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 Post Posted: Sat Nov 05, 2011 5:12 pm 
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Joined: Fri Oct 14, 2011 11:38 pm
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I have recently installed a different carb on my engine, and noticed that after running the engine and then letting it set for a day that the fuel level in both bowls dropped from the center of the sight glasses to just slightly below the bottom of the sight glasses. I know that the power valve in the primary and the power valve plug in the secondary were tight as I had checked them after main fuel jet changes. The power valve gasket seals are teflon and they looked to be in good shape. Any ideas why the fuel would drop the same amount in both bowls?

It has done this the past two times after having the car out for a drive. I tried opening the butterflies to see if I could notice extra fuel in the plenum floor of the intake manifold, but with the accelerator pumps squirting fuel in them I couldn't tell for sure but I suppose it must as there is no other place for the fuel to go as it hasn't leaked out external of the carb anywhere.


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 Post Posted: Sat Nov 05, 2011 7:47 pm 
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I'm bet that what you're seeing is fuel evaporation from decay heat after the engine is shut off and this would be especially true with pump gas with ethanol in it.

Most all those carb circuits are pull over type so there's not much way for fuel to leak into the engine if the PV and gaskets are good. And you haven't noticed anything leaking on the outside either?

FYI my methanol carbs will loose enough fuel from evaporation to where the engine wont' fire and this happens in just about a week.

You can test this by using a small glass with a measured amount of gas in it, or, just fill it half full and put a magic marker line at the level then set that glass on a hot engine after you shut it off to see what happens. I'm gonna bet your level will be down after everything cools off and it will probably go down even more each day.

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 Post Posted: Sat Nov 05, 2011 8:23 pm 
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Joined: Fri Oct 14, 2011 11:38 pm
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Hey Ken,

I bet you are probably right and thought that could be the case as well. The thing that threw me off with this assumption was that the fuel level dropped from half glass to about a quarter glass after three or four hours when the engine had cooled down which I thought could be the change in density and/or some evaporation. However, I checked it again a couple of days later after being gone out of town and it had dropped to just slightly below the bottom of the sight glass. This is when I thought that it was more likely that there was a leak, but it didn't make sense that both bowls had dropped to the same level and I really didn't think that the primary power valve and the secondary power valve plug were both leaking. I couldn't think of another way the fuel could leak internally and as mentioned it isn't leaking externally, so that is why I asked the question.

I think it must be just due to evaporation as you mentioned. Just didn't think it would be that much especially since the ambient temperatures are cooler here now. I may fill the bowls back to half a sight glass level without running the engine and see what the level does over 2 or 3 days.

Thanks for the input.


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