350 MerCruiser SBC smoking and erratic vac

[69GTOby]

This is a completely stock 260 HP MerCruiser (SBC) in an '88 Baja. It runs great - pulls skiiers up no prob, does 60 MPH, super smooth, planes in seconds, just a little rough and smokey at idle.

The engine is obviously old, but looks really clean on the outside and has fairly low hours for a boat. Last season my Dad overheated the engine. I replaced the impeller in the outdrive and t-stat and we used it all summer no prob, 140* always. Now it is getting hot again, I'll save that for another thread. The thing is, it smokes a little. Especially when it is run on the lift with the drive in the water and the exhaust above water, it is considerable and greyish. The timing is at about 12*. I pulled the plugs last season and some random ones were black and sooty. Put in new plugs, ran it, and pulled a few and some were black and sooty again. Also new wires, cap, rotor, coil at that time. The oil is always clean and at correct level. 150 psi compression in all cylinders. There's a little oil coming thru the PCV hoses. Vacuum gauge is at about 15"Hg and vibrating. Not sure if this is normal for a boat cam, but assume not. ? I'm having trouble looking at all these clues and finding out a place to start. Valve seals melted in overheating episode and worn guides? Maybe just carb dumping fuel? Intake leaking and sucking up oil whcih makes it to some plugs and fouls them out? Also, ever since overheating, when I rev the engine in neutral, right when the RPMs begine to come back down there's a clicking sound - melted rubber seals on flappers in y-pipe?

 

[RandyJ]

Re: 350 MerCruiser SBC smoking and erratic vac

Too much oil??? (overfilled)

 

[69GTOby]

Re: 350 MerCruiser SBC smoking and erratic vac

It's only like 1/2 qt. overfilled. I put in 6 qts, 5 for the pan, 1/2 for the drain hose, and 1/2 overfilled. That definitely would not cause these issues...

 

[John_S]

Re: 350 MerCruiser SBC smoking and erratic vac

Things that caught my eye:

- Timing is too advanced for initial. Should be 8 degrees BTDC, unless it is T5 ignition, which is 10.

- Clicking is probably exhaust flaps that had the rubber burned off.

- Pump out the extra oil. Make sure it is not causing the foaming or issue with oil in the vent hoses

- Vacuum may be ok, if light steady pulses of one psi. A little low, but you didn't mention elevation

- Determine if smoke is oil, water, or gas (after extra oil removal)

- If it is gas at idle, check for dribling in carb or overflow from fuel pump vent


Questions:

- Did you find all the pieces to the impeler?

- What temp is it running at now?

 

[69GTOby]

Re: 350 MerCruiser SBC smoking and erratic vac

I performed a compression test in the spring and came up with ~150 psi in all cylinders - good. I know the spec for the engine is 8* inital, but that is conservative for SBCs and a little more can be helpful. My vac was 12"Hg and erratic at *8 while at 12* vac went up to ~15"Hg and vibrating. It runs a little better with 12*.

Elevation is only a couple hundred feet above sea level. The vac is not light or steady, it is a little harsh in its vibration, but smooths out with higher RPM. I am not sure how to be certain of what the smoke is. It is greyish and rather thin. Thick and white is water, steam, so I don't think that's it, which leaves fuel or oil. Since the oil is always clean and at the right level, and the engine has lower hours on it, I'm leaning towards fuel as the culprit. Also, considering the carbon fouled black plugs - probably fuel rather than oil, but hard to say. Then again, it doesn't smell "fuely" and it's not black like fuel tends to be.

It runs at 140* cruising, or even at high RPMs, but gets up to ~200 while idling along. It seems to maintain ~200 while idling for several minutes, and goes right down to 140* once accelerated and planed. I did not find all the pieces of the impeller. But, after I changed the impeller and the t-stat in the spring '11 , it ran at 140* all summer long even on the hottest of days while putting along the shoreline. Now it's cooler outside, and it's struggling to stay cooled at low RPMs. Mind you, the recirculation pump (water pump) and exhaust risers are likely the original units from '88.

I am considering removing the y pipe and installing thru-hull exhaust. That way we eliminate the clicking, and can see water coming out of the exhaust for peace of mind in the future.

 

[John_S]

Re: 350 MerCruiser SBC smoking and erratic vac

At this point, you may have more than one issue. Gray or blue/gray would usually mean oil. Even though you have good compression, a leakdown, as mentioned should confirm good or bad. Did compression rise with oil in cyl? Holding a sheet of cold aluminum in the exhaust might help with ID (running on muffs). You should get some condensing steam or oil, or soot.

But, I'm thinking you might have a combination of steam and running rich. Looking down the carb at idle and checking vent line are quick and simple.

It doesn't sound like the impeller is supplying enough water at idle. It could be one of the missing pieces has dislodged and is now blocking flow. It is unlikely that the pieces will travel all the way through the system and out the exhaust. If you only replaced the impeller then the housing could have been worn and is starting to destroy the new impeller.

I am sure it runs smoother with timing at 12, but it needs to be at 8. You are just masking the idle issue. Your wot total advance is getting on the high side. With a boat, you can esily get detonation, and generly won't hear any pinging until it gets severe.

Oil level is determined via dipstick while at rest in water. If it is showing anything over the fill line, pump/drain some out.

 

[NHGuy]

Re: 350 MerCruiser SBC smoking and erratic vac

Have you set the idle mix? Best way is with a vacuum gauge. Set the timing to 8* BTDC, run the boat in gear at 650 RPM at normal running temperature - choke open/off (have somebody steer) and then set your idle jets for the highest steady vacuum reading.
You probably already know this but don't crank down the idle mix adjusters hard onto their stops, damages them.
Mercruiser method says to run the engine as above but without a vacuum gauge they want you to turn down the idle mix screws til the motor stumbles, back them out til it stumbles and then split the difference. Obviously, you can only do one jet at a time.
If your boat is like mine, and I bet it is, you'll need a carb adjuster tool with one of those springy linkages so you can reach in there. I got one off e-bay for $12.

 

[Volphin]

Re: 350 MerCruiser SBC smoking and erratic vac

Heating at idle is also indicative of a cracked head, inside the combustion bowl.

V

 

[69GTOby]

Re: 350 MerCruiser SBC smoking and erratic vac

Hey NHGuy, nice boat! I am thinking of converting mine to thru hull since my y pipe needs flappers and it would make working back there much easier and roomier. Does yours have thru hull, because I cannot figure out where to route it?

Anyway, yes, I tried to adjust the mix screws with a vac gauge, but couldn't get rid of the vibration in the needle. I'll bring the timing back down, but there are certainly a few issues here, timing isn't it since the problems were there at 8*, even worse, too. Total advance with base at 8 was only like 28. SBCs like as much as 38. I wouldn't go that high, but 28 seems awful conservative. I'll put base at 8* and fix the problems at hand.

When I did the impeller just this past spring, I did the housing since it was melted. I also did all the little shims and stuff. It's weird that I only got one season out of it. Maybe I did something wrong, or maybe the problem is elsewhere?? The circulation pump (water pump), and pressure pump on the side of the block are probably both original. If it was a cracked head from overheating, wouldn't that have presented itself early on, rather than at the end of a long hot summer of boating?

 

[Volphin]

Re: 350 MerCruiser SBC smoking and erratic vac

If it was a cracked head from overheating, wouldn't that have presented itself early on, rather than at the end of a long hot summer of boating?

Sometimes the crack is so small that it only presents itself later. Raw water systems are not under pressure in the cooling jackets like a car, so some seepage can go unnoticed for a while. I ran my 4.3L with a cracked head for two seasons. Upon removal, you could barely see the crack.

V

 

[69GTOby]

Re: 350 MerCruiser SBC smoking and erratic vac

Interesting. I was afraid of that answer. How can I test for a cracked head? I do not think that is the case, but it would be nice to know for sure.

 

[Volphin]

Re: 350 MerCruiser SBC smoking and erratic vac

Interesting. I was afraid of that answer. How can I test for a cracked head? I do not think that is the case, but it would be nice to know for sure.

In your original post, you mentioned that only some of the plugs were sooty. What did the others look like? Soot means a rich condition BTW, and a separate issue from the overheat. But if a plug is sparkling clean, it could be from water seepage into the cylinder. The plug gets steam cleaned a bit. The best way to determine if your problem lies in the heads or head gaskets is to do a leak down test. Here is a good article on how to perform it and why.

http://www.mobiloil.com/USA-English/MotorOil/Car_Care/DIY/Engine_Leakdown_Test.aspx

V

 

[69GTOby]

Re: 350 MerCruiser SBC smoking and erratic vac

I know that black and sooty indicates overly rich, but I thought that maybe it was carbon fouling from oil - not sure. I did a compression test a little while back and got ~150 pis in all cylinders. I do not think the clean plugs are steam cleaned. They look pretty normal - light tan - but a random set of 4 or 5 plugs are black and sooty. It is not one bank, or one set fed by the same barrels of the carb. There really is no pattern. I have done a leak down test before, I just didn't know if that would detect a slight crack, now that I think about it, it sure would. But, the more I think about it, the more I do not think I have a cracked head. I bet it is just the original, untouched '88 Q Jet dumping too much fuel at idle. For some reason it only makes it to some random cylinders... I will have to pull all the plugs again and take a closer look. I am really thinking the carb is at fault here.

 

[John_S]

Re: 350 MerCruiser SBC smoking and erratic vac

If the inlet needle is stuck, gas will be dribbling down the primaries. If fuel pump diaphram is leaking, it will be coming in on the plastic tube, and believe that feeds to the seconadies on q-jet. Well plugs leak from underniegth, and can be one or both sides. Raw fuel runs down the intake runners, depending on intake and motor elevation/pitch in the water. Stearn heavy boats typically cyl 8 or 5, and 7 or 6, but changes if pitched to one side or bow down. Running on muffs in the driveway will typically be different from in the water.

 

[69GTOby]

Re: 350 MerCruiser SBC smoking and erratic vac

I am working on talking my Dad into bringing the boat home, to the tools rather than bringing the tools to it. It will likely be a few weeks until we get down to the lake. In the mean time, I am devising a plan for the boat. I have decided to retain the thru prop exhaust. I will restore the y pipe and flappers (which I believe have melted). I will replace the impeller, and peobably the water circulator pump. I want to get it cooling properly before I look into the running issues (likely the carb). I have been reading about preventive maintenance. The issue with riser corrosion has got me a little scared. The boat, to my knowledge, still has the original risers, untouched since 1988. It has only been in fresh water, but it's going on 13 years old, and it has seen temps in excess of 250* during an overheating episode. Should I replace the risers? Also, is the concern just for the risers, or the manifolds below them as well? Thanks!

 

[John_S]

Re: 350 MerCruiser SBC smoking and erratic vac

In fresh water they seem to last a long time. 23 years is a while. Take them apart and inspect. If they look good, re-install with new Merc gaskets.

Plan on a complete pump kit instead of just an impeller.

PS: I completly agree with store it local. Otherwise you burn good vacation time working on it at the lake/remote location.

 

[NHGuy]

Re: 350 MerCruiser SBC smoking and erratic vac

Hi, I haven't checked in on your thread for a while...thanks for the kind words. BTW, I agree with what you are doing. Bring it home and give it a good going over.
No I do not have thru hull. I can't spend the money right now but I kind of want it. I would never get through hull with out the switchable setup (Captains Call). I like my neighbors too much and my boat lives in a quiet idyllic spot, plus I am a very early riser. So with normal exhaust I'm not loud and obnoxious.
On edit: I have seen folks more knowledgeable than I state that running excessive advance can cause detonation. I'd set it to spec. There's no knock sensor on a motor with that ignition.

 

[69GTOby]

Re: 350 MerCruiser SBC smoking and erratic vac

I'll pull the risers and see how they look and hopefully just seal them back up. I did a complete kit this past season (Spring 11), housing and all, because it was all melted. I will see how it all looks, and replace it all again if needed. Good point on burning good time at the lake working. That may be the deciding/convincing factor for my Dad.

NH, I totally understand the noise concern with thru hull. But, Captain's call is awful costly. My Uncle keeps his boat at our place, it's an '88 Century with the same engine we have, the 260, and he's got standard thru prop exhaust. So, we can use that one for when we want to be quiet. My only hesitation is adding potential leak spots in the hull.

It's true that detonation is not always audible, and can be a silent killer. What I do not understand is how SBCs on the street run 16-18* initial adv for the most power, detonation free, while in boats everyone runs the spec, which is 8-10* adv (Alpha, Mag, respectively). I've heard boats are more susceptible to detonation, but why? Not being argumentitive, just curious. I will turn it down to 10* or so, becuase there probably is a good reason.

 

[gbeltran]

Re: 350 MerCruiser SBC smoking and erratic vac

Simply put, running a boat is like having your truck loaded with cargo and driving around in high gear all the time, pulling a load at speed with your foot holding the throttle full open is not when you want a lot of timing. Street vehicles get away with more timing because they have several gears they go thru. You're right, timing curves are very conservative, and for good reason.

 

[joewithaboat]

Re: 350 MerCruiser SBC smoking and erratic vac

I know that black and sooty indicates overly rich, but I thought that maybe it was carbon fouling from oil - not sure. I did a compression test a little while back and got ~150 pis in all cylinders. I do not think the clean plugs are steam cleaned. They look pretty normal - light tan - but a random set of 4 or 5 plugs are black and sooty. It is not one bank, or one set fed by the same barrels of the carb. There really is no pattern. I have done a leak down test before, I just didn't know if that would detect a slight crack, now that I think about it, it sure would. But, the more I think about it, the more I do not think I have a cracked head. I bet it is just the original, untouched '88 Q Jet dumping too much fuel at idle. For some reason it only makes it to some random cylinders... I will have to pull all the plugs again and take a closer look. I am really thinking the carb is at fault here.

Have you ever looked at the welch plugs on the bottom of the carb? That carb sould have 4 I think. Not uncommon for them to leak. If leaking baddly they can foul plugs at idle and cause smoking.

<Edit> uppon re reading i see John S already mentioned this