Mercruiser 4.3 advice on problem

[QBhoy]

Hi all...been a while since I had a 4.3 or a carb. This relates to a friends new boat.
Its a maxum 19 foot bowrider with a carb 4.3. ‘99 vintage.
Been sittting unused for a while and not used much before.
I’ve only looked at it briefly when the engine wouldn’t run right. Told him to check for water in filter. There was loads. Told him to empty fuel tank. He pumped about 3l of water out the bottom of it and left about 30l of fuel in there....I know, I know !
its now haveing pre ignition issues where is sputters and runs on after switching off. Also pops a bit when revs in neutral. Since been diluted with 50l of fresh fuel. Apart from water still being an issue, what else might cause this ?
next step I’ve suggested is Defo ruling out water by totally draining tank and changing filter again. Then change the Diz cap. Perhaps then coil ?
the local Merc guy reckons it’s common for them to do this because of poor fuel from the petrol stations. I’m not convinced of this. I never had a problem with all the carb gm engines I had. Something to do with carbon deposits left smouldering on the pistons. Mmm

 

[Rick Stephens]

Water in fuel, especially if the fuel has ethanol, means all the fuel is garbage. With Ethanol gas, when you add water, the molecular electron charges make the water molecule combine with the ethanol. And it does it virtually instantly. Try pouring a full glass of alcohol into a full glass of water, they will mix instantly and the volume will be that of just one full glass. When ethanol mixes with water, it combines, gets heavy and drops to the bottom of the tank. Then you pump it out and all is good, right?

Wrong. The ethanol water mix pulls about 5-8 points of the octane down to the bottom of the tank with it. The octane level in the gasoline left is now less than the engine needs to run. Ethanol added to fuel is considered an octane booster, your water mix pulled enough octane to make the fuel that is left sub-octane level needed to run.

The other thing that happens is old fuel cruds up and leaves crystals and gummy deposits all through the fuel system. If it is bad enough you should pull everything, clean or replace anything not perfect. Fuel hose will have crystals in them that will drop chunks into the clean new fuel after you've done a carb rebuild, flushed the tank and replaced the filters several times.

With all the water pulled, the entire fuel system needs draining and a complete cleaning ending with the carburetor rebuilt.

 

[Maclin]

Rick just described in 2 short paragraphs what I have read in several full 3-4 page articles, about the octane crash phenomenon with the ethanol laced fuels. If you stay on top of the freshness with them there will not be a problem just due to the ethanol usually, but once you go past the freshness fence or have significant water intrusion then a full refresh is required. Be sure to at least dump the carb contents, best to take it apart and clean it all out

Good luck, hope you guys get it sorted.

 

[R055]

Clean fuel/fuel tank and lines plus a carb rebuild would most likely fix all/most of those problems.

My 4.3 also diesels(continues to run a bit after turning off) if I use 87 octane fuel and it's hot after water sports. Doesn't happen if I fill up with 92 octane gas because higher octane has has a hotter combustion temperture. Many threads on this already, look through those to get a better idea.

 

[gtfireftr]

I would get a separate fuel tank with fresh gas and try to run her off that. You still probable will need a carb rebuild though..

 

[Lou C]

If you had that much water in your fuel tank, then I bet you have water in the carb as well. Those water separating fuel filters can hold some water but when you have that much water in the tank, I think you will get water in the carb which will make it run really poorly. Time for a carb rebuild.

 

[alldodge]

IMO, you had 30L of bad gas, now you have 80L of bad gas

 

[QBhoy]

Water in fuel, especially if the fuel has ethanol, means all the fuel is garbage. With Ethanol gas, when you add water, the molecular electron charges make the water molecule combine with the ethanol. And it does it virtually instantly. Try pouring a full glass of alcohol into a full glass of water, they will mix instantly and the volume will be that of just one full glass. When ethanol mixes with water, it combines, gets heavy and drops to the bottom of the tank. Then you pump it out and all is good, right?

Wrong. The ethanol water mix pulls about 5-8 points of the octane down to the bottom of the tank with it. The octane level in the gasoline left is now less than the engine needs to run. Ethanol added to fuel is considered an octane booster, your water mix pulled enough octane to make the fuel that is left sub-octane level needed to run.

The other thing that happens is old fuel cruds up and leaves crystals and gummy deposits all through the fuel system. If it is bad enough you should pull everything, clean or replace anything not perfect. Fuel hose will have crystals in them that will drop chunks into the clean new fuel after you've done a carb rebuild, flushed the tank and replaced the filters several times.

With all the water pulled, the entire fuel system needs draining and a complete cleaning ending with the carburetor rebuilt.

Superb Rick. About as good a description of what’s going on as I’ve ever heard. I figure that’s what’s happening. He should have ditched the whole lot.

 

[QBhoy]

Thanks everyone. Not my boat, worth pointing out. Agree with all. Carb has now been stripped etc. Runs better, but still a little pre ignition on shutdown. No popping anymore.
He needs to get rid of it all likely and if unlucky clean carb again. Filter will be a must.
Thsnks again.

 

[Scott Danforth]

change the filter often, check timing, burn out all the fuel, add some ethanol free fuel and some isopropyl alcohol

 

[Maclin]

Letting it idle for a bit before shutting it off is a recommendation from OEM also on those I believe. Good idea for any marine engine. My 5.7 would run on every once in a while until I started to let it run a bit.

 

[QBhoy]

Thanks guys. Think we will get rid of fuel, Get pure fresh fuel (almost impossible to get non ethanol fuel) and dose with an octane booster...see how she goes. If not, new Diz cap and rotor.

 

[hoowahfun]

.. and some isopropyl alcohol

What is this for?

 

[Scott Danforth]

to dry up the ship-ton of moisture and not have the same negative side effects that ethanol has.

 

[hoowahfun]

to dry up the ship-ton of moisture and not have the same negative side effects that ethanol has.

Interesting, didn't know it would do that. Thanks