b72 Mercuiser 120 Power Loss

"I've been reading as much

"I've been reading as much as I can and am overloaded.

I'll try to explain this in as much detail as I can... hopefully one of you guys can help.

When I first got this boat (2 months ago), it ran pretty good - 25mph. It has gradually lost power - I'm lucky to get 10mph when the lake is calm. With the throttle cracked full open at the carb, I can only get 2500rpm in gear.

In neutral it'll rev fine.

I used carb cleaner and washed a ton of junk down - ended up rebuilding the carb last week. Had a cracked distributor cap and just replaced it. Installed new plugs - tonight, I will be installing new points/condensor and new plug wires.

I checked the compression last night, but forgot to warm the engine up first. Compression on all cylinders was 100-105psi. I put a teaspoon of oil in each cylinder, and rechecked compression - no difference.

I'm planning on trying the leak down test tonight - can someone explain exactly how to do this and what I should be looking for? I know I blow compressed air into each cylinder and then listen for leaks. Do I need to loosen the rocker arms and check both exhaust and intake valves?

Does anyone have any idea what these symptoms sound like?

Thanks for any help.

Glen"

"Sounds like your problem is n

"Sounds like your problem is not compression. 105 cold will be 140ish warmed up. As long as they are consistent, you should be fine. I wouldn't pursue the leak down.

From the info provided it's difficult to say, but usual suspects are bad fuel, water in fuel.

Does it idle smooth? What is it doing when it reaches 2500rpms under load?"

"I'll send you some leakdo

"I'll send you some leakdown testing info tonight from home. I have a Word document on the topic.
It was covered extensively about last October on this site. You may be able to go back thru the archives and find it.
You are not the only one who's boat has mysteriously lost power for no explicable reason. It usually comes down to breathing, compression, mixture, or spark. Have you checked your timing, plugs, correct gap, etc?
Your compressions sound a little low. They should likely be 125-135 psi for a perfectly healthy engine.
Rod"

"I don't know if this make

"I don't know if this makes a difference, but the engine does call for leaded fuel. I've been been running unleaded as my dad said that lead additive is not good for my engine????

It does seem to idle smooth and no problems trolling at ~3mph.

David - thanks... is there an additive I can add to the fuel tank like heet?

Rod - thanks... it sounds like my compression will go up if I had warmed the engine first. I'll try warming it up tonight and then re-check to verify."

"using unleaded is not the sou

"using unleaded is not the source of your problem. But use a lead additive found at any auto parts store. (lead acts as a lubricant as well as an octain booster in gas. Not using it will wear out valve seats on older engines over time)

Compression is not likely to be your problem.

Do make sure you have fresh points and condenser in the distributor. Gap to .020 (that will get you close enough) and set timing to specs.

Again - make sure you have good clean fuel. If you "rebuilt the carb" as you state, you may have a clogged main jet."

"David, you said:

Again - m

"David, you said:

Again - make sure you have good clean fuel. If you "rebuilt the carb" as you state, you may have a clogged main jet.

Thanks David... what do you mean by "main jet"? I assume you're talking about the jets on the carb - I cleaned and blew all that stuff out when I installed the rebuild kit.

I just want to make sure I know what you're talking about...

Thanks - Glen"

Octane boosters will foul the

Octane boosters will foul the plugs. Stick with Techron or SeaFoam to clean out the carb and combustion chambers and valves.

"Octane booster additives are

"Octane booster additives are not the same as lead substitute additives. In this case you are looking for the latter.

Yes, carb jets."