77 Merc 850 suddenly running poorly

[gjungle56]

For over 25 years, the family speed boat has performed almost flawlessly. Last weekend though, that changed. After picking up our last tuber, we proceeded to shore. As it is late in the season, the lake has weeds near the surface and close to mooring the water is shallow.We drove in with the motor up a bit but not so high that it didnt have cooling water. When it came time for more activities, the motor was hard to start (very unusual) and when did run, I had to use warm up lever. Also there where globs of two stroke oil coming up from under water exhaust and engine was misfiring. Took off air box cover and found puddle of oil at bottom. I blotted this up and tried engine again but not better, finally got running long and in gear and got up to speed hoping to clear out exess oil but when came back in engine still had no power and would not run when put into gear. Any help appreciated.

 

[daveswaves]

Re: 77 Merc 850 suddenly running poorly

For over 25 years, the family speed boat has performed almost flawlessly. Last weekend though, that changed. After picking up our last tuber, we proceeded to shore. As it is late in the season, the lake has weeds near the surface and close to mooring the water is shallow.We drove in with the motor up a bit but not so high that it didnt have cooling water. When it came time for more activities, the motor was hard to start (very unusual) and when did run, I had to use warm up lever. Also there where globs of two stroke oil coming up from under water exhaust and engine was misfiring. Took off air box cover and found puddle of oil at bottom. I blotted this up and tried engine again but not better, finally got running long and in gear and got up to speed hoping to clear out exess oil but when came back in engine still had no power and would not run when put into gear. Any help appreciated.

It sounds like a reed petal has broken off. The oil puddle in the air box is from fuel blown back through the carb throat. You may be able to see it looking in the carb throat with a flashlight.

 

[Chris1956]

Re: 77 Merc 850 suddenly running poorly

Dave has jumped to a conclusion. He may not be wrong, however, it would be unusual to have a broken reed. More likely the fuel spilled when the motor was tilted up.

Is this the first time you have used gasoline with alcohol in it? if so, it can loosen debris and clog the carbs. Inspect the fuel strainer, and the fuel screens under the brass fittings on the carb covers. Inspect the fuel line for deterioation.

The carbs may also be gummed up....

 

[daveswaves]

Re: 77 Merc 850 suddenly running poorly

Dave has jumped to a conclusion. He may not be wrong, however, it would be unusual to have a broken reed. More likely the fuel spilled when the motor was tilted up.

Is this the first time you have used gasoline with alcohol in it? if so, it can loosen debris and clog the carbs. Inspect the fuel strainer, and the fuel screens under the brass fittings on the carb covers. Inspect the fuel line for deterioation.

The carbs may also be gummed up....

Yes I did, which is unusual for me. The engine was still running when tilted up so I discounted fuel spillage from that source. Good possibility though. Thanks for keeping me honest Chris!
A simple test would be to hold a white piece of cardboard in front of the carb intake when the engine is running. If fuel is being sprayed back that "could" indicate a damaged reed.

 

[gjungle56]

Re: 77 Merc 850 suddenly running poorly

Thanx for some leads. Was wondering if the oil sucked up may be really fouling the 2 lower cylinders. I also noticed that I will have to replace some wiring with crumbling insulation. Is the flywheel hard to remove? Also the engine has been run on gasahol before.

 

[freshness247]

Re: 77 Merc 850 suddenly running poorly

gjungle56,

My family too has a 1977 Merc 850. Dad bought it brand new in august of '76. This motor has run to near perfection for the 33 years of use. However, a few weeks ago my brother and I had it out for the day, drove up river (approx 9 miles, slow and fast 5200rpm, 40mph) with no problems. Met up with some friends for about 4 hours. The last 200 yards of the trip we trim the motor up to the highest tilt setting due to low water. (this motor has never hit bottom).

When it was time to start up and head back down river, it struggled to start (nearly always just click the key and fires up) and struggled thru the low rpm range.

Long story short, since this problem we have replaced spark plug wires and fuel pump diaphragm (first time since the motor was purchased). Also have taken carbs apart, soaked in cleaner for an hour and reinstalled. We also put a new hose from the tank to motor as the old one hadn't been replaced in 7 years or so. I hear rumors of the new gas deterioratng fuel lines and this is 100% true.

I cut apart the old line and found the internal liner "flaking". We decided to do this after checking the fuel filter screen and finding small flakes of this liner there.

We have contacted Merc Classic Hotline who recommended Mercury Powertune.

Motor idles as slow as 500rpms but is rough from 500-1200rpm. Occasionally has a "surge" from 1200-1700 then plains off and runs great at higher rpms.

 

[merc850]

Re: 77 Merc 850 suddenly running poorly

I had problems with my '75 850, I cleaned the carbs and stumbled across this by accident, a hairball had collected at the bottom carb. You have to pop the tops and unscrew the fittings, a service bulletin recommends removing the screens and discarding.