Clunking noise when engine idles very low (~500RPM)

[loco]

Hi all,
Our AQ205A blew a cylinder head gasket. We had poor compression in a few of our cylinders, so we rebuilt the top end: skimmed the heads, put in new inlet valves, and had it all ground to fit nicely.

We got the engine back together and set the timing up correctly, and the engine was singing. However, the engine was idling a little higher than it did previously - we figured it was running more efficiently on the old idle setting, so wound it out a bit till it was down to ~750 rpm. We tested the timing at 750 and 2,500rpm and appears pretty much spot on.

We took the boat out and the engine was singing - sounded perfect. However, when we brought her back into harbor and were crawling in, the idle rpm had dropped to ~500rpm, and it started to make a knocking sound and eventually stalled. Starting it up again, if we allowed the RPM to drop to 500, the noise would come back, but as soon as we increased the RPM it disappeared, and sounded perfect again.

We figured it was just under-ignition through too little fuel, or the timing wouldn't work properly at that rpm. Do you think just increasing the idle to the proper RPM is our only issue here, or could we have some other issue?

Many thanks in advance!

 

[Adirondack]

Re: Clunking noise when engine idles very low (~500RPM)

This may sound a bit out there, but I would pull the plugs just to be sure you weren't still sucking water from a bad intake. Your original post doesn't mention checking it while it was off. One other consideration is does it crank hard, or "thump" when your starting it? The time it takes to check will be worth the insurance. If it is it will cause a lot of damage.

 

[Philster]

Re: Clunking noise when engine idles very low (~500RPM)

Sounds like water in one or more cylinders. Usually has the biggest effects at idle or low RPM, where the exhaust stroke is more likely to suck the water in (if it's sitting in or near an exhaust port). There's where you'll hear the dreaded low RPM el clunko sound. When revs build, the exhaust cycle really won't suck in exhaust water... but it will do so at low RPM.

How does she start up? Is she cranking on the first turn of the key and sounding perfect and starting right up?

I say that clunking at low RPM is water interrupting the cycle of the engine, as she gets ingested on an exhaust stroke/cycle, and I'd worry about a leaky exhaust (letting water in exhaust passage via riser/manifold/etc).

 

[loco]

Re: Clunking noise when engine idles very low (~500RPM)

Ok, many thanks, that's interesting. We did rebuild the exhaust system though, putting in new gaskets all around, and torquing up to the correct levels so that surprises me. Maybe one of the plates that live in between the risers, sandwiched between two gaskets is the problem. Might need to clean them up a bit better or something.

Which reminds me - what is the deal with those risers? Should there be a metal plate between two gaskets, or should there simply be one gasket joining the exhaust and the riser? this could be our culprit.

you say we need to take the plug out and check for water, but how would I go about doing that, exactly? The plugs are underneath the exhaust, so if I take one out, then what do I do?

Thanks again for your advice.

 

[Philster]

Re: Clunking noise when engine idles very low (~500RPM)

Different riser/mani joints use different types of gaskets, so I can't speak tothe ones you have... but this is the leading suspect right now.

I'm not sure how you can get to the spark plugs, but they are worth a look. You could play the odds and just go right after the manis/risers.