Warped phbg float bowl suction leak craziness

Totally normal JBOT /

I have been battling a weird hanging idle with my hobbit for quite awhile,

after much trial and error, realized the culprit was the used 19phbg's warped float bowl and gasket. It's Jetted spot on, and would be fixed for a few weeks after using Yamabond on the gasket, but then with any removal of the bowl the problem would resurface. Stupid hanging idle again.

There was no fuel spilling out of the bowl at all.

Today I really dove into the project and noticed how very warped the bowl was. I sanded it down on some flat marble to be very flat, made a new gasket and it ran fantastic, no idle hang, consistently returning to idle quickly after a high speed run.

Here's the question.

With no fuel spilling out of the bowl, I'm assuming that I had an air leak that was sucking in air in to the bowl. How would this cause the hanging idle I had? I thought since the float bowl was vented already that an air leak couldn't be the issue, and the gasket was just there to keep fuel from spilling out....

So what gives? Air bubbles in the gas in the bowl causing weirdness?

A vacuum issue (air or fuel or both?) causing the motor to not pull fuel at low vacuum? please enlighten me.

Re: Warped phbg float bowl suction leak craziness

gas cap vent plugged? checked fuel flow rate at petcock?

Re: Warped phbg float bowl suction leak craziness

Totally normal JBOT /

You didn't even read my post, did you? Lol

The thing is....I already know what happened, I know what was broken and I fixed it,

Isolating all other variables.

I wanna know wassup with a leaky float bowl Gasket causing hanging idle simply by pulling in atmospheric air without leaking any gas???? (edited)

Re: Warped phbg float bowl suction leak craziness

Totally normal JBOT /

Aha! Reading IS good, I did some.

Bernoulli's principle!

Air leak into float bowl chamber causes it to lose it's high pressure it needs to have,

Thus it can't use Bernoulli's principle to "pump" the gas utilizing the venturi. Interesting!

So with the small leak I had. it just couldn't pump as much during the low vacuum of idle, but still ran fine on top...

Science! (edited)

Re: Warped phbg float bowl suction leak craziness

Totally normal JBOT /

Wait a tic... Low pressure in the Venturi,

The "high" pressure in the float bowl is just atmosperic pressure, the same atmospheric pressure that happened to be leaking in thru the warped gasket...


I second guessed myself that the float bowl gasket/bowl warpage was the problem, but after eliminating everything else, it's the only repeatable solution.

Well, if any carb theory gurus come up with an explanation, I'm game.

Just glad it's running awesome now after fixing the float bowl's seal.

Re: Warped phbg float bowl suction leak craziness


Over analyze the gift horse and it will fucking kick you in the mouth

Re: Warped phbg float bowl suction leak craziness

yeah, I read your post, but since it didn't make any sense, I offered a couple of possible solutions. A leaking bowl seal does not affect the atmospheric pressure (you said it yourself) in the bowl, due to vents. You didn't say what you eliminated. You obviously didn't eliminate everything

Re: Warped phbg float bowl suction leak craziness

Totally normal JBOT /

I did eliminate everything else. The bowl WAS the prob.

I need to know why, cuz it took me so long to figure it out.

Rode it today again, no hang after the bowl repair.

I cleaned the carb over and over til I could eat off it, still hanging.

Made sure no vent was blocked in tank, no leak at connections,

Adjusted the mix screw every way possible, still hanging.

Replaced seals, propaned/carb cleanered the whole motor, no air leaks...

The only thing that would eliminate the hang was Yamabonding the bowl to the carb body, or fixed it for good when I sanded the bowl flat and just used a gasket.

So if it's not a pressure issue,, it must have been air bubbles getting into the fuel on the float bowl, maybe frothing it up causing a lean condition.

Crazy. But true....

Re: Warped phbg float bowl suction leak craziness

From wikipedia.....this is the issue....as I was saying before, just more clearly.....get it....more vacuum is created at idle..more throttle...less vacuum pressure...no leaky leaky....c'mon...this is solved ands explained...go to sleep...and make that evil hobbit even faster. :)

Anyway, Here it is:

As the throttle is progressively opened, the manifold vacuum is lessened since there is less restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle (i.e., as the velocity increases, pressure falls).

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