David Carter wrote:

Finn Lassen wrote:

Ok, I'm back to jetting my carbs and final engine runs before taking the
plane to the airport for assembly.

At 1000 - 1200 RPM I am getting 20 in Hg MAP. Is that normal or should I
look for air leaks (false air) before drilling up the idle (slow) and
main jets?

The engine runs rough if I don't apply near full choke and starts
missing at 2000 RPM with prop load (higher RPM w/o load). This with only
one of the three carbs - the center carb supplying both rotors.

Finn

 Finn Lassen wrote:

I have three Mikuni carbs from a '79 Yamaha XS1100 bike. The manifold 1/2" of
the original manifold with three 1.75" tubes welded onto it. Very close to
Teracy Crooks original set-up.
<snip>

It's easier to jet one carb at a time (or at least the center carb and then
the outer two carbs).
<snip>
My main point was that if my MAP is way off it could indicate some air leaks,
which I would have to fix before starting to drill up the jets.

Finn

Finn,

You are using one carb at a time?  Meaning the other two are removed and
the big air sucking holes are covered and sealed so air is only coming
in through the one carb you are jetting?

Or, are you running the engine with all 3 carbs on and contributing, and
only changing jets on one at a time?

If the former:  How could jetting one carb "for smooth operation" with
the other two removed turn out to be the correct mixture when you put
the other carbs back on and they make their (unbalanced) contribution to
total fuel/air mixture and volume to the rotors?

If the latter:  A real "trial and error" method.  Make 1 small change at
a time, take lots of data points (egt's, manifold pressure at set rpm
data points) & subjective evals of smoothness on engine operation from
low to high rpm, then make another small change.  Did you start off with
jets of same size as Tracy's, so as to be "in the ball park"?

Is it correct that the better the engine is running, the lower the
manifold pressure will be for a given rpm?  If so, you will be looking
for decreasing MAP as you get closer to optimum.

David Carter

 
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