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High RPM

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Ken W View Drop Down
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    Posted: August-20-2019 at 9:21pm
I have a '97 196 with a chevy 350 5.7 EFI TBI. It has run flawlessly this whole summer. Tonight I took her out and she started right up but idled high around 2000 RPM. It came down to around 1500 after running for a few minutes. At around 25 MPH the RPM was 4200! I turned off the boat and let the kids swim and when I restarted it was the same issue. No weeds on the prop.
I did nothing to the boat as far as mods. I ran her 1 week ago and it was fine. Idle was around 700 RPM and at 30 MPH the RPM was roughly 3100. I read the flame arrestor can cause issues but it appears clean. New plugs/wires.....complete tune up last year. Any ideas would be greatly appreciated. Thanks!

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MrMcD View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: August-20-2019 at 10:18pm
I guess first, is your tach correct? 4,200 RPM is very noisy so I guess you would know if it really is turning that fast.
The only thing I can think of that would allow 4,200 RPM @ 25 MPH would be the prop or tranny slipping. Prop can slip if you sheared a keyway. Properly lapped in this would never happen.
Tranny can fail allowing slipage. Check your tranny fluid first, maybe it is just a little low.
Your new idle issue is odd to show up with all these other issues. The tranny or Prop would not affect the idle speed at all. The engine should idle the same even if you disconnected the transmission from the engine. Keep us posted on your finds.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote quinner Quote  Post ReplyReply Direct Link To This Post Posted: August-21-2019 at 10:30am
On the Excal the Shroud that covers the flame arrestor can interfere with your throttle cable if you also have Perfect Pass, Not certain the same can happen with the Apex.
To rule this out just remove the shroud and see if the issue goes away, also check your throttle cable path to ensure nothing is pushing against it or that it runs neatly along it's path.
My excal had this issue and I trimmed the shroud a bit and problem solved.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MourningWood Quote  Post ReplyReply Direct Link To This Post Posted: August-23-2019 at 12:04pm
Originally posted by Ken W Ken W wrote:

. At around 25 MPH the RPM was 4200! I turned off the boat and let the kids swim and when I restarted it was the same issue. No weeds on the prop.


I think if the boat was truly turning 4200 rpm while moving at 25 mph, you would definitely notice that. It should have been going @ 40-ish.

Doubt that could have anything to do with throttle cable, Seems it would have more to do with 'slippage' or a mechanical problem in the driveline, i.e. weeds, prop spun on shaft, coupler, or trans itself.
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Ken W View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken W Quote  Post ReplyReply Direct Link To This Post Posted: August-31-2019 at 9:31pm
Thanks to all for the input. I cleaned the connections on the tach and she's back to normal. The engine sound didn't appear to be running any different when it was reading high RPM.
Bad tach is all it was. Man I feel stupid!
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MrMcD View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: September-01-2019 at 2:22pm
Glad you got it fixed. Put one it down as a win.
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