Sounds like BB's bouncing around in rocker compartment over 3500 rpm

[Gilby]

I read and re-read the "MisFire above 4000RPM after 20 min" thread and it sounds similar to the problem I have but not quite the same. I have a 91 four winns 170 freedom with a 3 liter OMC sterndrive. The engine is basically new and runs extremely well except for one thing - when it's opened up to 3500 rpm or more, it sounds like there are 2 BB's bouncing around the rocker compartment - except I'm not sure where the sound is really coming from. There is no loss of power and it's not really during acceleration, just when it hits a certain rpm and then it's constant. It's NOT an even tick-tick-tick, but really like something is loose and flying around randomly. When I take the power down, the sound goes away. At low rpm, it runs perfectly smooth with no such sound. I replaced the head (to solve a different problem) but that didn't affect this issue at all so I really don't think it's anything actually loose or in the head and the pistons were clean. A few mechanics have looked and listened but couldn't figure it out. Last fall I saw a post somewhere that discussed something similar and the cause was the starter. In fact, it does kind of sound like the starter splines bumping against the flywheel or something and bouncing back. Has anyone ever heard of this? Any ideas? Based on the "Misfire" thread, I need to check timing/advance, octane, coil, exhaust flapper and plugs but the big difference is this doesn't really sound like a misfire/detonation to me, it's not rythmic in any way and there is no loss of power when it occurs. I'd really appreciate any help or advice.

[Frosty]

Google anti knock device.

You can read about it.

[Bergalia]

Slack timing chain ?

[StianM]

If a mecanic can't figure it out standing there listening I think it will be hard for annyone else just reading about it.

[TerryKing]

Originally Posted by Gilby

(snip)... when it's opened up to 3500 rpm or more, it sounds like there are 2 BB's bouncing around the rocker compartment - except I'm not sure where the sound is really coming from.

This is easier with a boat than a car... You need to identify just where the noise is coming from... Get a cheap stethoscope, or a 2 foot piece of garden hose.

Someone else drives, and you move your 'probe' to find out where it is exactly..

Put your probe (other end to ear) on the head outside the valve cover, too.

Could be a bad valve lifter / bent pushrod etc, and is flakey.

Or high-speed ping (Try some high octane gas for a quick test)..

Or???

[Gilby]

Thank you all very much for your replies - problem solved! There is a small wire at the base of the distributor and a tiny section was frayed through to bare wire. Taped it up and the engine runs perfectly.

[StianM]

Originally Posted by Gilby

Thank you all very much for your replies - problem solved! There is a small wire at the base of the distributor and a tiny section was frayed through to bare wire. Taped it up and the engine runs perfectly.

Your mecanic did not find out? Well done