115 Merc Damage Control Detonation

L

Larryb1150

Recruit
Joined
Dec 18, 2007
Messages
1
I have a 1987 115 merc (sn OB 206359) on a 19' Crestliner. She lugs out of the hole and spins 4200 at wot, otherwise it runs good. Has a 22P prop. I was told by a local dealer that these engines will burn up pistons every time unless they are readjusted to compensate for todays fuels and that lugging them is certain death due to holes in pistons. I am rebuilding the carbs and replacing the floats and also want to make all the necessary adjustments and modifications to give this low hour motor a long life . The carbs are WMK 41's with .076 main jets and .094 vent jets. After reading this forum my questions are as follows:
Should I completely remove and leave off the float bowl vent jets?
Should I do anything with the small hose which runs from the float bowl cover to the upper port on the oppisite side of the main carb body?
Are the .076 main jets big enough to prevent lean out conditions?
How do I remove the idle stabilizer module? Just disconect wires?
Do I need to do anything with the enricher valve?
What should the timing be adjusted to?
Is there anything else I can do to protect this last of the in-line 6's. Thanx
 
Don S

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: 115 Merc Damage Control Detonation

Leave everything stock, get it running right, do a sync and link. Then get it propped to the proper rpm. Now you can start tinkering with it.
Just because you read something on the forums, doesn't mean it will all work for your 87 115 Merc.
 
D

DJ

Guest
Re: 115 Merc Damage Control Detonation

Larry,

Your prop is way to steep. A 22" prop on a 115, for a heavy boat, is a sure way to lug it.

One inch in prop pitch equals about 200 RPM, up or down.
 
hkeiner

hkeiner

Lieutenant Junior Grade
Joined
Oct 17, 2006
Messages
1,055
Re: 115 Merc Damage Control Detonation

When re-propping, you might consider getting a four bladed lower pitched prop. I found that the extra blade helps a lot in getting a better hole shot and running at lower speeds. You just give up a little of top end speed.
 
j_martin

j_martin

Admiral
Joined
Sep 22, 2006
Messages
7,474
Re: 115 Merc Damage Control Detonation

115 on a 19 foot bote ain't high performance by any means. Top speed don't mean much if the boat is hard to use and enjoy. Prop it for what the motor wants, I'd guess about 16-17 inch pitch. It needs to rev over 5000 rpm at WOT or you'll stress the engine. You need a ventilated prop to let it spin up for the hole shot. If it were mine, the next prop I'd try would be a 16 inch high five.

Don't mess with engine adjustments unless you can read the plugs, and have the instruments to check timing etc. Stock is good for longevity. Merc's are set up a little rich stock to stay cool and safe. If you can get non-oxygenated gas, I'd use it. (92-93 octane at most pumps)

hope it helps
John
 
OldMercsRule

OldMercsRule

Captain
Joined
Nov 30, 2006
Messages
3,340
Re: 115 Merc Damage Control Detonation

Larryb1150 said:
I have a 1987 115 merc (sn OB 206359) on a 19' Crestliner. She lugs out of the hole and spins 4200 at wot, otherwise it runs good.

There is no "otherwise". It does not "run good" at all, if it lugs n' then turns only 4200 at WOT! As the others have said you are likely propped wrong. When ya hit the throttle that engine should turn 5500 RPMs + or - in short order 'till ya back off the "juice". I would get the prop right before runnin' any time on that engine! My overpriced $.02 JR


Has a 22P prop. I was told by a local dealer that these engines will burn up pistons every time unless they are readjusted to compensate for todays fuels and that lugging them is certain death due to holes in pistons. I am rebuilding the carbs and replacing the floats and also want to make all the necessary adjustments and modifications to give this low hour motor a long life . The carbs are WMK 41's with .076 main jets and .094 vent jets. After reading this forum my questions are as follows:
Should I completely remove and leave off the float bowl vent jets?
Should I do anything with the small hose which runs from the float bowl cover to the upper port on the oppisite side of the main carb body?
Are the .076 main jets big enough to prevent lean out conditions?
How do I remove the idle stabilizer module? Just disconect wires?
Do I need to do anything with the enricher valve?
What should the timing be adjusted to?
Is there anything else I can do to protect this last of the in-line 6's. Thanx
Click to expand...
...
 
You must log in or register to reply here.
Top