Weird R16 tapping noise

[fish]

Hey y'all--

So I've been chasing this odd tapping noise around for a while and figured I'd ask if anybody else encountered this. Motor is an R16 Rebello stroker (1750) with about 7500 miles since the rebuild. Stock carbs, top end, cam, etc.

It's a pretty loud tapping noise that's very localized within the RPM range, right between 1700 and 1800. Doesn't make the noise above or below that, and doesn't taper off when you get outside that range, it just stops. Worse when it's cold, but never goes away entirely. First thought due to the extreme specificity of the RPM range was that it was something loose in the engine bay that was resonating right at that frequency. Didn't find anything. Second guess was badly adjusted valve(s), so I went through the usual adjustment exercise a few times with no luck. Top guesses at the moment are sticky valve/lifter or a cam chain/tensioner issue, but I haven't dug in on those yet.

Video (and audio) for reference: https://youtu.be/ijDly1Hlf8g

Any thoughts?

[Bwk2000]

Sooo … not valves/lifters you say? Strange If it hadn’t just been rebuilt, I would have guessed that maybe you flattened a cam lobe.

[Bwk2000]

Sorry Mods … Something weird was going on with my computer there

[redroadster]

It's not louder as revs go up ?
You've checked the valve clearance correctly ? A go no go style guage is best
Checked the oil for metal shavings
Pulling 2 plug wire at a time ,doesn't go away on one cyl.

[fish]

It's not louder as revs go up ?
You've checked the valve clearance correctly ? A go no go style guage is best
Checked the oil for metal shavings
Pulling 2 plug wire at a time ,doesn't go away on one cyl.

Nope, it's completely non-existant outside of that little 100 RPM rev range between 1700-1800. It's definitely good on valve clearance, oil is clean.

What condition is the "pull two plug wires" thing testing for?

I'm kind of leaning toward timing chain at the moment as that narrow band still suggests resonance to me and if the chain is loose and/or the tensioner isn't doing its thing, that might be a resonant spot for the chain. (I'm also thinking Rebello may have just re-used the old tensioner when they did the motor.) It really sounds like a metal-on-metal thing though, and I don't remember if the chain can plausibly get close enough to anything metal to give it a resonant whack...

[redroadster]

Supposed to be 1,a plug wire ,also my 1s are often Q's ,.......for rod bearing clearances or piston slap on plugs wire pull the knock will go awsy if not so extreme... may need to check the rocket shafts for wear.
can go .001 tighter How are you adjusting the valves ? Didn't set it on the clearance ramp ?

[Gregs672000]

Are you sure it's coming from the valve train? Check your fan.

[fish]

So I found my good stethoscope and did a little more investigating, and the noise was 1000% louder at the timing cover. Replaced the chain tensioner, and voila, no more noise. Everything looked OK in there so it wasn't really obvious where the noise was coming from, but the tensioner did fix it.

(And FWIW, apparently Rebello doesn't include new timing components as part of a comprehensive rebuild (or at least they didn't on mine), so if you're getting that work done, ask for that specifically I suppose...)

[Gregs672000]

Well done, congrats and thanks for closing out the thread!