OMC 5.7 will idle @10k but then slowly idles down and dies

[Bigmommasdaddy]

This is a 1978 omc sterndrive. It has a (professionally) rebuilt carb, new coil, new distributor, electronic conversion kit, and was recently tuned up.

The boat will start and run for about twenty minutes, but then if I leave it at an idle it will slowly die out. It will easily restart, but then dies...If I try to give it a little extra gas at this point, it will still die and then tries to backfire. It then gets to the point of not starting at all..I have a clear inline fuel filter and it was showing about 1/4 full of fuel at all times. The distibutor and conversion kit was also professionally installed.
By the way, thank you coast guard for your professional and effeciency in towing us back in!

I have tried all I can think of and I am out of ideas...any help would be greatly appreciated!

 

[havasuboatman]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

Pull a couple of spark plugs right after it runs down and dies. You are looking for very wet (fuel) plugs.
If that is the case, check float level, Needle valve, fuel pressure.
If they are not wet, but are kinda white, then your problem is a lean condition. check the same things, add looking for water in the carb bowl, and checking fuel volume from the pump, and if you have a water seperating fuel filter, check it for water by dumping it into a clear container. DO NOT reuse the water seperater. If you don't have one, you should install one.
By the way is your clear in-line fuel filter one of those glass ones that the ends screw on?
If so, get rid of it. They have a habit of leaking after a few weeks.

 

[Bigmommasdaddy]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

Pull a couple of spark plugs right after it runs down and dies. You are looking for very wet (fuel) plugs.
If that is the case, check float level, Needle valve, fuel pressure.
If they are not wet, but are kinda white, then your problem is a lean condition. check the same things, add looking for water in the carb bowl, and checking fuel volume from the pump, and if you have a water seperating fuel filter, check it for water by dumping it into a clear container. DO NOT reuse the water seperater. If you don't have one, you should install one.
By the way is your clear in-line fuel filter one of those glass ones that the ends screw on?
If so, get rid of it. They have a habit of leaking after a few weeks.

Thank you for the quick response! The carb was professionally rebuilt and installed no more than a month ago, but I will pull the plugs just to make sure...I haven't installed a water seperator yet, but it is on my list, would Heet help in the meantime? Also what would water look like in the carb bowl?
As far as the filter, it is a clear plastic inline filter (new) and the boat also has a canister filter (also new). I only installed the inline so that I could see at a glance if fuel was flowing.
It almost sounds as though the carb has a vaccuum leak...Could this be a possible cause?

 

[Bigmommasdaddy]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

This is a 1978 omc sterndrive. It has a (professionally) rebuilt carb, new coil, new distributor, electronic conversion kit, and was recently tuned up.

The boat will start and run for about twenty minutes, but then if I leave it at an idle it will slowly die out. It will easily restart, but then dies...If I try to give it a little extra gas at this point, it will still die and then tries to backfire. It then gets to the point of not starting at all..I have a clear inline fuel filter and it was showing about 1/4 full of fuel at all times. The distibutor and conversion kit was also professionally installed.
By the way, thank you coast guard for your professional and effeciency in towing us back in!

I have tried all I can think of and I am out of ideas...any help would be greatly appreciated![/QUOTE
----------------------->thread title should read 1,000 not 10,000 lol

 

[Alpheus]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

Sounds like your idle mixture needs adjusted. Running to rich...

 

[Bondo]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

The carb was professionally rebuilt and installed no more than a month ago, but I will pull the plugs just to make sure...I haven't installed a water seperator yet, but it is on my list,
As far as the filter, it is a clear plastic inline filter (new) and the boat also has a canister filter (also new).

Ayuh,... With Old carbs(31 years is pretty danm Old),...
You've got a 50/50 chance of sucess, or failure at Rebuilding it...
Your's could be just plain ole Junk...

The Cannister filter,.. IS the Water Seperating filter....
But,.. Get rid of the little Plastic in-line filter,... It's Dangerous,+ Don't belong there....

You also mention a possible Vacuum Leak,... That could be your problem...
It sure sounds like it's Starving for fuel...

 

[cr2k]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

Sounds to me like good old fashioned vapor lock. How is your fuel line routed?

 

[rbh]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

When you get the engine going, spray wd-40 around the base of the carb, you may have a leak between the carb and manifold???
if the engine picks up there it is

 

[havasuboatman]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

By the way, I doubt you are idling a 10k, unless you are running a turbine.
In your response you mention the back fire on accelleration. That should have been in the original post along with the vacuum leak suspition. Makes all the difference in the world. Too lean a fuel mixture causes backfiring thru the carb.
A vacuum leak, causes too lean a fuel mixture.
So does:
Insufficient fuel volume
Too low fuel pressure
Improperly adjusted floats.
You may also have a problem with your fuel tank vent. To test this, take off the gas cap and run the boat. If problem goes away, there you are.

 

[Bigmommasdaddy]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

Sounds to me like good old fashioned vapor lock. How is your fuel line routed?

The fuel line is rubber, and it runs from the tank to a canister style filter about four feet away from the tank. There is an inline filter before the canister filter. The line runs in basically a straight path.

 

[Bigmommasdaddy]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

By the way, I doubt you are idling a 10k, unless you are running a turbine.
In your response you mention the back fire on accelleration. That should have been in the original post along with the vacuum leak suspition. Makes all the difference in the world. Too lean a fuel mixture causes backfiring thru the carb.
A vacuum leak, causes too lean a fuel mixture.
So does:
Insufficient fuel volume
Too low fuel pressure
Improperly adjusted floats.
You may also have a problem with your fuel tank vent. To test this, take off the gas cap and run the boat. If problem goes away, there you are.

Yeah, I caught my typo (10k) after I posted and was unable to edit it..Nothing like showing of my intelligence! lol...
I apologize for the incomplete description of symptoms, this was my first post. I am going to find the vaccuum leak, and once fixed, I will try loosening the gas cap to see if this helps.

 

[Bigmommasdaddy]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

Thank you for all your help guys!....I will update as soon as I find anything..

 

[Bondo]

Re: OMC 5.7 will idle @10k but then slowly idles down and dies

There is an inline filter before the canister filter.

Ayuh,... You need the Cannister Filter,...
By your discription, there's atleast 2 in-line filters,.... Get Rid of those In-line filters... All of 'em...