Mercruiser 120 Issues

[sYnOnYx]

I have a Mercruiser 120 that I think is in great shape just something simple is wrong.
My first question is does this motor have some sort of choke or cold start mechanism?
Alright here is what it is doing:

It is very hard to start, I have to pump the throttle in neutral pretty good for about 15-25 seconds before it will start it will start very rough. Once it is warmed up it seems like it is ok and Idles at 1,000 RPM just fine. But then it struggles to go higher up in RPM while in neutral and it pops like a misfire. But eventually it will make its way to about 3500 ( I stop it there ) It seems like a misfire or not getting enough fuel issue. Mind you I lean more to the Ignition side cause I tried using starting fluid to help start it and it didn?t help.

So I splashed it this past weekend and while in forward gear it won?t go past 1300rpm?s any movement of the throttle after that it just seems to make a bogging down noise and won?t go any higher.

Where should I start? Does this seem more fuel or ignition related?

 

[45Auto]

Re: Mercruiser 120 Issues

The choke housing should be right in front of you when you look at the carb. It's the round thing mounted on the forward side of the carb (closest side to the front of the engine).

I have a Mercruiser 120 that I think is in great shape

First thing you need to do is verify that the engine is is great shape, not just think it is. Have you done a compression check, if so, what were the readings? If you an internal problem in the engine (stucky rings or valves, burnt valve, worn cylinder, etc) you're wasting your money and time until you fix that. A basic compression check will let you know where you stand.

Once it is warmed up it seems like it is ok and Idles at 1,000 RPM just fine.

If it's idling at 1,000 RPM it's idling way too high, and it's not just fine. You're going to take out your dog clutches (basically square-tooth gears) in the outdrive banging it in and out of gear at that RPM. Factory spec is 650-700 RPM idle.

When is the last time it had a tune-up? Water filter, points, condensor, plugs, distributor cap, timing, etc? If it's been a while that's where you'll have to start (after the compression check). The symptoms you describe could be either electrical or fuel related.

 

[sYnOnYx]

Re: Mercruiser 120 Issues

The choke housing should be right in front of you when you look at the carb. It's the round thing mounted on the forward side of the carb (closest side to the front of the engine).



First thing you need to do is verify that the engine is is great shape, not just think it is. Have you done a compression check, if so, what were the readings? If you an internal problem in the engine (stucky rings or valves, burnt valve, worn cylinder, etc) you're wasting your money and time until you fix that. A basic compression check will let you know where you stand.



If it's idling at 1,000 RPM it's idling way too high, and it's not just fine. You're going to take out your dog clutches (basically square-tooth gears) in the outdrive banging it in and out of gear at that RPM. Factory spec is 650-700 RPM idle.

When is the last time it had a tune-up? Water filter, points, condensor, plugs, distributor cap, timing, etc? If it's been a while that's where you'll have to start (after the compression check). The symptoms you describe could be either electrical or fuel related.

How is the choke activated though? I'm wondering if the choke could be stuck to the on position.

Anything below 1,000rpm the motor shakes like crazy and wants to die. It had new plugs, plug wires, dist. cap and points installed right before I bought it. Which makes me suspect the timing could be off. I checked the compression when I bought it and it was good all across the board. It ran great the day I picked it up, sat for 2 weeks and now these are the issues I'm having.

 

[45Auto]

Re: Mercruiser 120 Issues

Do you know what the compression readings were? For some reason it strikes me as funny that someone who doesn't know what a choke is and doesn't have a timing light would have a compression gauge and have any idea what the readings should be ....

You really need to start with the basics.

An engine needs compression, fuel, air, and a spark at the correct time to run correctly. Obviously your's is missing at least one of these.

If it has compression and all the new electrical parts in it and they were installed correctly, then all that's left is fuel and a spark at the correct time.

If it was running well at one time, the timing very seldom changes itself. Anyway, that's an easy one to check, takes about 30 seconds total with a timing light.

If that's good, sounds like you're down to fuel. Check all the fuel and water filters, and order a carb kit for your carb in case you need to rebuild it.

 

[macguyvermedic]

Re: Mercruiser 120 Issues

Where would you recommend purchasing a rebuild kit for a 1984 120 merc?? Sorry for the thread jack.