Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: von on January 21, 2011, 08:35:09 AM
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Is it possible for a stock mechanical fuel pump (small block Chevy) to have too much pressure and overcome a Q jet needle and seat (.120)? If so, how common?
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Yes, there are quite a few mechanical fuel pumps showing up these days that have WAY too much pressure for any carburetor......Cliff
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Cliff, I shipped you the problem carb Friday that I talked to you about on the phone Thursday (gasket seepage/slow flooding problem after shut off). I'm wondering now if the fuel pressure could be too high from the stock pump and overcoming the needle and seat after the engine is shut off, causing the fuel level to slowly rise. The fuel pump has been on for years and quite a few miles so if that's the problem it would likely be something that happened in the pump recently.
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If the carb passes pressure, vacuum and running tests, then look elsewhere for the problem(s). Could be high fuel pressure, a gauge in line is the best way to determine if this is happening?.....Cliff
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I installed an in-line fuel pressure gauge near the carb. when the engine is running it swings wildly and rapidly (probably with every pulse of the pump) from about 3 to 12 psi. When the engine is shut off it stays at 8.5 psi then very slowly bleeds down to 0. A second q jet on this engine is also getting the gasket so wet after a few hours it runs down the front of the carb.
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That's not going to work. The pressure should be steady, and not past about 7psi at any time. Some late model pumps were high pressure, but most were low pressure. I think in later years they were trying to move more fuel thru the return system to keep it cooler as those emission controlled engines had some pretty hot underhood temperatures......Cliff
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Well I replaced the pump with a new parts store replacement. Now the pressure is about 1 psi higher. Pulsates wildly between about 4 and 13 psi. When shut off it stays at 9.5. I give up.
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I'd would confirm this with another fuel pressure gauge.
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A plugged return line will cause this. Do you have one? Is it plugged? This happened to me once. Pressure would go above 9.
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The car doesn't have a return line. Factory fuel line.
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Update. This relates to the wet air horn to bowl gasket problem. I installed a Holley fuel pressure regulator and set it as low as I could get it, 3.5 psi per my gauge. After rebuilding and installing another carb ('73 Chevy carb), and spending hours on the air horn gasket surface, getting it to match up perfectly with the bowl. the gasket still gets wet. I tried a little experiment by putting the corner of a gasket in a pan of gas. It wicked like crazy, travelling vertically and soaking the gasket. My conclusion is that the wet gasket is caused by this wicking action and that they all do it. It's more noticeable on ones with gaskets that protrude in some areas.
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Vacuum build up in tank from improper venting.
Dirt or debris in fuel tank can cause fuel pressure
going up and down, there either something
restricting the flow were pressure builds then releases
when blockage temporarily moves out of the way, or
the carb. itself is not set up right. eliminate one part at a time,
you purchased a new pump so not the pump, you say you tried a different carb. so not the carb,
BUT you have not redone fuel lines and cleared tank and replaced in-tank filter.
fuel pressure is not going to be effected by air horn gasket slightly leaking at the
levels your getting on your gauge. fix the blockage and install fuel pressure regulator.
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I verified it isn't a fuel pressure problem by installing a regulator and gauge with pressure set at 3.5 psi. It's not a tank or line problem since there's no fuel delivery problem at WOT and the tank vents are functioning properly. The wet gasket problem is also present on other Q jets I've observed or know about. I'm guessing it's a common occurrence that's not real noticeable so mostly ignored.
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wet gasket means its not sealing because the air
horn and maybe the main body are warped,
with the warp certain channels dont work right.
Buy the thick gasket and if that doesnt works it must be sent to cliff
to be de-warped. Or second idea your power valve spring
is not the right one for your application and is closing
and opening sporadically.
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I disagree. I've had one unwarped by Cliff and also dressed the air horn on another myself so it fit the bowl perfectly. Thicker gasket made no difference. In fact I think it makes the wicking problem worse because of additional gasket volume. The power valve spring was not cycling. The test engine was a bone stock Chevy 350.
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So then you have fixed your problem, right?
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No. Both Q jets I have still wick gas through the edge of the air horn gasket after sitting a while after engine is shut off. My point is, I don't think there is a fix. I'm guessing it's caused by either today's gas or gasket material, or combo of both.
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Its not the gas, because the pressure of the carbs
air horn against the main body does seal it with no wicking.
You also mentioned that it wicks after you turn the car off,
if it were the gas it would also wick while running. You say
it wicks from the edge, sounds like your gas is boiling on heat soak
& the level is rising to soak the gasket. It should not
leak gas Cliffs carbs dont wick gas and there the same type of carb,
so it must be fixable.
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Have you verified your float level? I've seen this happen when everything else is good, but the float is a bit high. Running the float to high is not a good thing...might be worth checking.
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I've seen the wet gaskets alot too. I did a motor home carb. '87 chevy G-30. Last of the q-jets. Anyway, It had what appeared to be a plastic airhorn gasket. It also had the additional airhorn screws. The replacement gasket was plastic too. No wetness. GM must have addressed this problem toward the end of the q-jet run. I would assume the wet gaskets are normal to a point. You have a paper gasket over a bowl of fuel. Hit a bump, Hit the brakes, It's gonna splash.
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the fuel level should not be up that high to wick into the gasket and it does not splash sitting still.
sounds like fuel level getting up too high maybe due to hot temps boiling the gas after shutoff sometimes a carb insulator gasket helps this. I had a problem with fuel boiling on an edelbrock after i ran it and shut it off I had hard starting and the insulator cured this problem.