Gas smell

sti1471

Petty Officer 1st Class
Joined
Aug 2, 2011
Messages
312
Have a strange problem.

I have a 1984 mercruiser 140 3.0L. Mercarb 2 bbl Carb. Manual fuel pump. I started the boat and ran it for a bit on the water muffs and the boat runs good. I noticed that if I let the boat sit for a while in the garage, when I go out to start it the next time, it takes a little bit for the fuel to get up to the carb. SO it seems like the fuel is returning to the tank or something. That has been happening since I bought it.

Now, I just bought a new boat cover. I put the boat cover on the boat, and left it in the garage for a few weeks. I went out the other day and removed the boat cover to access the engine. As soon as I removed the boat cover and my nose was punched with the smell of gas. I lifed up the fuel door cover to access the tank, and there are no leaks around the tank. I checked all the lines from the tank to the carb and all have clamps on them are no cracks. Is that gas smell coming from the throat of the carb as the fuel bowl empties, or what else could it be.
 

Fishermark

Vice Admiral
Joined
Oct 19, 2003
Messages
5,617
Is this a new to you boat? It is possible that at some point in the past there has been a fuel leak that has gotten down into the bilge or foam. Otherwise, it is possible it is simply evaporating. On the start up, with a carb boat you need to put it into throttle only position, then pump the gear shift lever a couple of times and leave it at about 45 degrees or so till it starts.
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
I think the only way for the bowl to empty is if the power valve is leaking. The bowl is pretty well sealed and there is no way for the fuel to leak back into the tank because the level of the fuel is below the level of the inlet. Besides the anti siphon valve at the tank should keep the line full and prevent the fuel from flowing back. If the power valve is leaking it would be dumping gas into the manifold and the fumes could come back up through the carb throat. If you can't find a leak in the line suspect the carb. Just speculating here ;)
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
I have a different carburetor but my boat aldo has a full mooring cover. Mine goes over the tank vent too. I can smell a little fuel when I remove the cover after a few days of not using the boat. My carb has a vent to atmosphere on the fuel bowl. I believe the smell is evaporated fuel from the bowl along with some from the tank vent.
I allow the air to remove any smells above "deck" in my little 19 footer, then I run the blower to clear the bilge of fumes before I crank the engine.
Bet you are dealing with that unless it's really stinky and wet with gasoline. Check the junctions in the fuel system, before, during and after a run. If they stay dry you have a tight system.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
.... Manual fuel pump. ....

I've never seen a manual fuel pump in a boat... Maybe you mean 'mechanical'?

Would it smell if the new cover was pulled over the fuel tank vent?

If the temperature between days and nights was sufficient so cause enough expansion and contraction to force vapour out of the vent, most definitely...

If you think fuel is draining back from the fuel pump and the lines to the carb into the tank, then install a simple 1/4 turn valve in the inlet side of the pump. Once you finished for the day, close the valve. If the fuel now doesn't drain back, replace the valves in the fuel pump, or the complete fuel pump (depending if there are rebuild kits for that pump).... If the fuel is still gone, the carb is the suspect....

Chris......
 

JerryIrons

Petty Officer 2nd Class
Joined
May 27, 2010
Messages
125
In case this helps, I had something similar happen to me. I noticed that the boat, if it sat in the driveway with my boat cover on for a week or two, picked up a gas smell, and when I tried to start the boat it took a lot of cranking. Like the fuel bowl had emptied out. The smell was strong enough that I actually thought I had a leak somewhere that was under the floor, because I couldn't find a leak anywere on visible lines, pump, engine, etc. What ended up fixing my problem was a carb rebuild. In the process I also changed out the two fuel filters as well, but I noticed that when I rebuilt my carb, not only was the starting problem fixed, but the gas smell went away as well.

I used an ultrasound cleaner that I picked up from harbor freight, used the purple "simple green" cleaner, and it worked really well. I was amazed at how clean I was able to get the original carb cleaned in a 1986 engine. Plus the end cleaning fluid is basically soapy water, and easy to dispose of.

-Jerry
 
Last edited:
Top