Odd noise

[Charles P]

i've got an unpleasant rattle on a trailing throttle in a freshly rebuilt blown engine.
The video is here (a bit rubbish but you should be able to hear the noise)
https://www.dropbox.com/s/sihsfa02cua9zc...4.MP4?dl=0

Things that I've checked:
The crank isn't moving 
Depressing the clutch doesn't alter it
The blower isn't making the noise - uncoupling the blower doesn't stop the noise
The cam isn't moving (checked by removing the dynamo and looking in)
It's not the starter motor touching the flywheel 

Suggestions welcome please before I start stripping it again.

Thanks

Charles

[Alan]

Charles, what crank, con rods and camshaft have you?

[Ruairidh Dunford]

Collet seated incorrectly.

Soft cam follower.

[Alan]

It could be a tappet problem as R describes, but it could also be something hitting the crank as it turns. The last 2 or 3 inch and a half cranks I’ve installed, I’ve struggled to get clearance between the con rod and crankcase. The other thing I’ve noted is with new ( not original re ground) high lift cams, the cam lobe can hit the con rod every other rev. It sounds just like that.

[Ruairidh Dunford]

Eek!

[Ian Williams]

I had a similar noise once when using a cam that had been reground with a narrow wheel, the followers were riding on the shoulders left by the grinding giving false tappet readings. I am far more careful these days and always ensure the followers are sufficiently narrowed!

[Charles P]

Thanks chaps. All good ideas.

Phoenix crank, Barlow rods and a Bonewell blown cam & followers - which isn't particularly high lift. The crank is held at the rear main with floating front bearings. The Barlow rods are a tighter fit than Phoenix and number 3 rod has more thrust face float than the others from the balancing. Not excessive but it does warrant a look again. However surely a rod moving around axially on the pin wouldn't be limited to a trailing throttle?

I had the engine out last week and dived in from the sump. I couldn't see any evidence of rods hitting lobes with my mirror on a stick. I turned it over by hand to try and replicate a collision and drew a blank. Doesn't mean that it isn't it hitting.
The "off throttle " nature of the rattle makes me suspect anything driven by helical gears which leads to cam moving.
I too, have suspected a cam follower problem but why would the noise manifest itself on the trailing throttle, unless the cam is moving (which I didn't think that it was). The tappets were in the previous engine before I destroyed it so shouldn't be soft.

Keep it coming

Charles

[Alan]

To me, it sounds half engine speed, which is why I suspect it’s camshaftey. I’d be surprised if the amount of rod clearance you have is causing the problem, but if it is it should be rectifiable by relieving the rod or edge of the cam lobe. I’ll measure some cams tomorrow to get an idea of what the differences are.

[Charles P]

To me, it sounds half engine speed, which is why I suspect it’s camshaftey.  I’d be surprised if the amount of rod clearance you have is causing the problem, but if it is it should be rectifiable by relieving the rod or edge of the cam lobe. I’ll measure some cams tomorrow to get an idea of what the differences are.

Thanks Alan.

First thing tomorrow I'll whip the manifolds and valve cover off (not so easy on a blown engine).
If Ian's theory is sound my very carefully set valve clearances will be up the spout as a result of the shoulder issue. 
Following your comments about clearances with Barlow rods I did spend a vast amount of time aligning the crank, rods and cam to try and avoid a collision problem. It's possible that in aligning it that way I've overlooked the follower clearances.

Charles

[Ian Williams]

Pauls cams are ground quite narrow, it was one of his with which I experienced the problem, but only on one or two lobes. I should add that this is in no way a criticism of PB's cams, they are in my experience extremely good, Paul knows his stuff and the grinds he produces do exactly what he say's they will do.