1990 507F90C - Running On One Cylinder

[Bob La Londe]

I was fishing a tournament out of my little boat yesterday. I have a 1990 50HP Force model 507F90C on that boat. It suddenly started running crappy. I pulled it off the main river and pulled the cowling. I pulled both spark plugs to see what the condition was. The upper plug looked perfect, but the lower one was pretty clean except for some grey milky oil. Looks like water mixed in oil to me. I didn't have any choice, so I ran it back up river to the marina where I launched from. It ran on one cylinder all the way. <br /><br />I just checked compression on it this morning. I got 152 PSI on the top cylinder, and about 90 on the lower cylinder. I poured some two stroke oil in the lower cylinder to help lubricate the cylinder and see if the rings were bad. Even with a couple ounces of oil in the cylinder it still only read 90 PSI. <br /><br />I am guessing head gasket blown through to the water jacket. Pretty simple fix. Is there anything else I should look for?<br /><br />

 

[eurolarva]

Re: 1990 507F90C - Running On One Cylinder

Head gasket is easy. When you have the gasket off you will need to look to see what other damage was done. Hopefully it is just the gasket. When you get it fixed you need to find out what caused it. Force motors require periodic decarbs. See FAQ for details. Other reasons could be lean carb settings, improper gas oil ratio, impropper two stroke oil and also poor or no water circulation. IF it is just the gasket you should decarb and then do carb adjustments. If the motor requires re bore or new piston you will have to run 24 to 1 oil during break in.

 

[Bob La Londe]

Re: 1990 507F90C - Running On One Cylinder

I just pulled the carb the other day. I took it apart, sprayed out all the jetswith a good cleaner, and put it back together. It was running great right up until it flaked. In fact I was just admiring to myself how well it was running a couple minutes before it flaked out.

 

[eurolarva]

Re: 1990 507F90C - Running On One Cylinder

may be the carb was running lean. Turn the mixture screw on top of carb clockwise counting turns till lightly seated. If carb and fuel system is running normal this should be about 1 1/2 turns from fully seated. If it is less by a lot you were probably running too lean. Too lean a condition can give you the problems you are experiencing.

 

[Bob La Londe]

Re: 1990 507F90C - Running On One Cylinder

I don't think so, but its possible. A lean condition would be more likely to burn a piston in my opinion. I just pulled the head to take a look. No damage to the piston that I can see. The cylinders look very good. Not even a ridge. The head gasket was definitely blown. It was open straight through to one of the water jacket ports. I guess I'll be ordering a gasket first thing monday morning.

 

[eurolarva]

Re: 1990 507F90C - Running On One Cylinder

Here is a link. You can order parts from here or write the part number down and order it from a merc dealer. Not too bad 25 bucks for the gasket. power head

 

[Bob La Londe]

Re: 1990 507F90C - Running On One Cylinder

I just looked it up on the iboats parts finder. I'm gonna call the two local dealers in the morning and see if either one has one. One gives me small discount on parts for putting his name on my boats. If not I'll go ahead and order it.

 

[Bob La Londe]

Re: 1990 507F90C - Running On One Cylinder

Davidson Marine, a local dealer actually stocks that particular head gasket. I picked one up today. It was ten dollars cheaper than the look up through iBoats. No delivery charges either. I guess that make sit more like fifteen dollars cheaper. <br /><br />I guess it doesn't hurt to check the local guys first sometimes.<br /><br />