Pete,
You need the carb heat blocks to prevent any possible buildup of
ice in the carb area. Its "good" life insurance. Cheep at twice the
price.
Jim
N15JN
<peterdavis@ntlworld.com> writes:
> <peterdavis@ntlworld.com>
>
> I crave people's indulgence with this one.
>
> Last weekend I experienced a very slight vibration that lasted for a
> few
> minutes whilst flying. It was just audible and just noticeable
> through the
> control column. The frequency at 4800 rpm was about 2 or 3 beats per
> second
> and was very reminiscent of "interference" or "sympathetic"
> vibration that
> might occur if part of the 'plane was vibrating in sync. with the
> engine.
>
> I reduced the throttle to see if it would disappear, but the
> frequency just
> reduced with the reduced engine revs. I then opened up the throttle
> and
> experienced what I can only describe as something akin to going over
> "rumble
> strips" on a road in a car, after which the initial vibration
> disappeared.
>
> I have thought about this a lot and cannot come to any conclusion
> about what
> this could be. Has anyone else come across this, and if so could you
> share
> it with me? I really would like to avoid the "silent front end".
>
> My only, very amateur, thought, apart from the electrics going
> 'belly-up' or
> water in the fuel, is - with the under cowl temperature being uneven
> from
> side to side, is it possible for one carb only to get some slight
> icing
> which could produce the vibration which could then clear with
> opening up the
> throttle, with the ice being drawn into the cylinders, thereby
> producing the
> "rumble" effect due to misfiring?
>
> I have on two other occasions experienced this "rumble strip" effect
> and
> tended to put it down to air pockets or some such thing (ignorant
> idiot, I
> hear people say) but with the advent of an ANR headset :-) I was
> able to be
> more aware of the change in 'plane feel prior to the rumble on this
> occasion
>
> Any thoughts would be most gratefully received.
>
> Regards,
>
> Peter
>
> PS - 80hp 912 'classic' with cold air plenum
>
> :peterdavis@ntlworld.com
>
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