetor the only other place that I can think of where the engine could be drawing gas is through the fuel pump. If the diaphram is ruptured it may be pumping gas into the transfer cover. It may be loading up when you prime the engine. As for knocking it is never a good sign. This may mean your crank may be wiped and the needle bearings shot. Also could mean the wrist pins are worn down and the needle bearings there are shot as well. Don,t go tearing anything apart until you have exhausted all other measures and testing. See if any thing is rubbing or bumping around go over the engine with a fine tooth comb trying to find another source of the knocking

 

[Joe-Hunter]

Re: 1986 25 mercury Proble with knocking

Hi Gafisherman !

I am not a mecanic genie, but i do have encountered some problems in the past...

If your engine runs great for few minutes, then starts knocking at WOL, it sounds to me like gas detonation before the spark.

After your engine is warm / hot, gas auto ignite on compression cycle, maybe because you get oil / carbon residues dried on your head, this gets hotter than the rest and ignite gas when compression happens. Or maybe your air / gas ratio is not good, maybe too poor, causing easier ignition and auto-ignition.

If it would be bearing problem, i think it will do it all the time, cold or hot. Like it happened at a car i owned few years ago.

Are you using "Cheap" unleaded gas ?
Like i do... or you also tried higher grade gas too ?

Gas with more Octane will resist more efficiently to auto-ignition. Octane is an anti-ignition. People are thinking than Octane generate power, it's WRONG ! What generates power is HEPTANE. Higher octane ratio gazoline is required in high performance engines, with high compression, fast rpm, and other situations.

If you did not try it, i would say give it a try with 94 octane gas, to see if it will get OK or keep knocking.

Ask a marine mechanic first...

Good luck,

 

[oldman570]

Re: 1986 25 mercury Proble with knocking

Other than through the carburetor the only other place that I can think of where the engine could be drawing gas is through the fuel pump. If the diaphram is ruptured it may be pumping gas into the transfer cover. It may be loading up when you prime the engine. As for knocking it is never a good sign. This may mean your crank may be wiped and the needle bearings shot. Also could mean the wrist pins are worn down and the needle bearings there are shot as well. Don,t go tearing anything apart until you have exhausted all other measures and testing. See if any thing is rubbing or bumping around go over the engine with a fine tooth comb trying to find another source of the knocking

As Max has stated, I would check the fuel pump out first along with the small hoses from the intake transfer covers for detereation and cracks. While there you can pull the intake transfer covers off and look at the pistons for scoreing and ring ware. Check the fuel filter to see if it could be getting pluged. You need to go thru the fuel system and check for air leaks and that the vent on the gas tank is open and working. Water in the fuel will make the motor run bad and could be causing the knocking also. Your motor sounds like it is running hot from a lean fuel mix and could be causeing the knock also. Look at the top of the pistons thru the plug holes for any burning on the piston tops. You might have more than just one problem causing your trouble. After looking everything over, it might be the best to rebuild the carb, and decarbon the motor with Power Tunethat can be bought from the local Merc dealer. Reading the p;ugs will give you a good idea of just what is happening. JMO
Oldman570

 

[Maxz695]

Re: 1986 25 mercury Proble with knocking

sounds like your prop is about to be spun. If it always worked with you and your buddy in the boat but now theres no powr at WOT then something happened to cause the occurance which in turn affected the engine performance. ou could try retorquing the prop if a newer modle and hopefully minimum to no damage was done to the engine (unikely) or get a new prop. Then see if the engine works properly. If not then it ebay time