A problem that has us all stumped! Any advice is welcomed please!

[robacks300W]

Hello and first off, thank you for allowing me to join this great forum. So anyway, I have a sea Ray Weekender 300 with twin 5.7 Merc w/V drives, that we absolutely adore when its running right. So the boat ran perfectly all last season and this season until last week. I started it and it stalled and the only way i could get it to idle was if i picked up the throttle about an inch. So my mechanic looked at it and told us it was a choke issue need new carb. So, we replaced it with a new reman one from Guaranteed Carb, we did not rebuild it. So the carb was replaced on the starboard side and now i have throttle issues as well as black smoke coming from my exhaust only underway and my engine syncro is way off. We have checked compression and its perfect, oil pressure with new carb on starboard now reads almost 80 psi underway which is up from before which was normally around 50 psi just like my port side. We cant figure it out. Why black smoke, also the smoke is coming from both sides, port and starboard exhaust. It doesnt run hot, oil is clean as can be etc. So when i go out up to 2500 rpm both engines sync perfectly and the throttle position is equal, however once you throttle past 2500 rpm the port throttle position has to be about an inch below starboard for the engines to sync. Crazy huh! So cruising at 3200 rpm my throttle positions are way off and they dont sound in sync at all. I'm really concerned any ideas! Maybe 2 problems? Exhaust totally separate maybe it is fuel related, air, also fyi,gas in tank is from last season still( no smoke at idle ever on underway). I cant figure out this throttle position thing at all. Any advice would be so appreciated.

 

[nickdcj7]

First off, a choke problem is no reason to replace the carb, the choke parts are cheap and easy to jnstall.

Black smoke means you're running rich, as in too much fuel. It sounds like your secondaries are opening early, and your carb is jetted wrong for secondaries. Did you get an exact replacement carb, or a different model, and did you check that it matched before you installed it?

I would also advise you to stop running it immediately until the problem is resolved. The increase in oil pressure is most likely being caused by blow by from the extra fuel. This causes two problems. The gas is pushing past the piston rings and getting into the crankcase, which washes all the oil off the cylinder walls and can damage them, and it also thins the oil which can cause damage to the rest of the engine. Pull your dipstick and smell the oil on it. If one side smells more like gasoline than the other, you're getting blowby.

I would have that carburetor gone over thoroughly by a good mechanic and then go from there.

 

[dpmulvey]

I can't speak to all the stuff nickdcj7 states. I'm not that knoledgeable.

From experience however, he is spot on regarding choke parts vs. a new carb.

Fire your mechanic. He did you no favors.

 

[estoltz]

I hope you at least got to keep the old carb for spares.

 

[Mike Blake]

Sounds to me like you still may have a choke issue. Have you looked at the choke after the engine has come up to temp? That would mean see if the choke plate was open.

BTW welcome to CSR

Mike

 

[Mike Blake]

Almost forgot also check carefully for vacuum leaks also.

 

[julietbravomike]

Agree with all that's said here. You probably have four barrel Quadrajets. There's a ton of info about them all over the internet, they were widely used in the marine and automotive industry. Pretty simple to understand their operation (and overhaul if you're so inclined), but there is a certain process to set them up. In my experience if the secondaries open early, the engine will bog - known all over the land as "quadrabog" - that may well be what is happening to you when you get to a certain RPM and they open on one side but not the other. I would suspect a vacuum leak, choke issue, float level set incorrectly and/or the wrong jets. Also, definitely fire your mechanic. A choke issues does not = needing a rebuilt carb.

There are many variations of this carb, but the simplest form has a bimetal spring (inside a housing attached to the intake manifold) that opens the choke (via various linkages) as the engine warms. Kind of like the pic below. You might start there - let the engines run a bit and see if the chokes open.

Also, clean your spark arrestors. Those things can hold a lot of crud and restrict airflow, resulting in an overly rich condition.

 

[PlayDate]

I believe there may be a few problems. Like most things jumping into the middle of a story means you really don't know how things got to be where they are.

Let's start with the carb....bad decision by the mechanic. The carb explains the black smoke, throttle issues and poor performance....you are literally washing the cylinder walls with gas. Go back to the old carb or buy a new one. Saving money on this is keeping you off the water and costing you more money in gas.

Of greater interest is the black smoke now coming from the port engine. Hopefully that is not the case. If it is, then you have a fuel related problem that could be affecting both engines. Is this last year's gas?

Finally, the increase in oil pressure......that too is odd. However, I'm guessing the starboard rpms are higher than the port which accounts for the discrepancy between on the engine sync and oil pressure.

John

 

[becnme]

A friend had this same problem a year ago and after a new carb and tune-up it was found to have water in the gas from phase seperation.
Cleaned the tank and bought new gas, problem solved.

 

[dpmulvey]

If you go buy a new one, I would recommend a Holley vs. a Rochester. Superior product.