Older Merc 150 2 stroke really rough below 3000 RPM

[db4570]

I just bought a boat with a 2000 Merc 150 2 Stroke, carbureted. Compression is good, and consistent between cylinders.

It runs really rough at idle, and all the way up to almost 3000 RPM. Above that it runs smoothly, sounds great, and pulls hard. When I pull back on the throttle and drop the R's down, it starts running rough again when it gets below 3000. Like it's missing on a couple of cylinders, shuddering a bit, vibrating the boat.

I've had an I/O for the last 20 years, and only small outboards before that. So I'm getting to know this one.

My guess is clogged carburetor(s), especially in the low speed circuits. I assume these carbs have an idle/ low speed circuit. I have rebuilt car, motorcycle, and small engine carbs, and am hoping these aren't any more complicated. Also hoping a full disassembly and cleaning may fix them.

Any other suggestions on where to start before I start pulling carbs?

Thanks,

David

 

[Chris1956]

It sounds like you are on the right track. The carb model of that year looks to have std carbs on it. Each carb will have a idle mixture screw for each barrel. There should be carb kits both OEM and after market on the web.

 

[racerone]

On a 2 stroke you need to make sure that carburetors are clean.----Blockages may starve a cylinder of oil.-----Run with a timing light to see if one or more cylinders lose spark.

 

[db4570]

I went to 3 different boat repair places today and the third guy was exactly what I was looking for. An independent guy who seems to be well regarded. I liked that I could chat with him about it, rather than just surrender the boat over at some marina service counter.

He seemed to know the motor, and said it sounds like bad possibly ethanol-damaged carbs, or possibly ignition. I said I thought ignition issues would get worse at higher RPM and load, but he said bad electronics etc. could disrupt ignition at any point in the RPM range. But he was leaning toward carbs.

I am taking it to him next week and he is going to diagnose it. It sounds like it isn't anything major.

 

[Chris1956]

When he says "ethanol damaged" carbs, I hope he means dirty carbs. When you first run gasoline + ethanol, it cleans the fuel system and this loosened dirt can get into the carbs if the filters are missing or damaged. After the first use of gas + ethanol, it has no effect on the carbs.

However, like the political spectrum, there are all kinds of misinformation about ethanol in gasoline. Some consider it to be the ultimate evil. I, like millions have run it for years w/o issue.

Be prepared for sticker shock on the price of the carb cleanings. OEM Carb kits were $125 each. So $375 + 2 hours work....

 

[racerone]

The term " good compression " means nothing.-----What are the actual values.-----It is amazing to me how many folks are willing guess , and spend hundreds of beer tokens on coils , stators , switch boxes.----Yet are scared to spend a few dollars on head gaskets to ensure cylinders are in good condition.

 

[db4570]

Compression is all within a few pounds of 120.

This boat may have sat for a year or more with untreated Ethanol fuel in it. This is a recipe for damaged, not just dirty carbs.

 

[ThomW]

Ethanol won't damage your carbs. It can eat away at fuel lines and primer bulbs which in turn can lead to clogged jets, stuck float valves and other carb issues...but the carbs are not actually damaged. Just get the carb kits, tear them apart, soak and clean them, and they are good to go.

I would also check the condition of the fuel lines and primer bulb. If they are old and starting to break apart, cleaning the carbs will only last until another spec from the hose hits the carb and clogs a jet.

 

[db4570]

Well, it was carbs, clogged with debris from the ethanol-damaged fuel line. I had them rebuilt and it runs much better now.

The boat I was considering before this one had a brand new 140 Suzuki on it that ran incredibly smooth and quiet, and I got a bit spoiled. Outboards have come a long way in 22 years!

But this one seems solid now, and my mechanic says it appears to be in good condition.