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Old 09-17-2020, 07:19 PM   #1
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Carburetors! What do you know about them?

I have a 454 on my 86 Winnebago and I really want to understand this carburetor. https://images.app.goo.gl/9JPGFMKeFZR2eL438 It looks identical to this. My question is with a mechanical fuel pump is it suppose to squirt gas every time I push the pedal even with out starting it or will I eventually dry out the fuel pump and result in no gas squirting into the carb.
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Old 09-17-2020, 07:24 PM   #2
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Mechanical fuel pump ONLY pumps when engine is running

Example of mechanical fuel pump ---cut way view
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Old 09-17-2020, 07:31 PM   #3
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Originally Posted by Chieftain27 View Post
I have a 454 on my 86 Winnebago and I really want to understand this carburetor. https://images.app.goo.gl/9JPGFMKeFZR2eL438 It looks identical to this. My question is with a mechanical fuel pump is it suppose to squirt gas every time I push the pedal even with out starting it or will I eventually dry out the fuel pump and result in no gas squirting into the carb.



Yes your carburetor will pump gas into the manifold every time you step on the pedal when the vehicle is not running, it pumps the gas out of the float bowl, once your float bowl gets low or empty it will stop until you turn the engine over and the fuel pump pumps more gas into the float bowl. The accelerator pump on the carb is what does this.
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Old 09-17-2020, 07:33 PM   #4
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Yes, you will see gas when you press the pedal. Gas will squirt as lkong as fuel is in carb. when it gets low in carb, it wont. When you start the engine the pump will refill bowl in carb.
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Old 09-17-2020, 07:39 PM   #5
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I believe you mean the accelerator pump on the carburetor, correct? If so, the carb has a fuel bowl that is filled with gasoline by the fuel pump. When the engine is running, fuel is continually pumped into the carb bowl to keep it filled to the proper level. There is a float and needle valve at the carb inlet that shuts off fuel into the carb even if the pump is still active.

Engine off, the carb accelerator pump will still shoot fuel down the carb throat if it is repeatedly depressed. Eventually, the carb bowl will be empty until the fuel pump is restarted and refills the bowl.
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Old 09-17-2020, 07:43 PM   #6
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Wow, I used to rebuild Rochester four-barrel carburetors back in the '80s. I could buy parts and replacement jets at any good auto parts store.

This link may be of value: https://www.gmcmi.com/wp-content/upl...ice-Manual.pdf

From the responses above, so have other people.

HTH,

Ray
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Old 09-17-2020, 07:53 PM   #7
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It takes only a few squirts to empty that Quad carbs fuel bowl

The fuel chamber is relatively small in volume to reduce
fuel evaporation during hot engine "shutdown".

So w/o engine running/cranking just a few squirts available
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Old 09-18-2020, 12:16 PM   #8
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Quote:
Originally Posted by Chieftain27 View Post
I have a 454 on my 86 Winnebago and I really want to understand this carburetor. https://images.app.goo.gl/9JPGFMKeFZR2eL438 It looks identical to this. My question is with a mechanical fuel pump is it suppose to squirt gas every time I push the pedal even with out starting it or will I eventually dry out the fuel pump and result in no gas squirting into the carb.
Gasoline has mass, and therefore it has inertia. When the throttle opens suddenly, it takes time for the gas to accelerate through the internal passages in the carb and the metering jets into the venturi to provide enough fuel to match the airflow. This leans the mixture. The accelerator pump squirts gas directly into the venturi to make up for this shortfall. It's actuated by downward pedal motion. The farther you press the pedal down, the more the throttle plates open, and the more gas the accelerator pump delivers, up to the point where the pedal is on the floor or you stop pushing it farther toward the floor.

The accelerator pump is a piston-based pump with check valves. Because it's mechanical. it relies only on fuel that's in the float bowl, as others have posted.

A secondary function is to provide some priming for cold starts. If you overdo it, though, the engine gets enough gas to wet the spark plugs, usually called "flooding" the engine. Your owner's manual should have cold start instructions.
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Old 09-18-2020, 03:12 PM   #9
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The accelerator pump on the carburetor will squirt gas into the engine when the pedal is pressed even with the engine not running if there is gas in the float bowl.

Your engine may have a mechanical fuel pump mounted on it to supply gas from the gas tank to fill the float bowls in the carburetor .

You could have an electric fuel pump in the tank or along the frame rails to supply gas to the carburetor , or a combination of electric and mechanical pumps.
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Old 09-18-2020, 03:49 PM   #10
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The accelerator pump on the carburetor will squirt gas into the engine when the pedal is pressed even with the engine not running if there is gas in the float bowl.
Right. Its purpose is to immediately make fuel available if needed rather than waiting on the fuel pump to catch up so there's no lag between stepping on the gas and moving.

Wow, that memory really came out of the cobwebs.

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Old 09-29-2020, 05:34 AM   #11
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Wow, I used to rebuild Rochester four-barrel carburetors back in the '80s. I could buy parts and replacement jets at any good auto parts store.

This link may be of value: https://www.gmcmi.com/wp-content/upl...ice-Manual.pdf

From the responses above, so have other people.

HTH,

Ray
Iím trying to locate my air fuel mixture screws and cannot locate them. Ive adjusted the idle and choke but canít find the air fuel screws.
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Old 09-29-2020, 05:56 AM   #12
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Air fuel mixture screws may be sealed behind round lead plugs. Look for them.

You can turn a sheet metal screw into the lead and then pull the plugs out.
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Old 09-29-2020, 07:24 AM   #13
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Air fuel mixture screws may be sealed behind round lead plugs. Look for them.

You can turn a sheet metal screw into the lead and then pull the plugs out.

Most of the Q-Jet carbs which had sealed mixture screws had hardened steel plugs blocking access to the screws. (Tamper proof) (We used to call them "God's Metal") I still have the tools including a small hole saw to cut the base of the carb to get the plugs out. I believe they were made by a company called "Tomco".
With the base turned over you will see two dimples in the base next to where the screws are located. Using the dimples as a guide you can cut into the base with a hacksaw and knock a chunk of the base out to expose the idle mixture screws. Also some of these screws on later models had a head on them which required a special socket to turn the screws.
If you do gain access to the screws I highly recommend you turn the screws IN while counting the number of turns and record it when the screws are seated. Then remove the screws if you are going to clean the carb.
When you reassemble the carb turn the screws in until they are seated then back them out the number of turns you previously recorded. Fine tuning for best idle quality can be adjusted from there. Usually you will be backing the screws out some to richen the air / fuel ratio. Most Q-Jets were adjusted lean from the factory to meet emission standards. Back in the day we often used a tachometer and a vacuum gauge to adjust the Q-Jet idle screws.
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Old 09-29-2020, 09:19 AM   #14
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The way we adjusted the old Quadrajets was to take them off and put a Holley in their place.

They have retro fuel injection now.

Ethanol fuel doesn’t play well with carburetors. Especially if it sits for any time between use.
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