Carb sight glass - acceptable fuel level variation?

[SteveH]

While troubleshooting a hot-soak problem on my '78 FJ40 (Federal spec, stock), I lowered the float level to below the 'dot' on the sight glass - it had been well above. But, while idling, the fuel will drop to nearly the bottom of the sight glass, and then (when the float drops) it fills to nearly the top of the sight glass. The needle-and-seat are ~5 years old when I last rebuilt the carb, and other than the hot soak issue, it runs very well.

Here's the question: On a properly functioning carburetor, how 'tightly' does the needle-and-seat keep the fuel regulated in the carburetor? Should the fuel sit at the 'dot' at idle, and basically not move?

Bonus question: Does the fact that my carb is slightly taken apart (vacuum lines removed) affect my measurement of this fuel level?

Thanks much! - Steve

 

[cbmontgo]

Steve, I am not an expert, but have rebuilt 8 or 10 of these Aisin carbs over the years with success. The fuel should remain steady in the center of the sight glass and the needle valve regulates this in "real time" by the height of the float, opening and closing the needle valve by this precise amount and regulated by a tiny spring inside the valve. It may be possible that your float needs adjustment or that the needle valve is somehow sticking. I have found some Aisin carbs that had a filter screen prior to the needle valve at the fuel inlet, and perhaps this has some debris in it that made it past the main fuel filter and is holding up fuel flow until pressure builds up. If it has no filter, then I'd bet that there is a piece of crud stuck in the needle valve if your float is adjusted properly.

I can't imagine how vacuum would affect it.

HTH since no one has responded.

 

[bikersmurf]

Sounds like the needle and seat aren't working correctly. Back when I rebuilt my Asian I had it adjusted to below the dot and it worked beautifully. I never saw the level fluctuate... don't know if it was right to have the level lower, but had done so to reduce the possibility of problems off Camber... and 25 years ago, info was very limited.

 

[SteveH]

Great info - thanks!

Guess what I found when sweeping my garage floor today? - the tiny brass insert that rides inside the spring that lives inside the needle. So, I will tear down and reassemble with this missing part. I'll update this thread with the results.

I also ordered another rebuilt kit and have a virgin '78 Federal carburetor I may rebuild to also try.

 

[SteveH]

Reassembled and adjusted the float height last night, and all is well - fuel level is rock-steady and only goes up a bit when I turn on the supplemental electric fuel pump.