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Dieseling

dog

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I have a 1997 4.3 L engine. It

I have a 1997 4.3 L engine. It diesels some times after it has been run hard. Mid grade gass seems to eliminate the problem. Is there an anti-spark knoock switch on the engine?
 
"No anti knock with a carb. Me

"No anti knock with a carb. Mercruiser had enough problems with this issue to come up with a service bulletin in '97 and they revised it in '99. Look for a thread titled "Change from carb to injection" or something similar. I posted the conditions and suggestions from the bulletin there. The problem gets worse in hot, dry weather.

If it ever runs on and won't turn over when you try to restart, don't keep trying. You'll need to remove the spark plugs and crank it so you can clear the water out of the cylinder(s). If you keep trying, you'll kill your starter. Make sure to have a spark plug wrench onboard at all times, along with the rest of your tool kit."
 
"Doug.
Check on the carb thro


"Doug.
Check on the carb throttle linkage at the carb and see if there is a solenoid mounted by the throttle plate lever. If so check that is closing the throttle plates when the ignition is turned off.
If there is no solenoid look into getting one. This will help a great deal in stopping the dieseling.
On the other hand,make sure you do not have a high compression engine that requires higher octane fuel.
If mid grade fuel helps stop the dieseling the fuel may not have enough octane in it. This is not good for the engine at all. The pre-ignition caused by the lower octane can damage the pistons.
I know fuel is high but buying a cheaper lower grade is not a good idea if your engine needs the top grade.
It will cost much more to repair the engine than the extra cost of the fuel."
 
"The solenoid part number is 8

"The solenoid part number is 862656A3 (4 BBL Weber carb) and 862656A2 (2 BBL Mercarb carb). The list price is $294.35 for either version. I would call around for price on this.

Again, look for the thread I mentioned before so you can do as much as possible to eliminate this. The higher octane and cooler plugs should help a lot. I have gone through the whole list in that bulletin on these motors and on quite a few, it worked- on a couple, I needed to install the solenoid. That took care of it every time.

I've never heard that there was a detonation issue, just run-on, either from running the boats with this condition myself or hearing it from others, including the Mercruiser technical dept. That doesn't mean it isn't happening but I don't know that there have been a load of piston failures. Still, better to play it safe."
 
"My fault. I meant run on, not

"My fault. I meant run on, not dieseling. I saw the thread and printed it out. Are the spark plugs you show on the thread the colder heat range plugs? There are two adjustments on the carburetor. I've always refered to one as the idle speed adjustment(on linkage)and the other as the mixture (on carburetor). Which one do I adjust? Which way do I turn it?"
 
"Actually, it is technically d

"Actually, it is technically dieseling, just without high compression- run-on is just the way it's usually referred to by manufacturers and is the way I learned it once I got into training.

If you saw the thread and are referring to the AC MR41T, yes. The info on the thread is right from the bulletin, I just didn't want to get nailed for any kind of copywrite problems by scanning it.

The idle mixture screw is at the borrom front and the idle speed stop is the screw where the lever is, controlled by the throttle cable. The idle mixture will have nothing to do with high speed running since that one is sealed. Unscrewing the idle mixture will enrich it (they mentioned 1/8 turn) and the idle stop works the same way- unscrewing it decreases the idle speed.

If you make a note of where these screws were when you start, you can always put them back to the original position."
 
"To finish out the thread, you

"To finish out the thread, your suggestions seem to have fixed the problem. The lower temperature thermostat seems to have made the difference."
 
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