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502 Carb Runs Fine Except Won't Turn Over 3500 RPM

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Old 10-12-2017, 08:22 AM
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Default 502 Carb Runs Fine Except Won't Turn Over 3500 RPM

31' scarab twin 502 Carb motors, motors have low hours, ran good previously with no changes to setup, one motor now won't turn over 3500 rpm under load but runs fine otherwise, will rev higher out of gear, seems almost like Carb secondaries aren't opening but they are, hard to tell what's happening because it's loud (dry exhaust) and other motor kind of compensates and it only does it on plane around 50 mph, has msd distributors with no boxes and locked timing, any ideas on where to start?
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Old 10-12-2017, 08:25 AM
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swap carb's
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Old 10-12-2017, 08:33 AM
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Check fuel pressure and than compression and leak down test.
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Old 10-12-2017, 08:47 AM
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Thanks for replies, it runs fine getting up on plane where it's under load but rpms are lower which makes me think no internal damage to motor
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Old 10-12-2017, 09:34 AM
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Ran good previously. How long ago was previously and how was it stored?
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Old 10-12-2017, 09:51 AM
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Previously was earlier this summer
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Old 10-12-2017, 09:56 AM
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I wouldn't rule out an ignition issue.
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Old 10-12-2017, 10:25 AM
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ck timing make sure it has not slipped
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Old 10-12-2017, 11:49 AM
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i did verify distributor was not loose but I will check with light
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Old 10-12-2017, 01:29 PM
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Sock on the bottom of pickup tube in the tank.
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