1977 Mercury 850 not reaching max rpms

[SkaterRace]

I while back the motor overheated so I took it into the shop to have it repaired as well as the power trim. Turns out they fixed neither of the issues. I have since decided I will do it myself unless it is a big job. Max rpm I can reach is about 3300-3500 that is trimmed up for the 2-3 seconds it stays up. Ideas on why it is not reaching max rpm or how to test it? My father thinks he saw a gas leak in the carb would that do it? How do I test/ check for that myself?

 

[Mercurylips]

You could start with a compression check on all cylinders. Then I would gain access to the carbs and try to tap on the bottom of the float bowl firmly to see if the float is sticking. Access to rebuild the carbs is a ***** but if you take your time and have a service manual handy, a rebuild can be done. If the compression test is bad news, the decision to spend big bucks or part out the motor on craigs list is next.

 

[Chris1956]

An overheat often damages compression. You need to rule that out.

 

[SkaterRace]

An overheat often damages compression. You need to rule that out.

And if it is low compression? What does that mean for repair?

 

[Mercurylips]

Our 1977 motors are somewhat similar. If you get into the wiring you may find that the insulation on the wiring is crumbling (very likely) and you may need a new harness and at least a reworked stator wiring system. I did this myself and had help with the carburetors. When you start talking about the head and getting that work done. Your expenses there may be more than what you can buy another engine for. Prices on used engines vary, by area, in the US. I haven't had experience with reworking the head and pistons on an outboard so someone else will have to help with more specifics. A cracked block would be a deal breaker for sure.

 

[Chris1956]

Compression on Mercury outboards should be over 100 and even across all cylinders. a lot of compression gauges are wildly inaccurate, but are usually reliable. So if you get any PSI reading, but even compression across all cylinders, the motor is almost always good.

 

[SkaterRace]

Compression was fine on all but #3 before I took it in and had a gasket replaced. Still having issues so what next?

 

[racerone]

Remove bypass cover and inspect the pistons and rings from the side.

 

[Chris1956]

Gee, replacing a gasket will not fix a compression issue, as those motors do not have a cylinder head. You need to recheck compression to decide if motor is worth fixing.

 

[SkaterRace]

After reading the thread here I have decided I will likely sell the boat and motor for what I can get and then buy a small bowrider with a 3.0L in it hopefully next spring or maybe this fall.