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  Evinrude 50-HP: Fuel Sputtering from Carburetors

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Author Topic:   Evinrude 50-HP: Fuel Sputtering from Carburetors
dg22 posted 08-04-2015 10:42 AM ET (US)   Profile for dg22   Send Email to dg22  
I have 1973 Evinrude 50-HP outboard [on a Boston Whaler 13-footer]. I was noticing gasoline in the belly pan after taking it out last year. Over the winter I re-built the carburetors. I thought I had solved the problem, but, after taking the kids tubing, I noticed fuel in the belly pan, again. Before taking my week-off this year, I decided to replace the fuel pump, hoping to solve this problem, and it made no difference.

When I investigate the motor running with cover off there is no sign of gasoline leaking from the carburetors or fuel lines, but when I take it out for a boat ride and run it WOT, and when I return to the dock and remove the cover, I notice there is a small amount of gasoline running down the belly pan from where the air silencer is mounted to the front of the carburetors. Fuel appears to be sputtering out of the carburetors when running WOT. Do I simply need to replace the gasket on the air silencer so this excess fuel drains via the drain hose? Or, is there more going on here? Other than this, the motor runs smoothly and performs well. It idles well and tops out at 5,600-RPM WOT with a 17-pitch SS propeller.

jimh posted 08-04-2015 01:20 PM ET (US)     Profile for jimh  Send Email to jimh     
The flow of air in the air silencer should be toward the engine intake. If you are seeing a flow of air back from the engine, through the carburetors, the cause of that might be a problem with the reed valves. The reed valves should prevent back flow of air from the intake passage of the cylinder, backwards through the carburetor.

It is also common that some fuel in the fuel-air mixture drops out of the air stream. Usually there is a means to collect this fuel and re-circulate it back into the engine. I don't know the particulars for your 1973 Evinrude 50-HP.

dg22 posted 08-04-2015 01:56 PM ET (US)     Profile for dg22  Send Email to dg22     
Yes there is a drain hose at the bottom of the air silencer to the engine but because the gasket is quite old I'm thinking the fuel that drops out of the air-mixture is making it's way through the old gasket instead of going down that drain hose.

What are the performance symptoms of faulty leaf/reed valves?

knothead posted 08-04-2015 02:56 PM ET (US)     Profile for knothead  Send Email to knothead     
dg22,

If your carburetor bowls are plastic, as they are on several OMC models, they may have become warped and therefore leak fuel. If leaking fuel at WOT perhaps the carburetor floats are set too high.

Symptoms of reed valve problems can be spitting back through the carburetor at idle, starts but will not idle slowly,low power, stalls on acceleration, loss of WOT RPM. You don't mention any of these symptoms, so perhaps reed valves are not the problem.

Good luck and you might want to try the engine forums at iboats.com

regards---knothead

dg22 posted 08-04-2015 03:20 PM ET (US)     Profile for dg22  Send Email to dg22     
When I did the carb re-build I made sure the floats were set right and the bowls are metal. At this point, I'm not ruling out the leaf or reed valves. It does stall occasionally going into gear and it will sometimes not start easily after running for a while. The next time I'm at the cottage, I'll try removing the air silencer and see if it is spitting back fuel while in idle.

I appreciate the feed back.

dg22 posted 08-17-2015 08:29 PM ET (US)     Profile for dg22  Send Email to dg22     
I removed the carburetors of the 50-HP engine, and I removed the intake manifold where the reed valves are located. I inspected the reed valves, and they are perfect. I performed the light test and they are good. I was totally puzzled. [Upon] further inspection of the carburetors and fuel lines, I checked the clamps on the fuel line that goes from the upper to lower carburetors. I noticed a hair line crack in the fuel line where the clamp is located. I wondered how I missed this, so I checked the photos I took when I re-did the carburetors. Sure enough, I only removed this fuel line on one side to separate the carburetors and therefore did not remove that clamp where the hair line crack was located. Man I hate Murphy’s law.
jimh posted 08-18-2015 11:53 AM ET (US)     Profile for jimh  Send Email to jimh     
Was this newly discovered hair line crack in a fuel hose at the carburetors the source of the fuel that you earlier described as sputtering from the carburetors?

What is the test procedure you described as "the light test"?

dg22 posted 08-18-2015 02:06 PM ET (US)     Profile for dg22  Send Email to dg22     
Yes, it turns out my assumption of fuel sputtering from the carbs was wrong. I think what is happening is at higher throttle (with increased fuel pressure in the line) causes the leak to occur via this hair line crack. I'm replacing all the fuel lines and gaskets while I'm at it.

The light test is simply taking the intake manifold with read valves attached and looking through with a light on the other side to see if there are any gaps between the leafs and the plate. If you see a lot of light coming through you probably have a leaf that is turned up and not laying flat against the plate.

dg22 posted 09-21-2015 09:42 AM ET (US)     Profile for dg22  Send Email to dg22     
Replaced that bad fuel line and put the motor back together with new gaskets. Ran the motor for 2 weekends now and no fuel leaking into the belly pan. It is now bone dry in the belly pan under the carbs. Glad to get this problem solved.

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