Rough start with black smoke

[shoot]

I have a 1997 Mariah 5.7L 2 bl carb with bravo 3 drive. Cold starting has become very rough beginning a few weeks ago. Lots of black smoke and rough warming up. Have to keep the rpm's 1500 to 2000 or it stalls. My normal routine is to push the throttle all the way down once and back to 1/4. Had my mechanic look at it and he said he found a lose and dirty connection at a plug on the back of the motor that would cause the choke to remain open. He said he cleaned and secured it and all is well. However, my start yesterday was just the same. Lots of smoke and rough.

Seems to run ok after warming up but not sure that it sounds quite as good as it did before the hard starts began. It starts fine after that even if turned off for a few hours. However, the last three times out, the warning signal will not shut off for approx. 60 seconds after non-cold restart. Then when applying throttle to plane the warning signal comes back on but goes off when reducing throttle to 1000 - 1200 rpm.

Oil pressure gauge reads around 30-35.

 

[Haut Medoc]

Re: Rough start with black smoke

Welcome to iboats......
I'd look at the choke with the flame arrestor off, before the next cold start & see what it does......
Ya might want to look at the plugs now, too.......​

 

[shoot]

Re: Rough start with black smoke

Yes, I did take off the arrestor and look at the choke before I called the mechanic and seemed ok. I had a complete tuneup done at the beginning of the season but have not pulled one since.

 

[Haut Medoc]

Re: Rough start with black smoke

They might be a bit fouled if the choke was acting up.....​

 

[Don S]

Re: Rough start with black smoke

Was your flame arrestor plugged up? If it's all clogged up with black belt dust it will act just like a choke being closed.
You also said your choke "Seemed" ok, what exactly does that mean. Would you know what it looks like if it didn't work properly?

 

[shoot]

Re: Rough start with black smoke

No, but a mechanical boater friend told me that it worked properly as we were trying to diagnose at the dock. The flame arrestor was clean. That is when I called the mechanic to look at it. He said after cleaning and tightening some wiring to a plug on the back of the motor the choke worked correctly.

 

[Boatist]

Re: Rough start with black smoke

Adjust the choke to the point it really does not close when cold then start it up. If you still get black then has to be a carb problem but really sounds like a misadjusted or sticking choke.

Black smoke means way too much gas and that is the first sign of a choke set too rich or stuck. For sure a flame arester can cause the same thing but if it goes away sounds like the choke finally got hot enough to open up.

 

[Don S]

Re: Rough start with black smoke

It is also quite normal for a carbed engine to have some black smoke when first starting a cold engine when the choke is closed.
Merc always sets the choke too tight for my liking. I set them (on a cold engine) so the choke plate just barely closes. That is plenty of choke for a Chevy engine in warm weather.

 

[shoot]

Re: Rough start with black smoke

Thanks for the info. A liittle black smoke at cold start seems normal and has been my experience with this engine over the past year. But in the last few weeks it is excessive and the motor shakes badly and struggles to stay running. Do you think the warning signal could be related?

 

[Limited-Time]

Re: Rough start with black smoke

If its not the choke gone out of adjustment, you'll need to look closely at the carb as the engine is idling. ANY visible fuel flowing from any venturi or other section of the carb will cause your condition. It may be time for a carb rebuild.

 

[thrasher]

Re: Rough start with black smoke

I would still strongly recomend the previous recomendation of changing the spark plugs. Don't just clean them, if the choke was not operating correctly in the past, then there is probably soot deposits inside the spark plug and this can cause the engine not to run correctly and idle very badly. My first recomendation would be to change the 8 spark plugs and then check the carburation.

Gary

 

[shoot]

Re: Rough start with black smoke

Did some diagnosing this evening based on the feedback. The warning signal is almost certainly from the outdrive resevoir falling below the minimum level. Was able to get the alarm to sound by pushing down slightly on the float.

The carb is another matter. Took off the arrestor to watch what happens. the first start attempt resulted in setting the choke closed with the throttle and watching it open as it should without any black smoke or roughness. Let it run for a few minutes and shut it down.

After about 20 minutes tried starting again. It was difficult to get it going. The choke was about 1/4 inch open at the start and did not move even after about 5 minutes of rough idling and lots of black smoke.

Any feedback appreciated.

 

[shoot]

Re: Rough start with black smoke

Spoke with the mechanic today and he is quite certain that the electric choke is working intermitently because of a poor connection at the back of the motor. He is going to look at it again to try and clean it better and get a tighter connection by spreading the connectors. If that does not work he has bypassed that modular connection in the past on other boats and created his own water tight secure connection as he indicated the wiring an connections an get very expensive.

 

[shoot]

Re: Rough start with black smoke

Power to the choke is only registering 1 amp. Mechanic says 12 is needed. Jiggling the harness does make the meter jump around but not much more than 1. The harness looks about the size of a half dollar with many pins inside. Everything else associated with that harness works ok. The harness is secured inside the engine compartment very tightly. Mechanic thinks that any movement of the engine caused stress on the harness damaging the choke wire. He suggests jumping over the harness to get a clean connection to avoid the purchase and installation of a new harness.

The choke does work properly when supplied with enough power. What do you think?

 

[Don S]

Re: Rough start with black smoke

I don't think your mechanic knows what he is doing. There isn't a seperate wire in the harness main plug for the choke. He should also be measuring VOLTS, not AMPS.
The hot wire for the choke comes from the alternator, and goes directly to the choke. Check the connector on each end. Corrosion will kill your voltage. I would also make sure the ground wire is in good shape.

 

[shoot]

Re: Rough start with black smoke

Thanks Don. That may be my mistake on volts vs amps. If power to the choke is NOT associated with the harness, why would the power meter change when the harness was moved around?

 

[Don S]

Re: Rough start with black smoke

If that is happening, the purple wire is in that plug. So you should see the same voltage fluctutions in the purple wire (Ignition).
Might be time to clean that plug and spread the pins SLIGHTLY to make better contact. DO NOT overtighten the clamp holding the plug together. That does more harm by distorting the connections than it does holding anything. Just a bit snug is all that is needed.

 

[shoot]

Re: Rough start with black smoke

I didn't see it done but the mechanic said he cleaned and spread slightly to see if it helped. Don't know about the clamp tightness. He said that it was successful after initially done. Worked fine for me on first start last night, no good since.

Rather than mess with the wiring harness, I'm thinking that it might be better to just provide power to the choke from another source to be certain some other issue is not effecting operation.

 

[Don S]

Re: Rough start with black smoke

There is nothing wrong with the source, unless you have alternator problems.
Why not fix it right, instead of re-engineering it?
Your boat is warning you of impending problems...... Heed the warning.

 

[shoot]

Re: Rough start with black smoke

End of the story, hopefully.

After questioning the mechanic on the source of the power, he checked the wiring diagram and found that the source is in fact the alternator. Then he tested the alternator and found that power was intermittent and needed to be rebuilt with a rectifier. Should be back and installed tomorrow. Hope this is the fix.
Thanks very much for your assistance.