Rebuild, or run it?

[k10bolts]

Hey fellas

I just purchased my first outboard and need an opinion. I've read as much as I can about the topic, but I'm still in question over compression test values for my motor. I have a 1969 Merc 20HP. I tested it today and got 108/102. What range should I be in? I've read that the different years have different numbers. The previous owner complained of hard starting, so I planned to do the fuel pump, carb, and replace the impeller. I inspected the points, and they look good. I'm up in the air whether an engine rebuild is in order, or if I should continue as planned.

Thanks in advance,
Matt

 

[kabcpapc]

Re: Rebuild, or run it?

First do the things you have already mentioned before you do the rebuild. The difference in the compression is not a major thing. The compression can be as much as 10% difference before a rebuild may be needed. If you can get the motor running without a rebuild, you may get many more hours out of it before you have to spend the bucks.

 

[JB]

Re: Rebuild, or run it?

There is zero evidence for an overhaul. Your numbers are fine. You are just suffering from outboard hypochondria, which is a simple complication of cabin fever.

Do the routine maintenance stuff and before you know it it will be Spring.

 

[k10bolts]

Re: Rebuild, or run it?

Hahaha. I've read a lot about the paranoia surrounding these numbers. I'm a Honda tech by trade, so I've become custom to set specifications. Paranoia perhaps, but I want to make sure I dive into my first boat the right way. I just purchased the boat, so I have no knowledge of how it performed. Just the mentioning of hard starting. As advised, I'll do the maintenance I mentioned. Thanks guys!

 

[JB]

Re: Rebuild, or run it?

Yes. A lot of folks don't understand why 2 strokes don't have psi specs when 4 strokes do.

The reason is that 4 strokes have valves, which cause most losses of compression.

 

[Silvertip]

Re: Rebuild, or run it?

Search this forum for "Decarb Procedure". This process involves running a very rich mixture of SeaFoam and gasoline which removes carbon buildup in the engine -- specifically on the pistons and rings. Carbon buildup in the ring grooves causes them to lose their seal and they can eventually catch on a port and break. This process may very well bring that low cylinder back up closer to the others.

 

[Camlocker]

Re: Rebuild, or run it?

Agreed with the decarb. I was quoted $1600 for a rebuild because of a low comp piston, which was more than I paid for the entire boat...$1100. I changed the plugs, rebuilt the carbs, sprayed Seafoam Deepcreep into each piston, let it soak overnight, ran the snot out of it the next day.....runs absolutely perfect now. My FIL is jealous because my old '84 classic 50 runs better than his '06.

 

[Silvertip]

Re: Rebuild, or run it?

A one shot spray deal is not the answer. Do the actual procedure I referenced. Then continue to run Seafoam in every tank of fuel to keep the fuel system clean AND reduce carbon build up.