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Old 06-22-2020, 12:22 PM   #1
1929
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Default Downdraft Carburetor question?

trying to figure out why, when I start the car after sitting in the garage after a week or maybe two, with out using the choke, the car will start up in 3 seconds. I go for a 30 minute ride or more, shut the car off, come back in 15 minutes or more, and the car takes much longer to start, why?
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Old 06-22-2020, 01:04 PM   #2
Bob C
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Default Re: Downdraft Carburetor question?

Possibly the gas in the carb boiling and flooding the engine.
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Old 06-22-2020, 06:56 PM   #3
Synchro909
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Default Re: Downdraft Carburetor question?

Mine is the other way around. After being left for a while, I have sit and wait a while till the fuel pump refills the carb after the fuel has evaporated while the car was sitting. If I come back to it within a day or two, it fires up immediately.
Your situation seems odd to me.
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Old 06-22-2020, 10:30 PM   #4
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Default Re: Downdraft Carburetor question?

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Possibly the gas in the carb boiling and flooding the engine.
The intake is never hot though.
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Old 06-22-2020, 11:08 PM   #5
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Default Re: Downdraft Carburetor question?

So it's gravity feed. Do you turn the fuel off?
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Old 06-23-2020, 12:16 AM   #6
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You should use a fuel pump,strombergs like 2psi.when warm or hot starting do not touch the throttle,just bump the starter and when its right it will fire in less than a full turn.Stromberg float level is critical @15/32"..if the power valve leaks it will cause a hot hard start condition,its located below the accelerator pump plunger,its opened when the throttle opens half way by the accel pump plunger to continue add fuel after the pump is out of stroke.
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Old 06-23-2020, 10:53 AM   #7
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Default Re: Downdraft Carburetor question?

Newer cars had a fuel return line so that when the engine was shut off, the fuel pressure in the line was bled off and returned to the tank. This prevented the condition you describe. The Ford 300 CID truck engines did that until Ford sent a bulletin telling owners to drill a very small hole in the fuel pump body that did the same thing. If you are using an electric pump, turn it off a few hundred feet before shutting the engine off. Don't forget to turn it back on after re-starting and heading out, it can be embarrassing. Don't ask how I know this!
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Old 06-23-2020, 11:50 AM   #8
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Newer cars had a fuel return line so that when the engine was shut off, the fuel pressure in the line was bled off and returned to the tank. This prevented the condition you describe. The Ford 300 CID truck engines did that until Ford sent a bulletin telling owners to drill a very small hole in the fuel pump body that did the same thing. If you are using an electric pump, turn it off a few hundred feet before shutting the engine off. Don't forget to turn it back on after re-starting and heading out, it can be embarrassing. Don't ask how I know this!
Jim, I stopped using the fuel pump a year ago, works fine with out it.
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Old 06-23-2020, 11:52 AM   #9
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So it's gravity feed. Do you turn the fuel off?
Shut the fuel off every time,if its going to sit for over an hour.
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Old 06-23-2020, 10:28 PM   #10
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Default Re: Downdraft Carburetor question?

Try shutting it off every time. You don't want fuel leaking into the engine, as what happens with a downdraught if the needle valve doesn't hold.

They don't call me "updraught" for nothing ...
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Old 06-24-2020, 07:53 AM   #11
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Item 9594 is the power valve,it can leak fuel into the intake...to check run the car and create the condition,shut down,remove the accelerator pump rod and open the throttle plates..look into the air horn and see if fuel is leaking.
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Old 06-24-2020, 11:06 AM   #12
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Default Re: Downdraft Carburetor question?

Shut the valve off about 100' before coming to a stop.
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Old 06-25-2020, 07:15 AM   #13
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Shut the valve off about 100' before coming to a stop.
Thanks, I will do that.
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Old 06-25-2020, 08:59 PM   #14
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Default Re: Downdraft Carburetor question?

Another thing I have found with the downdraught carb is that in cold conditions, the throat can ice up, especially if there is humidity about, like fog. The car still runs but quite poorly. Diagnosis took some doing but when I worked it out, I smacked my palm on my forehead. My solution has been to use a piece of exhaust pipe, slit it length wise and clamp it over the exhaust manifold and direct the warm air upwards and into the air cleaner. Problem solved!
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Old 07-02-2020, 09:30 PM   #15
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Default Re: Downdraft Carburetor question?

While on the issue of down draft carbs, can anyone tell me what is an ideal starting point for jet sizing is? Its a stock engine with a Lyon 6.5:1 compression head, down raft tube intake and a tube header.
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Old 07-03-2020, 11:47 AM   #16
Jim Brierley
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Default Re: Downdraft Carburetor question?

Always start with the stock jet sizes for the carb being installed. Carbs are stupid, the only thing they know is how much air is going thru them, and they add the appropriate amount of fuel to produce the correct air/fuel ratio.
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Old 07-03-2020, 12:55 PM   #17
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Default Re: Downdraft Carburetor question?

repro

Last edited by Benson; 07-14-2020 at 09:21 AM.
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Old 07-03-2020, 04:50 PM   #18
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Default Re: Downdraft Carburetor question?

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Always start with the stock jet sizes for the carb being installed. Carbs are stupid, the only thing they know is how much air is going thru them, and they add the appropriate amount of fuel to produce the correct air/fuel ratio.
Jim is right, stay with the stock jets, I had issues when I changed the stock jets 3 times on my Stromberg.
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Old 07-03-2020, 05:49 PM   #19
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Default Re: Downdraft Carburetor question?

Also there is a problem

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