High idle after a run - sometimes
#1
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Heres the deal.
Coming down off plane from a run, back to idle speeds (and this is sometimes), when I pull back into neutral the motor could crank up to about 1500, with no prop load on it now. Its random, and, even if i took the idle screw all the way out it still keeps going.
When I shut off after 15min or so, its fine. (powervalve?).
Carbs are Holley 9029 (716cfm marine, vac secondaries)
Coming down off plane from a run, back to idle speeds (and this is sometimes), when I pull back into neutral the motor could crank up to about 1500, with no prop load on it now. Its random, and, even if i took the idle screw all the way out it still keeps going.
When I shut off after 15min or so, its fine. (powervalve?).
Carbs are Holley 9029 (716cfm marine, vac secondaries)
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#2
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That's the engine trying to catch its breath after a hard run
Seriously speaking, I have seen the same thing happen on several boat engines after coming off plane. Mine does it too, but not 1500rpm, perhaps more like 100-150rpm more. My engines idle right around 750-800rpm in neutral and see them at right around 900rpm at times after coming off plane. I will sometimes put one engine in neutral as I am idling in and will notice it doing that on occasion. It seems to be a common thing, but I don't really know what causes it.
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Seriously speaking, I have seen the same thing happen on several boat engines after coming off plane. Mine does it too, but not 1500rpm, perhaps more like 100-150rpm more. My engines idle right around 750-800rpm in neutral and see them at right around 900rpm at times after coming off plane. I will sometimes put one engine in neutral as I am idling in and will notice it doing that on occasion. It seems to be a common thing, but I don't really know what causes it.
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I have the same issue with my 29' Powerquest (Fuel injected 7.4L). I have had it in the shop twice and its back again. The Starboard engine will stay at 1000 RPM in gear with the throttles back. If I shift it into neutral it will spike to 1600 and slowly after 30 sec. it will come back down to 700 (idle). I will let you know what the mechanic says when I get it back (if it fixes the problem).
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#4
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What kind of Ignition System are you running? If you have mechanical advance, it may not be returning your advance back to your idle setting? Maybe a broken spring? Just a thought?
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Panther. These are the original Accel DualPoints with a MSD Blaster2.
Weird thing is, with the primaries closed (and I mean disconnect the cable and idle screw out, all-the-way closed), the engine could still be at 1500. Then I could wait 10minutes and its fine. And it is totally random.
Bob
Weird thing is, with the primaries closed (and I mean disconnect the cable and idle screw out, all-the-way closed), the engine could still be at 1500. Then I could wait 10minutes and its fine. And it is totally random.
Bob
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If the butterflies are shut, you are pulling air into the engine from another place. Maybe as paradigm shift said, a vacuum leak may be your culprit. You said Carbs are Holley 9029's, is this motor blown or tunnel ram?
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I would vote for the advance weights. Easiest way to find out is put a timing light on when it's doing it.(They may drop back before you can take apart the dist to look)