Piston degradation -115 hp Merc

[jpolizotti]

The piston in cylinder 2 of my 115 merc is fouling plugs and deteriorating the piston. All cylinders except #2 are at 120 PSI. Cyl. #2 is 130 psi. The engine runs fine for 10 minutes with a new plug then fouls with metal deposits. I have heard carb probs, timing maybe? Any ideas before I take drastic measures? Thanks

 

[jimmbo]

Re: Piston degradation -115 hp Merc

Have youu looked inside the cylinder when you have the plug out?

 

[ZmOz]

Re: Piston degradation -115 hp Merc

Have you decarbed the engine anytime recently? I'm no expert, but it could be a carb issue, timing, or maybe some debris in the water jacket not allowing #2 to cool properly? I would start by checking the timing and removing the carbs, clean them, and replace all the carb gaskets.

 

[emckelvy]

Re: Piston degradation -115 hp Merc

If you have a distributorless model, verify that the timing pointer is set accurately. Set the timing to no more than 21 deg BTDC at cranking speeds. If you have a late enough model with idle stabilizer, disconnect and remove it. They can fail and mess up the timing.<br /><br />Use a premium oil (Penzoil full synthetic is good, and cheap at Wally's); use mid-grade or premium unleaded non-alcohol fuel. If all you have is gasohol, use regular.<br /><br />Be sure to take that carb off and blow it out. check the main jet specifically. Make sure the jet is of the correct size.<br /><br />Also check the float adjustment and take the needle assy out entirely to check for restrictions/debris. Check the fuel lines for internal deterioration which could act as a 'check valve' to restrict fuel flow.<br /><br />If you have 'back drag' carbs, remove the 'vent jet' from each of the 3 carbs. This jet is visible externally and sticks out the side of the top of the float bowl cover. If you have one fuel pump mounted on the stbd lower side of the engine cowling, you'll have these. Don't believe the older style with engine mounted fuel pumps had them but I could be mistaken. If your carbs have the single 3/8" screw holding on a filter cover, you don't have the vent jets.<br /><br />Be sure you are running the correct heat range of plugs, L-76V in a Champion, V40FFM in AC Marine.<br /><br />Also ensure there's sufficient cooling water flow, the exhaust plate should be no more than lukewarm when running.<br /><br />If you have an older inline with distributor cap, pull the cap down and check it for crossfiring, even if it's got just corrosion products it can cause problems.<br /><br />Check for vacuum leaks around the intake port covers, spray some WD or similar around the gaskets while it's idling and if it changes speed you've got a leak. Check vacuum hoses for breaks.<br /><br />Don't run it any more until you address these problems or the damage will be even worse than it is already. 10 psi diff on a Merc is not good. Maybe it's carboned up and the decarboning will help with cylinder balance.<br /><br />HTH and Good Luck.......ed

 

[rodbolt]

Re: Piston degradation -115 hp Merc

hello<br /> if your getting metal on a plug its time for a leakdown dest. odds are its also time for a teardown. metal is bad when its attached to the plug. normally indicates that some bad things have happened. any good shop should be able to perform this test. if leakage is more than 10% its time for that teardown.<br /> good luck and keep posting

 

[emckelvy]

Re: Piston degradation -115 hp Merc

He may be lucky and the metal is only coming off the deflector, and not from the ring lands' area yet. But you're right, the compression diff is disturbing. <br /><br />Strange that the suspect cyl is 10 psi more than the others. Unless all the other ones are bad, maybe #2 has a ton of carbon built up??

 

[kev_79]

Re: Piston degradation -115 hp Merc

Metal is always bad. I would suggest a tear down to inspect before something breaks and messes up the head. No removable heads on the Merc Inlines.<br /><br />It is very interesting that # 2 has more PSI. I'm with Ed, seems like carbon could be the only thing to cause this.<br /><br />k

 

[jpolizotti]

Re: Piston degradation -115 hp Merc

Originally posted by jimmbo:<br /> Have youu looked inside the cylinder when you have the plug out?

I know muchtime went by since I posted this, but winter is longand I am jst getting around to thinking about boating again. To answer your question - yes I looked in the cylinder with the plug out and I can view the top of the piston which shows a sort of erosion. Material has come off the top of the piston but it is still in tact.<br /><br />May be just a matter of time before it blows

 

[jpolizotti]

Re: Piston degradation -115 hp Merc

Originally posted by Ed Mc.:<br /> If you have a distributorless model, verify that the timing pointer is set accurately. Set the timing to no more than 21 deg BTDC at cranking speeds. If you have a late enough model with idle stabilizer, disconnect and remove it. They can fail and mess up the timing.<br /><br />Use a premium oil (Penzoil full synthetic is good, and cheap at Wally's); use mid-grade or premium unleaded non-alcohol fuel. If all you have is gasohol, use regular.<br /><br />Be sure to take that carb off and blow it out. check the main jet specifically. Make sure the jet is of the correct size.<br /><br />Also check the float adjustment and take the needle assy out entirely to check for restrictions/debris. Check the fuel lines for internal deterioration which could act as a 'check valve' to restrict fuel flow.<br /><br />If you have 'back drag' carbs, remove the 'vent jet' from each of the 3 carbs. This jet is visible externally and sticks out the side of the top of the float bowl cover. If you have one fuel pump mounted on the stbd lower side of the engine cowling, you'll have these. Don't believe the older style with engine mounted fuel pumps had them but I could be mistaken. If your carbs have the single 3/8" screw holding on a filter cover, you don't have the vent jets.<br /><br />Be sure you are running the correct heat range of plugs, L-76V in a Champion, V40FFM in AC Marine.<br /><br />Also ensure there's sufficient cooling water flow, the exhaust plate should be no more than lukewarm when running.<br /><br />If you have an older inline with distributor cap, pull the cap down and check it for crossfiring, even if it's got just corrosion products it can cause problems.<br /><br />Check for vacuum leaks around the intake port covers, spray some WD or similar around the gaskets while it's idling and if it changes speed you've got a leak. Check vacuum hoses for breaks.<br /><br />Don't run it any more until you address these problems or the damage will be even worse than it is already. 10 psi diff on a Merc is not good. Maybe it's carboned up and the decarboning will help with cylinder balance.<br /><br />HTH and Good Luck.......ed

Ed Mc,<br />Thanks for this detailed look, I really appreciate the time. These are all great suggestions and look at these one at a time - Thanks again, you are a really useful resource.<br /><br />jjp

 

[rodbolt]

Re: Piston degradation -115 hp Merc

hello<br /> is this a 6 or 4cyl 115?<br /> and if its a 6 and only the deflector is eroding you may be lucky. I have seen the deflectors(crowns) eaten some with no running damage. usually its due to carb lean out or detonation or overheat. follow the advice and hope for the best.

 

[Clams Canino]

Re: Piston degradation -115 hp Merc

Ditto.....<br /><br />These pistons sometime become "self-saving" by slinging a little melted aluminum off the deflector and fouling the plug.<br /><br />Check advance carefully, and check the carb / fuel deliverry for lean-out.<br /><br />But you need to find out **why** it's doing that before running it. And as with all inlines, de-carb well and often.<br /><br />-W