Carberooter?

[spydermann22]

I have a problem with my 5.7 alpha 1, I believe the issue is the carb. I recently purchased the boat and the previous owner did state that he thought the carb needed to be rebuilt.
The symptoms are at idle, the motor runs very rough, the RPMs vary from about 800 RPM up to 1300 RPM. The motor also starts to chug and burn oil when it gets down at the 800 RPM. I have a few things that I suspect but am looking for input.

What I suspect
A: Carb Base Gasket. I will test this tonight by spraying carb cleaner around the area - with the motor running.
B: Low Idle Jet
C: PCV Valve stuck

above 1500 RPM it runs strong & smooth

Thanks in advance...
Kevin
2000 Maxum SC

[mmwjr]

A bad PCV may cause oil to be draw into the carburetor and then burned. It will also foul the spark plugs and may lead to a ruff idle.
A small leak at the base of the carb can also cause idle issues but not oil consumption.

Also how old are the plugs, wires, and cap?

IHO I would do a tune up with a boat this old but still may need to redo the carb.

[pascavone]

I bet you that your manifolds are leaking water into the engine at the ruff idle is a wet plug that goes away when your have high RPM.

Do yourself a favor and pull all the plugs in the morning and check for water in the cylinders.

look for white foam on the under side of the oil filler cap.

This is why people sell used boats....

[ss3964spd]

People seem to have a tendancy to blame all sorts of symptoms on the carb itself. In my experience poor running symptoms are usually ignition related. The fact is that the carb on a boat engine suffers far less wear and tear than does a carb on a automobile engine. If the boat is equipped with a fuel filter and a fuel/water separator chances are the carb is fine.

I think you're on the right track Kevin. Pull the PCV hose from the carb and look for signs of oil in the hose. A PCV is basically a controlled vacuum leak and if it goes bad it can create a big vacuum leak - producing some of the symptoms you described. Pull all of the spark plugs and lay them out in the same order that they were installed. If the tips all look the same then whatever is causing the poor running is affecting each cylinder. If you find one that looks significantly different than the rest then the problem is associated with that particular cylinder(s). Check for vacuum leaks all around the carb as you described. Consider a general tune up like Mike suggests.

IMO, taking the carb apart would be the last resort. Since you don't know the complete history of the boat someone may have fiddled with the carb's idle mixture screws. What carb is on the engine?

Dan

[spydermann22]

I was able to pick up plugs, wires, cap/rotor kit and fuel filter/separator. I did get the fuel filter installed. I forgot the PVC valve although it looks very clean, the ball rattled freely and the hose looked pretty clean.
I did look at the dipstick oil which looked ok (needs changing ASAP but not foamy). I will check for the foam on the filler cap, thanks pascavone, I will also see if I can determine if there is water in the cylinders while I am at it. I am thinking I would have to crank the motor with the plugs out and see if a mist comes out?
mmwjr & SS3964spd: The carb is a 2 BBL and looks very clean. The entire engine and compartment looks very clean with original paint & decals.
I will install the plugs, wires, cap and get the PCV Valve tomorrow and send an update.
Thanks for all of your help!

[mmwjr]

Kevin,
you mentioned starts to burn oil, how are you determining this? Oil level goes down, how much? Smoke out exhaust, what color? As Pas mentioned a small leak between the manifold and riser gasket will allow water to be drawn into the engine through the exhause back pressure. The water will cause a rough idle and turn to steam and go back out the exhaust not showing up in the oil. Eventualy the problem will worsen and if not correct kill the engine. Just another possibility.

[spydermann22]

At the low rpm, the engine starts missing and the exhaust becomes filled with blue smoke smelling like burning oil. I have burned about 20 gallons with Sea Foam thinking it may clean out the carb but nothing has changed yet.

[mmwjr]

I assume you mean 20 gallons of gas and not oil. Carb has nothing to do with burning oil.

Sounds like it's time to perform a compression test. How many hours are on the engine?

[jrsick]

Have you tried spraying seafoam directly into the carb? Remove the flame arrestor and spray into carb using a spray nozzle.

[mmwjr]

What jr suggest is a better way for cleaning the carb but as I have already stated this has nothing to do with burning oil. As Dan mentioned above the carb gets a bad rap when ever there is an issue and most of the time it's not the problem.

spydermann, my senses are tingling (sorry could not resist) that the reason the previous owner sold the boat is because he knew the issue was more serious. I would be a rich man if I got a dollar for every time someone said it's just the carb needs a cleaning. Heard this from folks when they have the engine half torn down saying Oh it just needs to be put back together and clean the carb. Why was the engine torn down in the first place.

I am loosing it here but I am tired of folks being taken advantage of because they just don't know what to look for when buying anything that has an engine in it.

While I have recomended to have a boat surveyed in the past the best money spent is to have the engine surveyed expecially if the seller is saying Oh it just needs...

[ss3964spd]

Kevin,

No need to crank the engine with the plugs removed in order to see if there's any water in the cylinders. IF there is a water ingestion issue one, or more, spark plugs tips will be far cleaner than the others. If it is burning any water the water basically steam cleans the tip of the plug, as well as the combustion chamber and top of the piston.

The surging at idle (800 to 1300 and back) is a symptom but does not necessarily suggest a vacuum leak to me, it also does not suggest water ingestion. The carb on that engine probably has only one vacuum line on it - for the PCV, so you can easily eliminate that as an issue by simply installing a cap on the carb fitting.

You can also check the carb idle mixture screws. There are two, left and right, on the front of the carb. Pick one of them, and turn it clock wise until it stops - but use only light pressure. Count the exact number of turns it makes until it stops (my guess will be from between 1 1/2 and 2 1/2 turns). Then return it to it's original position. Repeat on the other one. The number of turns for each should be very similar.

Another quick check. Make sure that the choke is completely open when the engine is up to it's normal operating temp. Another quick check. With the engine up to temp and idling, and after you've verified that the choke is completely open, use a flash light to look down into the carb opening. If you see any gas dripping/discharging then you do have a carb issue.

Dan

[spydermann22]

I agree mmwjr. I did look the boat over pretty well. Well enough to pull the trigger on the purchase after a pretty good test drive and look over. I do wish I would have had an expert look at it. It did run good but the idle was turned up higher than I would like to run it. Once I turned it down I found out why.. That being said the boat is in such good condition and after negotiating a bit, I will be ok, even if I have to do some work. I am very handy and have friends as full time "auto" mechanics and am a pretty good mechanic myself. I have worked on my own boats for years but that being said, it sure is nice to have your experiences and advise to help me - rather than just start throwing every part under the sun at it.
The oil burning is not bad and really only happens when it starts missing/chugging at the low rpm. It does not sound right but above about 1200 RPM the exhaust smells normal and the engine runs very smooth, quiet & strong. I am not overly concerned about rings or valve seats or that type of thing - but - the missing/chugging has to be corrected. I can not keep the idle high at docking and cant have it missing/chugging either. I will see how much I can get done tonight. Keep your fingers crossed for me!

[spydermann22]

Dan, thanks!

[mmwjr]

If th engine is starting to burn oil it can foul the plugs and cause the rough idle. At higher rpms the oil is burned and blown out, as the amount of oil being burned increases it will impact performance even at high rpms.

Still suggest a compression test jst to check it out, if nothing else it will be a baseline on the engines condition.

Best of luck and keep posting the progress.

[spydermann22]

I got the plugs, cap & rotor changed. The plugs were all exactly the same, black from oil. They looked like they were the original AC plugs. The idle was much better but it still does blow out some blue smoke - not terrible but not desirable either. My buddy the Auto mechanic works at a Wagamon Brothers (this is not a ad for them) who does sell or rebuild marine longblocks. I may pursue this over the winter. They are at $2250 for a long block. From what I understand they build them there. Does this sound about right?

[mmwjr]

First were the plugs dry black (just old) or kind of wet oily? The price you mentioned is about right may do better online but then ned to pay shipping so probably a wash.

Still suggest a compression test to get the real story before spending $'s.

Will you be replacing it yourself? If so can talk you through it but it will be very helpful if you get an aftermarket merc manual.

[spydermann22]

The plugs were actually dry. It did look like oil burned to them though. I did not move the idle adjustment screws, I ran out of time. I talked to the previous owner and asked him about the oil. I did expect him to deny any knowledge of it burning oil but he did sound surprised stating that it never used any oil and maintained that the carb should be rebuilt adding that the boat sat for 8 years.
I will have my mechanic buddy come over with his compression gauge and take a look. It will cost me a few beers..
If I end up replacing the motor, I will do myself - with my buddy's help (more beer). I would get a manual.

[mmwjr]

ETP "Enjoy The Process" ;-)

[spydermann22]

Well I had my mechanic buddy over. It seems I can not diagnose like a pro. Right off the bat he stated the smoke was not oil. It is running so rich that the exhaust was very black. He adjusted the carb one way then the other. Each time after adjusting and letting it run, the RPM would start to change. It would range from 1200 rpm down to less than 800 stalling out a couple of times. It was pretty apparent (even to an -thought I was an ok mechanic- like myself) that the carb was not working well. He pulled the carb off and took it with him. He is hoping to have it re-built today and back to me this evening. I will send an update. From now on I will ask the pro to diagnose. I can still turn wrenches.

[mmwjr]

OK that's why I asked you earlier and you said th smoke was blue. Yep black smoke is running to rich.

Turning wrenches is easy, it's know what needs to be wrenched that makes it challenging ;-)

Hope this takes care of the problem.