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Old 06-16-19, 06:50PM   #1
vicgto
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Default Fuel leak from rear of Q Jet puddling on intake manifold

I'm getting fuel puddling on both left and right sides of intake manifold, I think it might be coming from the secondary throttle shafts as they are quite loose and the puddles are just below the secondaries. Anyone ever run into this? Anyone ever installed bushings in the secondaries?
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Old 06-16-19, 10:16PM   #2
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Iíve never run into that, trying think what would cause it. Is it after it sits for a while or after taken it for a cruise ?
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Old 06-17-19, 12:20AM   #3
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When was the last time this carburetor was serviced?
Who touched it last?
How long does the car sit between use?
daily driver
once a week
once a month
occasional use
once a year

Leaking fuel onto the manifold exterior:
Float level wrong or sunken float.
dirt/crud in the needle and seat area.
fuel pressure possibly to high
possibly damaged or poorly seated gasket between top/air horn & main body.

The other problem on early Q-jets was the main well plugs would loosen up and let fuel drain out of the float bowl when the car was parked.
The fuel would drip right past the throttle body and into the manifold causing over rich start or hard start because the fuel bowl was emptied out.( over night or quicker depending on severity of the leak(s) on the main well plugs).
Lot of carburetor rebuild kits had different fixes for this condition some used epoxy and some used a dense foam gasket that seals the main well plug cavity between the main body and the throttle body.

If you need the carburetor expertly restored try:
RECARBCO
http://recarbco.com/
They can bush worn throttle shafts and take care of a lot of other problems.


If you want to DIY the throttle shafts
This company sells kits for it
http://www.throttleshaftrepair.com/index.htm

But if you have never done this it can be tricky .
Getting the screws that hold the throttle blades to the shafts out must be done without snapping them off.
If it gets botched then you are sending it out to get saved.

"good judgement comes from experience & experience comes from poor judgement" LOL
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Last edited by sprint250 : 06-17-19 at 12:45AM.
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Old 06-17-19, 01:01AM   #4
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Quote:
The other problem on early Q-jets was the main well plugs would loosen up and let fuel drain out of the float bowl when the car was parked.
The fuel would drip right past the throttle body and into the manifold causing over rich start or hard start because the fuel bowl was emptied out.( over night or quicker depending on severity of the leak(s) on the main well plugs).
Lot of carburetor rebuild kits had different fixes for this condition some used epoxy and some used a dense foam gasket that seals the main well plug cavity between the main body and the throttle body.
Thatís what I was thinking had that problem, canít remember the fix though, think there was a kit, were you drill them out and plug them. Been a while
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Old 06-17-19, 10:39PM   #5
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Thanks for the response Sprint250.

I overhauled the carb this year, I used a kit from Cliffs Qjets. Car is driven barely once a week. I'm thinking it might be fuel pressure related as I recently went to a Mallory 140 pump with regulator set to about 7 psi. However, if the level were high, I would expect to have a rich running engine, which I dont think I have, no black smoke. I know my main well plugs do not leak as I have tested by letting fuel sit overnight in the bowl.
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Old 06-18-19, 10:48AM   #6
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Additional info
FUEL LEAK BY THROTTLE SHAFT(S)
A common complaint today is fuel dripping out of the throttle body by the throttle shaft AFTER the engine is switched off. While a number of issues may cause this problem, by far the most common issue is the volatility of modern fuel. Mechanical fuel pumps have a check valve which prevents fuel from moving back to the fuel tank. The problem is as follows:

(1) After the engine is switched off, heat from the engine heats the fuel in the fuel line.

(2) The expanding fuel (increased volatility) creates pressure in the fuel line from the pump to the carburetor.

(3) The check valve prevents the fuel backing up through the fuel pump.

(4) The pressure increases to a point the float/fuel valve combination in the carburetor cannot withstand the pressure.

(5) An amount of fuel (usually from a teaspoon to a couple of tablespoons) flows into the fuel bowl of the carburetor.

(6) This raises the fuel level in the bowl above the main discharge nozzle(s).

(7) Fuel flows through the main discharge nozzle(s) and drips onto the throttle plate(s) which is/are closed, and exits out beside the throttle shaft(s) dripping onto the intake.



Possible solutions:



(1) IF POSSIBLE, AVOID ETHANOL LACED FUEL! Sometimes you can buy real gasoline at a marina

(2) Buy the lowest octane name-brand fuel that does not ping or detonate in your engine (the higher grades often have more ethanol)

(3) Install a “vapor return line” (take a look at return lines used on many factory air-conditioned cars)

(4) Learn to live with the issue.

Above material copied from : THE CARBURETOR SHOP trouble shooting section
http://www.thecarburetorshop.com/Troubleshooting.htm
__________________________________________________ _____________________
#4 -learning to live with the problem is not the best solution.
Nothing worse than getting behind the wheel and smelling raw fuel leaking,
Right combination of conditions could be disastrous or even tragic.

One thing I have had to resort to is insulating the fuel line where it passes close to the exhaust system and all fuel lines inside the engine bay.
It makes a huge difference given the propensity of today's pump grade gasoline's to percolate at higher temps.Fuel injected cars are not as sensitive to this.
If the fuel system gets heat soaked (stop & go traffic) the fuel will percolate and cause cause low speed drive-ability issues until the temps drop sufficiently and allow the fuel to cool. Excessive heat soaking of the Carburetor & fuel system when the car is parked after running will also cause problems as described by the CARB SHOP info.

Find a way to vent the hot air out from under the hood.
If the ram air scoops on the hood are non functional open them up to get some cooler air entering the engine bay.
Sometimes a 1/2" 0R 3/4" spacer between the the hood and the hood hinge pads will help to get rid of excess under hood heat. Not pretty but have had to resort to it and the results were positive.
A carburetor heat shield and thick carb base insulator may be in order .
The heat shields are EZ to make at home out of sheet aluminum ( Moroso & other companies used to sell them ready made ,just trim to fit).
Mr. Gasket still sells the base insulator kit for the Q-jet

Hopefully the condition your carb is exhibiting is a simple fix or a combination of simple fixes.
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Old 06-19-19, 07:41PM   #7
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Still available through Mr. Gasket
https://www.holley.com/brands/mr_gas...spacers/page=2
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Old 06-19-19, 09:57PM   #8
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Thanks for the detailed description of fuel leak by throttle shaft sprint250.

How does fuel inlet seat size affect what you described below. What would you say is the maximum seat size, I enlarged mine to .145".

Also I am running an electric Mallory 140 pump with a return line tee'd into the fuel inlet a the regulator, wouldn't this allow the fuel pressure to dissipate?
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Old 06-20-19, 12:22AM   #9
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I notice you said 7 lbs Pressure Itís been a while but I thought you couldnít run anymore then 6 lbs pressure with a Qíjet
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Old 06-20-19, 02:42AM   #10
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The larger the diameter of the seat size the more fuel pressure force against the corresponding needle size that the float is connected to.
If the float cannot generate enough force overcome the incoming fuel pressure the needle will be forced off the seat/ held open.

If the needle and seat sizes were changed to larger sizes the original fuel pressure setting should be left alone to see how things respond by the additional volume of fuel now available from the larger the needle & seat size.

Q jets are very fuel pressure sensitive carbs.

PSIG = pounds per square inch (above sea level atmospheric 14.7 psi)

That's the total pressure force exerted on an area of 1 square inch.
If you calculate the area of a .145" hole the total area is far less than 1 square inch

Take the decimal area result and multiply it by 7 psig and that is the total force being exerted against the closed needle and seat assembly.
The float has to be able to generate enough leverage on the float arm to exceed that value. That is why a conical needle valve is used as it gives more surface area to effect a tight shut off and more closely regulate the flow of fuel through the seat area.
A flat disc would not work well.

7 psi through an unrestricted .145 orifice will generate a stream 20ft long similar to what a super-soaker squirt gun does. So that's what is going on inside the carb float bowl if the needle and float cannot keep the fuel flow under control.
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Old 06-20-19, 10:19PM   #11
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Think the most I was ever able to run with the bigger seat was 6-6-1:2 lbs. something you might want to check maybe try 6 lbs. see if it eliminates the problem, just a thought
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Old 06-22-19, 01:44PM   #12
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7psi would be tops.

Max fuel pressure should be measured @ WOT , the engine under load with the gauge fitting as close to the carb fuel inlet as practical.
If you are doing the test on the fly the gauge has to be readable from the driving position. You can mount the gauge temporarily on the cowl in front of the windshield.
DO NOT run the fuel line and gauge inside the driver's compartment.
To do so safely requires installing a gauge line isolator in side the engine compartment between the live line and the gauge line.

The carb does not need 7 psi at idle or even part throttle cruise.
The return line flow rate can be adjusted either by a changeable orifice size or an adjustable needle valve.
What you are looking for is the lowest maximum fuel pressure required to sustain proper operation @ WOT ( without leaning out/shooting ducks going down track due to lack of fuel in the float bowl).

Not knowing the intended purpose of the car & top end combination & CID its hard to recommend anything. If the engine has mild performance upgrades from stock, is occasionally driven and occasionally run @ WOT the engine probably does not require 7PSI of fuel pressure at the carb inlet.
Next questions :
Did this problem happen before or after the carb rebuild?
After the seat diameter change?
After the fuel pressure increase?

These guys sell all kinds of different needle & seat sizes from stock .110" up to .150".
@.145" you are almost maxed out as far as largest available seat size.

Quadrajet Parts
https://quadrajetparts.com/quadrajet...-c-128_30.html

Their take on fuel pressure
https://quadrajetparts.com/-a-3.html


Could this be the cause?
Cliff's take on fuel pressure & float types:
"There are several different styles of Quadrajets out there. There are also different size/type of floats, and quite a few different fuel inlet seat diameters.

The early carbs with the short hinge pin and large float are the WORST set-up, and will not tolerate much pressure with a large fuel inlet seat.

Max with a .135" seat is about 6psi.

The later designs are far superior when it comes to leverage on the needle, so they are able to use a much smaller float and larger fuel inlet seat at higher pressure.

No problem at all running 6-7 psi on a .149" seat with a small float on the later units. Some racers run more than that.

On my own carb, which is a 1977 Pontiac part #17057274, I use a .145" seat at 6.5 psi with a small float, zero issues anyplace....Cliff "

More discussion
http://www.carburetor-blog.com/roche...et-carburetor/

There is a lot of discussion on this. you have to sift through the info.
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Old 06-23-19, 02:55AM   #13
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Man that was excellent!
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Old 06-23-19, 10:52PM   #14
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Aw shucks!
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