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Old 04-28-2019, 07:22 PM   #1
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Default Carb Problem

Just put a re-built carb on my 2006 5.0, and I can't get the engine started.
The inside of the carb is wet with fuel. However, I don't see any fuel squirt out of the discharge holes when cycling the throttle repeatedly from full throttle to quarter throttle. Is this an indication that the accelerator pump is bad?
I also checked that the choke was closed before starting, and, that the idle mixture screw was out 1.5 turns.
I went back and forth on whether to shoot some starting fluid down the carb, but decided not to after seeing no fuel squirt on the throttle check.
Any recommendations and questions welcome.
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Old 04-28-2019, 10:18 PM   #2
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Yes it seems the accelerator pump is not working.you can try some starting fluid or a small amount of gas down the carb throat, just not too much.
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Old 04-28-2019, 11:58 PM   #3
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Thanks for the reply Mike. I'll use some starting fluid on it and try to get it running before deciding if the accelerator pump is not working.
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Old 04-29-2019, 01:13 AM   #4
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Remind me, this is a 2bbl; correct?
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Old 04-29-2019, 01:27 AM   #5
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Yes, two barrel
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Old 04-29-2019, 11:49 AM   #6
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Get Mercruiser manual # 24 from Documents of this site and go to section 5B which covers your carb and includes troubleshooting.
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Old 04-29-2019, 10:04 PM   #7
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Thanks Mike, always a good idea to read the service manual.
I've been using service manual #24 through this process. In section 5B there is an 8 point carburetor checklist, and my rebuilt carb passes 7 of the 8 checks. Fails the "does it spray fuel from the venturi discharge nozzles when the throttle lever is pumped?" test. I was hoping there was someone on the forum with some experience with this problem. I'm trying to determine if there is a problem with the carb and I need to return it.
I'm going to the boat on Wednesday and pour some gas in the carb to get it started, hopefully. Buying a rebuilt carb is always an adventure.
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Old 04-29-2019, 11:45 PM   #8
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Quote:
Originally Posted by jrsick View Post
Thanks Mike, always a good idea to read the service manual.
I've been using service manual #24 through this process. In section 5B there is an 8 point carburetor checklist, and my rebuilt carb passes 7 of the 8 checks. Fails the "does it spray fuel from the venturi discharge nozzles when the throttle lever is pumped?" test. I was hoping there was someone on the forum with some experience with this problem. I'm trying to determine if there is a problem with the carb and I need to return it.
I'm going to the boat on Wednesday and pour some gas in the carb to get it started, hopefully. Buying a rebuilt carb is always an adventure.
This is definitely a carb issue. Either the accelerator pump is not being compressed, linkage issue or a passage is blocked. Thought you rebuilt it, did not realize it was a remanfactured one.
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Old 04-30-2019, 04:44 AM   #9
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I would say return it as you paid for a working carb.
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Old 04-30-2019, 09:13 PM   #10
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Thanks Don and Mike. I'm going to the boat tomorrow with a bottle of gas to try and get it started. If no start, I'll remove the carb and take it back. I'll let you know what happens.
If you don't hear back from me, that means I used too much gas......
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Old 05-02-2019, 10:41 PM   #11
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Update, the carb was the problem. The needle inside of the seat, near the fuel intake, was not allowing fuel through. This was a new needle and seat installed with the rebuild. The mechanic figured it out by blowing into the fuel intake and almost no air got past the needle.
They gave me a second newly rebuilt carb.
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Old 05-03-2019, 01:36 AM   #12
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AND???? Does it run now?
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Old 05-03-2019, 02:00 AM   #13
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Thanks for the update, hopefully this one works fine and you can tune it probably.
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Old 05-11-2019, 09:37 PM   #14
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I got the second rebuilt carb on and gave it 8 start cycles with no luck. I didn't hear the engine kick or try to turn over at all.
This second rebuild passes the checks in the service manual. The only check I forgot to do was to check the idle mixture screw setting.
I could not hear the electrical fuel pump come on when the ignition was turned to "on". Is it possible that I could have a malfunctioning fuel pump but still have fuel in the carb? Fuel squirts out of the discharge holes in the venturi cluster when pumping the throttle.
While waiting on the carb rebuild, I checked the fuel and fuel pump by connecting a plastic hose to the fuel line, and leading this hose to a jar. I turned the key to "start" for a few seconds and the electrical fuel pump pushed a good amount of fuel into the jar. The pump did not pump any fuel when the key was turned to "on", only in "start". I thought "at least it's pumping fuel" but thought it odd that no fuel came out with key in "on" position.
I'll go back at it tomorrow. I'll also check for spark.
I'm focusing on the carb but don't want to ignore other potential causes. The engine started fine before I took off the original carb for rebuild; and that was with a malfunctioning fuel enrichment solenoid and throttle frozen closed. Stuck throttle was reason for carb rebuild in first place.
Your advice and troubleshooting steps are most welcome.
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Old 05-11-2019, 09:57 PM   #15
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You wont get any fuel with the key in the on position only in start, the engine has to start and run for the oil pressure switch to close and let the fuel pump to run and supply fuel.

good luck hope everything works ok.
Don
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Old 05-12-2019, 03:09 AM   #16
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Don is correct the fuel pump gets power from the starter soleniod when cranking and then once running an oil pressure switch provides power. This is a safety requirement for electric fuel pumps in case the engine stalls the pump looses power so the carb cannot be overfilled and spill fuel.
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Old 05-12-2019, 03:11 AM   #17
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Did you ever try pouring a small amount of fuel in the carb bores to see if it will run for a minute on that?
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Old 05-17-2019, 04:19 PM   #18
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thanks for the clarification on the fuel pump. I misread the sequence of operations in the service manual. I appreciate the info and know the fuel pump is not the problem.
I did try pouring a small amount of fuel down each carb barrel. I tried starting fluid also. The engine did not fire at all. I've got a newly rebuilt carb and possibly a spark issue.
Going down tomorrow and going through the "Troubleshooting Thunderbolt V Ignition" and "8 point carb checklist" from the service manual. I got my nifty HEI spark tester, power probe, and multimeter.
Have you ever heard of disconnecting the wiring harness from the tachometer as a troubleshooting step in a "crank but no start" condition? It sounds like a short in the tach wire (gray wire) will cause a no start.
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Old 05-17-2019, 08:05 PM   #19
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I understand how this can happen but never heard of an actual case. Gray wire is is the same trigger to the coil so if it’s shorted the coil cannot be pulsed to create spark.
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Old 05-19-2019, 02:19 AM   #20
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Any findings from your testing today?
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