Mercruiser 120 rochester carb problem (w/ pics!)

[italianstal27]

hey guys,

Boat ran fine 3 weeks ago, and now we have problems.

So current symptom on boat is she won't rev past 2000 rpm in gear. She'll do 3000 no problem, in neutral, in the water. Compression on all cylinders between 100-110.

Spark plugs are looking a little sooty. In the last 3 years, we've taken the boat out maybe 9 times? Generally, she gets run once every 2-3 months.

On our last outing, we were using the main, and consumed 8 gallons of fuel at 2000rpm in 30 minutes - 1 hour. We have spark on all cylinders.

Suspected carburetor problem, so we took it apart and found one of the gaskets had started disintegrating. Does this look like a smoking gun?

 

[Scott Danforth]

the rust in the bowl may be from your tank. you probably have water in the tank

the inside of the carb looks like crap, carb rebuild time.

compression in the 100-110 range is motor rebuild time.

 

[italianstal27]

We have an inline water/fuel separator installed. One of those $60 ones. Does this mean our filter isn't working?

 

[Scott Danforth]

My guess the filter is saturated. when was the last time you replaced it and checked the contents?

 

[italianstal27]

2 years ago. we've run maybe 30 gallons of fuel through it.

The engine is a '76. Do they need to be run more often than once every 3 months? Also, does the fuel evaporating out of the carburetor bowl cause the majority of problems?

 

[Tycer]

Yes. Rebuild the carb. Get a new float, I prefer brass. How filthy is the flame arrestor? That’s a lot of black inside the venturi. Are you using the recommended oil? Quicksilver or Rotella 15w-40. After the rebuild do another compression test. Make sure the motor is up to operating temp -140°. It can be a challenge to get the temp up out of the water. With all plugs removed do a test on all four. Then do a second test with squirting a couple of tablespoons of oil in each plug hole right before testing that cylinder. If compression jumps then your rings are not sealing.
Good luck!

 

[GA_Boater]

The oil in the crankcase had nothing to do with what may be a backfiring motor. This may be the most soot filled carb I've ever seen and I've seen a few over the years.

The gasket isn't falling apart. The carb has probably never been opened and gaskets are disposable because they stick and tear.

Get a gallon can of carb cleaner and soak the carb bits and blow out the passages with compressed air. No compressor? Cans of keyboard dusting air do a passable job.

Try another gauge to recheck compression. Major auto parts stores offer free rental.

After she's running again, check the timing. Probably change the plugs and try fresh gas, too.

 

[Tycer]

The oil in the crankcase had nothing to do with what may be a backfiring motor. This may be the most soot filled carb I've ever seen and I've seen a few over the years.

Diluted oil can cause excessive blow-by and low-ish compression readings, rings could cause the 100-110psi and excessive blow-by. I’ve been through many carbs and you’re right, that’s the blackest I’ve seen too. Hadn’t thought about backfire.

So Sly,
Get yourself a vacuum gauge and add it to your arsenal. If your 120 is like my 140, there's a brass plug just aft of the carb on top of the manifold, the vac gauge kit will have a fitting for it.
Here's a good page for diagnosing with a vac gauge: http://www.secondchancegarage.com/public/186.cfm
GA_Boater might be correct that the soot is coming from below. The vac gauge will help determine that without having to go deeper.

 

[italianstal27]

We've been experiencing some backfiring recently at startup. Having the throttle at 2500 rpm at startup seems to help though. I ordered a rebuild kit and will get this put back together asap. We need to do a better job of running the engine at a higher rpm during startup. It's not EFI

 

[italianstal27]

Oh! We checked the oil and it definitely smells gassy. Sooty even? The oil has good color but has steaks of spot when running it across a white rag

 

[Scott Danforth]

if your oil smells of fuel.....change oil, spin motor, change oil again

re-check the compression. a good motor is in the 140-150 psi range, an OK motor is in the 120-140 psi range, below 120, your down on power and will have lots of blowby

find source of fuel:

A, the carb was dumping fuel down the manifold (check the welsh plugs)
B, the fuel pump diaphragm ruptured and the fuel pump is dumping fuel internally
C, the fuel pump ruptured, and the fuel pump was dumping fuel up the clear tygon hose to the carb throat.

 

[Tycer]

We've been experiencing some backfiring recently at startup. Having the throttle at 2500 rpm at startup seems to help though. I ordered a rebuild kit and will get this put back together asap. We need to do a better job of running the engine at a higher rpm during startup. It's not EFI

You should be able to run 1000-1200 immediately after starting.

 

[italianstal27]

Ah, yeah what I meant was we were starting the engine with very minimal throttle which resulted in that sneezing. I think that's why the carb is so black.

Our starting procedure now is 2 pumps of the throttle and move to half throttle. Seems to start well every time now! Learning curve with the older engine is tough sometimes

 

[Spiderhole]

That carb is definitely filthy. If it has been sitting up for some time and you've never done a rebuild on top of running fuel with ethanol in it it will gum up like that. I bet once you simply pull all the jets/needle/seat and clean the carb out real well with carb cleaner the boat will instantly run better. You can soak it and really make sure all the ports are completely clear. I wouldn't even mess with a new float or needle to be honest if they still look ok after you clean them thoroughly - in my experience if they are the original from the factory and they are still functionally ok, I use them. I've had problems even with the correct kits in getting the float height and need to seat like the factory ones, especially on these older carbs where you only have the one adjustment on the base of the carb and the idle screw. Primarily I use the gaskets and the other components of the kit for the rebuild. Also, how does the fuel filter look? I don't understand how rust would have even gotten past that screen. Once you do that and verify it works again, then I'd move on to the other maintenance items like the fuel/water filter just because they should be done. If there is indeed water in the tank and it's not a ton I'd probably put some fuel emulsifier in there like heet in lieu of pumping all of it out.

 

[kenny nunez]

From the picture of the float chamber it appears that the power valve needle is stuck in the down position because of all the debris from the tank. Is there a filter under the inlet fitting? If so it probably got so clogged up that the spring behind the filter compressed and the debris by-passed the filter. As everyone has posted give the carburetor a good cleaning and replace all the parts that come in the kit. Why is there a electric choke when your model used the heat tube from the manifold. The electric choke has to be wired to a 12 volt source and not on the + side of the coil.
.

 

[GA_Boater]

Judging by the aluminum oxide or crud on the heat tube connection, the choke was converted to electric and some time ago.

 

[Tycer]

We've been experiencing some backfiring recently at startup. Having the throttle at 2500 rpm at startup seems to help though. I ordered a rebuild kit and will get this put back together asap. We need to do a better job of running the engine at a higher rpm during startup. It's not EFI

I told you a couple of tablespoons of oil. The manual recommends ONE tablespoon of 20 wt. oil.

 

[italianstal27]

OH! That's not rust in the bowl!

It's actually the red gasket that has disintegrated below the power jet (I think that's what it's called?). It's the port that allows more fuel to flow through the carb when the plunger has a vacuum.

That red stuff is gasket material