High idle after a run - sometimes
 

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)




:rolleyes: :rolleyes:

That's the engine trying to catch its breath after a hard run :p :D

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.

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).:mad:

Ignition?
 

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?

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

Vacuum Leak? :confused:

Mine does it too. I've wondered about the float level or the transfer circuits leaking additional fuel into the carb bores somehow.

Hey FindMe, got any thoughts on this one?

Scott

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?

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)

check the butterflys on the secondaries. they are probably sticking. mine does the same thing from time to time but lift the hatch and push em shut and its back to idle. it is random and seems to happen to me if the boat has not run for a week or so.