Help! Gas overflow, wild idle variance, pressure in gas tank

[herbsteiner]

Newbie needs help!

Weird things started happening to my Z and before I go to the mechanic, maybe I can get some info from the forum. I'm a newbie, so please be patient with me. I've already done the search thing but it's still not clear to my dense, uneducated brain.

First, the car: A 1973 but with '72 round-tops and the emissions stuff removed and holes plugged. 3-speed automatic. Last visit to the Z mechanic was about 2 months ago. The car is a daily driver.

Now the problem: I recently noticed some intermittent hesitation and stuttering when accelerating, so I thought some kind of fuel delivery problem. Extremely strong smell of gas. Then the idle would vary wildly in neutral from 800 up to 1400, but intermittently. When put in gear it would sometimes run great and other times want to stall in stop-go traffic, cycling down to 500 rpm before stalling. Seems like these symptoms would occur after driving for awhile. Previously... last week... the idle in neutral was around 900rpm and in gear around 700 without stalling.

Today when I opened the hood and pushed on the throttle, gas would spew from the overflow hoses onto the tranny housing. I opened the gas filler spout and back pressure released either gas pressure or actual gas from the spout.

Fuel filter looks okay. So what does the wisdom and knowledge bank of the forum think is the problem these symptoms resemble? I really don't want to drive much with raw fuel draining onto the ground.

Please excuse my ignorance, I didn't get much info I could digest from the search.

Edited March 20, 2011 by herbsteiner

[d240zx2]

It appears you've got a vapor return line/fuel return line blocked. Look on the passenger side toward the front under the hood on the fender wall. You'll find three steel lines there, one of which is the fuel supply line (the larger one). Remove the rubber from the other two and using compressed air, blow each of the lines. If this doesn't solve the problem you've likely got a collapsed or deteriorated hose at the tank causing blockage.

[esmit208]

HERBSTEINER

If it is possible try to send a picture of exactly where the fuel is spilling from under the hood. "GAS OVERFLOW HOSES"? Hmmmm. Can't say that I have ever seen those. A pic would help with diagnosis.

[steve91tt]

Is it overflow from the float bowls?

[beandip]

If you have fuel overflowing out of the carbs. it is because the float shutoff has failed. Now this failure could be caused from excessive fuel pressure. Yes there is a fuel recovery/return to the rear tank. But your pump should not be putting out more than5psi. tops.

There is a valve connected to the fuel system that supplies fresh air to the tank and also releases pressure. It is located right under the ignition coil towards the front of the engine bay. Make sure that it is not clogged. Check the pressure the pump is delivering. If 5psi and under this is good. 5psi. is max. then check the needle and seat in the carbs float bowls. Even with a blocked return line the pump should not deliver over 5psi.

Information: the gas return line that is part of the fuel rails function, is to keep the gas flowing so it stays cool. The whole fuel rail is right above the intake and exhaust manifolds. The heat from this causes the gas to boil in the rail coupled by the heat from the mechanical fuel pump. By having the gas that is not used return to the tank it helps to keep all cool.

[herbsteiner]

Yes, it's overflow from the float bowls.

I have the FSM from 1973, but of course it deals with flat-top carbs and my car was reverted to 72 round-tops. I downloaded the FSM for 1972, but the file can't open without winRAR and my 'puter says it's a malware file.

[steve91tt]

Sounds like a float or fuel valve issue. Does it overflow from both carbs?

[Gary in NJ]

Sounds like a float or fuel valve issue. Does it overflow from both carbs?

I'd agree, sounds like the needle valve (float valve) is stuck. However, if both carbs have fuel coming from the float chamber vent that might be an indication of another problem because it would be unlucky to have both fail at the same time.

[Bruce Palmer]

So when both carbs are doing their deal, the engine is no doubt running rich..... blubbery with copious quantities of black smoke right? Sounds like to much fuel pressure (3 1/2 # max) but hard to fathom why it would start with too much fuel pressure all of a sudden. But I guess the pressure in the tank could be an additional driving force behind the pressure thing.

When all is said and done you might want to throw a new set of plugs in. Fuel fouled plugs don't run well and may never clean up.

[herbsteiner]

Can't tell at the moment if both carbs are stuck or only one, since the hoses are joined by a "Y" connector. But I can localize each hose and that would provide an answer. More info later, dudes, and thanks so much for all the help so far.

[Gary in NJ]

Can't tell at the moment if both carbs are stuck or only one, since the hoses are joined by a "Y" connector.

I believe that might be the source of the problem. They are VENTS and need to be vented to ambient conditions.

Run them individually and see if it clears-up the issue.

[Bruce Palmer]

So if it turns out to be just one carb, it could be nothing more involved than a piece of grit holding one needle and seat open. Figure out which it is and take that lid off remove the needle and seat and flush it out with carb cleaner. Order float bowl lid gaskets ahead of time as your will no doubt turn into potato chips when you remove the lid.