Carb tuning and idle setting
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=50964
Printed Date: April-25-2024 at 3:55am
Topic: Carb tuning and idle setting
Posted By: uncle-buck
Subject: Carb tuning and idle setting
Date Posted: October-26-2022 at 9:54pm
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Was tinkering with my boat engine and carb today. When the engine idle was set at 600 rpms, the engine would start right up. When the idle was reset to 650 rpms, I had to give it throttle to get it to start. How do I get it to start with a simple turn of the key with idle set at 650 or 700 rpms? Turn the mixture screws leaner? ------------- 1990 Ski Nautique (original owner) PCM 351W with D.U.I. |
Replies:
Posted By: MechGaT
Date Posted: October-29-2022 at 6:55am
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If it starts when set to idle at 600 rpm, that’s because the butterflies are choking off enough air to give a rich mixture with the amount of gas that is pulled while cranking. Setting the idle higher obviously increases the amount of air that gets by the butterflies at idle. The engine needs a rich mixture to start, hence the reason for the choke plate. You definitely don’t want to change the mixture screws to make starting a simple turn of the key. Those should be set to make a warm engine run well, use a vacuum gauge to set them. My first recommendation would be to make sure the choke plate is closing as much as possible and take it as normal to bump the throttle once to start the engine and forget about it. If you want to tinker and don’t mind the possibility of doing something that most wouldn’t then it sounds like you could try to reduce the gap between the choke plate and the air horn to make the mixture a little richer. How do you do that without cutting off to much flow? Good question. No idea if it will have some unintended consequences. You don’t want to add something temporary that could fall off and get sucked into the engine. That’s why I would say just be happy your engine runs well, bump the throttle once when starting, and enjoy. ------------- '92 Sport Nautique |
Posted By: uncle-buck
Date Posted: October-29-2022 at 7:42am
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I should have clarified. This is with a warm engine so the choke plate is not a factor. ------------- 1990 Ski Nautique (original owner) PCM 351W with D.U.I. |
Posted By: KENO
Date Posted: October-29-2022 at 10:24am
Quit fiddling Buck 
But since you're fiddling, I'd try a little farther open on the mixture screws as a start.
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Posted By: Jonny Quest
Date Posted: October-29-2022 at 11:12am
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I’ve had good luck with starting at 1 1/4 to 1 1/2 open on idle mixture screws (from lightly seated). Start engine. Allow engine to attain operating temperatures. Set curb idle to 650 to 700 rpm. Then I hook up a vacuum gauge to a full manifold vacuum port. Then I adjust the idle mixture screws to achieve max vacuum on the gauge. At that point, I usually richen up the mixture 1/8 to 1/4 turn. Just the way I do it. It’s worked well for me. Your mileage may vary… JQ
------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
Posted By: uncle-buck
Date Posted: October-29-2022 at 3:20pm
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Many thanks! ------------- 1990 Ski Nautique (original owner) PCM 351W with D.U.I. |
Posted By: Tim D
Date Posted: October-30-2022 at 6:43pm
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Is the boat in the water or on the trailer? ------------- Tim D |
Posted By: uncle-buck
Date Posted: October-30-2022 at 9:36pm
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on the trailer ------------- 1990 Ski Nautique (original owner) PCM 351W with D.U.I. |
Posted By: Tim D
Date Posted: October-30-2022 at 10:04pm
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Do you have the hull positioned close as if sitting in the water? If the bow is high or low that could affect the float position. ------------- Tim D |
Posted By: uncle-buck
Date Posted: October-30-2022 at 10:11pm
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Yes. I adjust the trailer jack height. Thanks ------------- 1990 Ski Nautique (original owner) PCM 351W with D.U.I. |