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Old 12-07-2016, 01:43 AM   #1
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Default wot rpm fluctuation

Hey all ,
I recently purchased, a well maintained, 1997 sc2300 under 500 hrs.
260 5.7 weber carb 9781s
Runs great, except under wot rpm fluctuates from 4800 down to aprox 4000.
Sometimes rpm will recover slightly, to say 4400.
At initial throttle increase to wot max rpm 4800 can be achieved then after 30 seconds or so drops back to 4000.
All mechanical linkages appear to be in proper working order.
Although i have never really thoroughly inspected the choke.
any input would be greatly appreciated.
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Old 12-07-2016, 01:59 AM   #2
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Sounds like a fuel filter to me. Change the fuel filter and see if it improves.
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Old 12-07-2016, 12:55 PM   #3
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Can you hear and feel that the engine is changing RPM, or is it just the gage telling you it's changing?
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Old 12-07-2016, 02:40 PM   #4
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I have changed the fuel filter, and also remove the fuel tank check valve for inspection.
The rpm change is audibly and physically noticeable, and my max mph drops from 45-41 measured using gps.
Usually this condition occurs only at wot but on occasion it has happened just under way 6-7 mph, rpm dropping. Rpm estimated as tach is not a precise instrument.
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Old 12-07-2016, 03:07 PM   #5
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Could be the coil as well.
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Old 12-08-2016, 12:36 AM   #6
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I wonder if that's your Thunderbolt V ignition acting up and retarding the timing... it's supposed to do that if it senses knock.
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Old 12-09-2016, 01:13 AM   #7
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Thanks for the info, definitively a couple of things to look into.
I wasn't even thinking ignition. I'm not a gas wiz, a bit more base engine/ diesel savvy.
I have a few months for a little research, since she's on blocks in the back yard until May.
Trying to be proactive so as a minor problem doesn't become large problem 5 miles off the coast of Maine.
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