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View Full Version : 454 magnum would not shut off



tdlong98
06-27-2011, 08:33 AM
I was out in the boat yesterday. I ran it at 3000 rpm for 20min (30 mph) and when brought it down to a 700 rpm(idle). When I turned it off, it kept running some what. It happened 4 time over the day. I noticed if I woulld let it idle for for more than 45 sec before turning the key, it was fine. (engine would stop) the boat runs great and idles great but I am getting some black sute on the back of the boat. Any ideas on what is going on? It is a 1997 donzi 454 magnum 365 hp with rm revolution marine exhaust and 450 hours.

pipnit
06-27-2011, 11:17 PM
Dieseling or running on can happen for tons of reasons. Being that you're seeing soot, it's probably too rich, causing lots of carbon deposits to build up, and burn then get hotter and so on. Be careful because it can also happen from a lean condition but I doubt that's the case because of the soot. Idle setting and idle air mixture can cause it too.

Planetwarmer
06-28-2011, 04:32 AM
I agree with the "dieseling". The carbon buildup in the cylinders is glowing hot for a bit after you run it. It acts as a glow plug and can continue the process of the engine running even though you have cut electrical power to the engine. It points to this since it only does this if you havent given the engine a chance to cool down after running at 3000 rpms. Obviously after the engine has idled long enough, the carbon has a chance to cool to a temp to not be hot enough to ignite the fuel.

I would have a mechanic give the boat a tuneup. It probably needs some minor electrical maintenance (ie-plugs, cap, rotor, wires, etc). It also needs the carb adjusted, rejetted, or rebuilt.

This is just a thought from what I gathered from your statements. It is also an idea that I have simply restated that pipnit already suggested.

RedDog
06-28-2011, 07:50 AM
fuel injected or carb?

if fuel injected which I think a 97 would be, shouldn't the injectors shut down when the key is switched off? What happens if you pull the lanyard kill switch?

Rumblefish
06-28-2011, 08:17 AM
I agree with the "dieseling". The carbon buildup in the cylinders is glowing hot for a bit after you run it. It acts as a glow plug and can continue the process of the engine running even though you have cut electrical power to the engine. It points to this since it only does this if you havent given the engine a chance to cool down after running at 3000 rpms. Obviously after the engine has idled long enough, the carbon has a chance to cool to a temp to not be hot enough to ignite the fuel.

I would have a mechanic give the boat a tuneup. It probably needs some minor electrical maintenance (ie-plugs, cap, rotor, wires, etc). It also needs the carb adjusted, rejetted, or rebuilt.

This is just a thought from what I gathered from your statements. It is also an idea that I have simply restated that pipnit already suggested.


Nicely said.....As long as its confirmed you have no igntion (spark) when the key is cycled off. do these items ( especially the carb)

MOP
06-28-2011, 08:47 AM
Before you plunk down any serious $$ give OMC or Merc Engine Tuner a try, if it is just carbon build up it will do a decent job of cleaning things up. Sea Foam also does a decent job on light carbon.

tdlong98
06-28-2011, 12:55 PM
Before you plunk down any serious $$ give OMC or Merc Engine Tuner a try, if it is just carbon build up it will do a decent job of cleaning things up. Sea Foam also does a decent job on light carbon.



My 454 magnum has a carb, it is not injected. I pulled out 2 sparkplugs and the center looks to be dark brown and the outside is black with little chunks of black crap all over it. I am sure I have carbon build up in the heads. I am leaning towards the carb running rich. I was told the carb was rebuild yearly last year. I pulled the cap to look at the rotor, and it looks to be ok but I will replace it any way. Should I just get the card rebuild or buy a new one. I realy dont want to spend $400 on a new one. What do you guys think. Carb, plugs, cap and rotor? Or what..... thank you

silverghost
06-28-2011, 01:19 PM
How old is the gasoline in your tank ?
Stale, year old+ fuel can cause all sorts of nasty spring running issues.

tdlong98
06-28-2011, 01:32 PM
How old is the gasoline in your tank ?
Stale, year old+ fuel can cause all sorts of nasty spring running issues.


It is a 1997 454 magnum 365 hp carb. I started the season with no gas in the tank. I put 25 gallons in it and it ran and started great. I have ran around 75 more gallons through it and it just started to act up. It still starts and runs great but it started to diesel when I turn the key off. The boat idles great too. the engine runs cold. I only see the temp come up to 100 if I idle for a while and it drops down when I take off. I think it has a Rochester carb but the book says Webber???

Rumblefish
06-28-2011, 01:41 PM
As Said (Mop) you do need to attempt removing carbon build up...

But the cause will remain unless you proceed with your plan on carb replace (or repair ) and finalize the correct tune settings.. or at least determine the problem.

Replace it.. unless you can rebuild and save

RedDog
06-28-2011, 02:00 PM
It is definitely running way too cold which could lead to carbon build-up. Address the carbon issue and get a new thermostat. You should be good then.

Why would you rebuild a carb every year?

CHACHI
06-28-2011, 02:28 PM
What is the heat range of the plugs?

They might be too cold.

Ken

DonziJon
06-28-2011, 03:10 PM
As RedDog suggested..100 degrees at idle, ...and Colder at speed, is WAY TOO COLD. You need to put a thermostat in the motor. Maybe there is No thermostat at all. If you have "closed cooling", or are running in fresh water the temp should be at least 165..or even higher to burn off the carbon buildup.

If you run primarily in salt water (w/o closed cooling) the temp should be around 145 to prevent "Salting" up the engine.

Put a thermostat in First, and run it in the water at cruise speed or higher for awhile to burn off the carbon.

Save the carb work for later..unless you have an urgent need to spend money. :nilly: DJ

Planetwarmer
06-28-2011, 06:04 PM
As RedDog suggested..100 degrees at idle, ...and Colder at speed, is WAY TOO COLD. You need to put a thermostat in the motor. Maybe there is No thermostat at all. If you have "closed cooling", or are running in fresh water the temp should be at least 165..or even higher to burn off the carbon buildup.
If you run primarily in salt water (w/o closed cooling) the temp should be around 145 to prevent "Salting" up the engine.
Put a thermostat in First, and run it in the water at cruise speed or higher for awhile to burn off the carbon.
Save the carb work for later..unless you have an urgent need to spend money. :nilly: DJ

You said it!!! I put a 165 deg thermostat in mine this year for the 1st time. The boat runs better than ever. Without the thermostat, my 454 would get no warmer than 140, and that was on a 100 degree day when the lake was bathtub temps.

I could only imagine that if your boat has an electric choke, it may not ever be completely off (the choke) because your engine never warms up. This would definitely lead to a buildup of carbon all over the place. Think of how the smoke looks on your lawnmower if you leave the choke on too long.

This is what I would start out doing;

1) Replace the thermostat. (I say replace, because I'm sure it had one from Merc even if there is no longer one in there.) This is a 5 minute job. Replace the gasket in the thermostat housing!!! It will most likely tear if it is old at all, and 1 side is sticky. When I reinstalled everything, I thought it all looked great. Well, at the boat ramp, it sprayed water EVERYWHERE from a pin-sized hole!!! The gasket was 2 dollars at west marine, by the way. I ended up going to some NAPA parts store in some rinky-dink town by the lake and cutting one out of gasket material with my pocket knife.

2) Put some liquid "Tuner" or Seafoam in the tank like MOP suggested.

3) Run the boat around for a while. I would go to full throttle and "blow out the cobwebs" for a bit too.

Be sure to monitor the temp gauge well at first, since you installed a thermostat.

BUIZILLA
06-28-2011, 07:27 PM
goofy gas