<VV> Engine Knock only at low RPMS

[Tony]

At 0954 05/05/2004 -0700, Brian Goldin wrote:
>I was wondering this myself but I've always thought that broken cranks are
>unlikely.  

Usually... unless you help it along, like running the engine without a
harmonic
balancer and keeping the rpm in the mid-range areas...  

>Once you get above the low RPM knock she revs up nicely and has
>plenty of power.  Luckily the knock started when I was close to home so I
>didn't dive it much after it started.  

There's also the possibility that you may have a flywheel end main bearing
wallowed out, which can sometimes cause an odd intermittent knock, usually
more
noticeable at low rpm and disappears when the engine is buzzed up.   Early on
in Vair R&D, engineers found such a knock that would manifest itself on some
engines and no bearings would be worn or defective in any way and the engines
were up to spec, yet at certain low rpms the engines would demonstrate an
intermittent bottom end knock.   Playing around with different combinations of
main bearings (as I recall) pretty much corrected this obscure occasional
knock
noise which evidently was never satisfactorily explained or determined.    

In your case, the sudden appearance of the knock indicates some other source,
I'd bet.   


>Another interesting note might be that it threw the fan belt at pretty much
>the same time that the knock started.  This rarely happens and there might
>be a correlation.  

Maybe...  look around to see if perhaps the flying fan belt might have smacked
something and knocked it askew or loose, maybe rattling against a shroud part
or something, anything, and possibly making a noise...?    



>Another point to note is that I don't have a harmonic
>balancer.

Uh oh...   if your engine is a 145" version it's likely not a problem  but if
it's a 164"... that's another matter, especially if it's a manual gearbox
car.   


>Is there a way to test for a broken crank without splitting the case?


Yes.   Just remove the top shroud and then the crankcase top cover.
You'll be
able to see the crank and rods etc.  Usually when a Vair crank breaks, it's
diagonally across a throw and the crack should be visible, especially if the
engine has been run a while.   A cracked crank can either knock a little bit,
or sound as if the engine is coming apart, and yes the crank can be broken
into
two separate pieces and still propel the car normally as if nothing was
wrong... except for the noise.    

Here's hoping it's a loose piece of shroud and not a broken crank.  


tony..