Merc 25 1986

[FatBack]

Engine runs great to half throttle. Beyond that, cuts out and bogs down. Checked-carb&fuel delivery OK-Checked ingnition OK- Compressiom is exactly 44 lbs each cyclinder checked rings and appear to be OK- no scoring anywhere- Reeds OK-No leaks in crankcase. Compressions jumps to 80lbs when oil is put in cylinders. Pistons OK. What in the heck is going on?

 

[FlyBoyMark]

Re: Merc 25 1986

Carb could be dirty, check "on engine" fuel filter if it has one and check fuel pump diaphrams. Compression sounds low, are you using the rite fuel mix?

 

[FatBack]

Re: Merc 25 1986

Rebuilt the carb-on engine fuel filters fine-yes the compression is low but everything looks fine on disassembly. Took rings off pistons and put in cylinder and they don't look worn-nothing looks worn in the whole engine. Timinig was OK. Oil mix 50:1 OK. This is tuff one!

 

[FlyBoyMark]

Re: Merc 25 1986

If you changed the location and orientation of the piston rings during disassembly in any way, the engine has to be broken in again. Check fuel line for VACUME leaks and the pick up in the tank or jus try a different tank AND fuel line from a friend. Some of the Mercury fuel lines and bulbs have a check valve problem. On the rings, the 20 and 25 HP mercs have chrome cylinders. Now that the engine WAS broken in the cross hatching in the bore from the factory to seat the rings is very worn. New breakin process could take forever.....You may have opened a can of worms...

 

[FatBack]

Re: Merc 25 1986

Rings have a check and can only go on one way. Have ordered new ones anyway. This is a 86 Merc and cylinders aren't chromed that I can tell. New engine to me previous owner had same problem. Gas line,bulb and tank are all new.

 

[FlyBoyMark]

Re: Merc 25 1986

How about fuel pump diaphams, in addition some pumps have a small inlet screen. Have you tried squirting fuel into the engine(carb) at speed to see if it speeds up when it starts to stutter? If you have and the nothing changed then the problem is ignition related.

 

[FatBack]

Re: Merc 25 1986

Diapham is OK Ignition passed all the test.Ground OK. I was hoping someone would say what I suspect but don't know can happen-previous owner ran engine with just the right amount of oil to cause no damage to anything but the cylinder walls. I don't have a mic for that yet. To have the same low compression on each cyl is remarkable. It should be above 100lbs on each.<br />In other words, the cyl walls are worn and the rings are not.

 

[FlyBoyMark]

Re: Merc 25 1986

The engine should not bog down though...try the fuel in a bottle squirt first.....

 

[FatBack]

Re: Merc 25 1986

You are right. But also, it should hardly run at all with 40lbs. of pressure. Three gages all read the same#@&^%!!!! I'll put it back together and hope for the best. Got to go now, thanks for help.