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  #1   IP: 69.255.8.44
Old 07-06-2009, 05:50 PM
uberbigun
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Engine runs fine, then dies after throttling back to idle

My 1976 Atomic 4 starts fine, and without a choke in the summer. It runs fine, throttles normal at low RPM getting to the fuel dock and pump out station, works just fine at high RPM getting out of the harbor. When I throttle back to idle after being under 3/4 load for a while the engine dies. It's hard to start the engine afterward. It needs at least half choke and a lot of throttle to get started and will not run under low RPM without leaving the choke on. This is a newly rebuilt carburetor with about 10 hours on it. I have an electric fuel pump, polishing filter, and Racor main filter. I have recently changed the Racor filter. It was dirty. The tank needs to be cleaned thoroughly very soon.

I am assuming that the issue is a dirty carburetor, and am planning on cleaning it out and replacing the polishing filter. Can anyone confirm my diagnosis?

Scott Carpenter
Kaze Mizu - P30
uberbigun@gmail.com
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  #2   IP: 64.12.117.76
Old 07-07-2009, 10:54 AM
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gary gerber gary gerber is offline
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similar problem

Once I start the engine w/ choke and she warms up she runs fine with part throttle ( no tach ) however when I throttle it back to idle it dies. If I adjust throttle stop setting I can keep it running smoothly, but at what seems a high idle speed ( I am going to install a tach...after 30 years ! ). Carb is clean and all fuel system components have been removed and cleaned before remounting.

I have adjusted the idle screw both in and out but no change.
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  #3   IP: 138.162.0.43
Old 07-07-2009, 11:32 AM
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vabiker23518 vabiker23518 is offline
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Same Story

My Columbia 8.7 has the same symptoms....I put in a new PVC system last year, so I need to check the manifold/carb bolts are tight. I've adjusted the slow speed adjustment with no luck either.
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  #4   IP: 12.47.208.34
Old 07-07-2009, 12:39 PM
tenders tenders is offline
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Good behavior at start, but bad behavior when warm that doesn't respond to carb adjustment, suggests to me a failing coil (or, maybe, condenser).
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  #5   IP: 63.239.69.1
Old 07-07-2009, 01:38 PM
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Quote:
Originally Posted by uberbigun View Post
My 1976 Atomic 4 starts fine, and without a choke in the summer. It runs fine, throttles normal at low RPM getting to the fuel dock and pump out station, works just fine at high RPM getting out of the harbor. When I throttle back to idle after being under 3/4 load for a while the engine dies. It's hard to start the engine afterward. It needs at least half choke and a lot of throttle to get started and will not run under low RPM without leaving the choke on. This is a newly rebuilt carburetor with about 10 hours on it. I have an electric fuel pump, polishing filter, and Racor main filter. I have recently changed the Racor filter. It was dirty. The tank needs to be cleaned thoroughly very soon.

I am assuming that the issue is a dirty carburetor, and am planning on cleaning it out and replacing the polishing filter. Can anyone confirm my diagnosis?

Scott Carpenter
Kaze Mizu - P30
uberbigun@gmail.com
It seems to me that if you can start the engine without the choke, then you are running a very rich condition to start with -- closing the choke riches up the mixture by cutting the air flow when the engine is cold.

Could it be that once the engine gets warm your mix is too rich, and idling back floods it out? That would also account for the difficulty restarting and the need for a partial choke at that point.

Have you adjusted your idle mixture yet? Maybe a half-turn counterclockwise, to let more air in and lean out the mixture might help.

The best procedure would be to get the engine warmed up, idle down in neutral as low as you can go without stalling, then give the idle mixture screw a half-turn counterclockwise. If the rpms come up, your mixture is too rich. If they go down you're too lean. If the screw falls out -- well, you were going to take the carb off anyway, right?
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  #6   IP: 24.107.127.82
Old 07-07-2009, 05:50 PM
jetmech jetmech is offline
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I had this same scenario recently. I knew my fuel tank was grungy, so I added a racor primary filter, then there's the screen in the mechanical pump, and I also added a in-line filter just before the carb. In spite of all these filters, my carb also clogged in about 15-20 hrs of operation.

Your idle jet is clogged, that's why it dies when you throttle back. Above idle, the main jet takes over.

Since your carb is almost new, all you need to do is order the 2 gaskets (carb flange and carb) split the carb and thoroughly spray out all the passages with carb cleaner.
Also take a small piece of wire and make sure the 2 small holes by the butterfly plate are clear. These are your idle ports and they must be clear or it won't idle.
After you get her back on, I would recommend rigging up a remote outboard tank to use temporarily, until you can clean your tank. The sediment that I cleaned out of my carb was extremely fine and the filters obviously were not picking it up.

I'm using an outboard tank on mine right now too. Unfortunately, most tanks did not seem to be built with an inspection port. I'm gonna remove my tank from the boat, cut an inspection port in the top, then make a plate to cover that with a cork gasket.
The new style inboard mounting plastic fuel tanks are not terrible expensive either. So if mine has corrosion on the inside, I will install a plastic one.

Good luck!
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  #7   IP: 71.252.20.96
Old 07-07-2009, 07:54 PM
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I just replaced my OEM tank on my '77 C-30 with a plastic Moeller. It was fairly cheap, all things considered! It is translucent for sure, but not clear enough to 'see' bad gas.
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  #8   IP: 69.255.8.44
Old 07-22-2009, 09:24 PM
uberbigun
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Thanks, everyone

I just had the tank scrubbed by Petrochem and replaced the filters and cleaned the carb. I have a new issue now that I started a new thread on based on the engine dying when I throttle up now.


http://www.moyermarine.com/forums/sh...5913#post15913
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