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Finn Lassen wrote:

Paul wrote:

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?

The two outer ports are covered (piece of tyvek inside hose clamped over manifold
tube).

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?

This is per Tracy's advice and the logic is sound: air/fuel mixture coming out of any
carb (whether one or ten carbs) should always be ideal (about 1 fuel to 15 air I
think). With the throttle full open on the one carb you'll be getting the maximum
horsepower that one carb can supply. Adding one or two more carbs will simply supply
more correctly mixed fuel.

Did you start off with jets of same size as Tracy's, so as to be "in the ball

park"?

Yes, however the carbs are slightly different.

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.

This is an excellent question, which I hope the experts here can answer. Again, my main
question was what the expected MAP at near idle is.If I had a leak somewhere between
the carb and combustion camber, I would expect a lower MAP reading (less vacumn).
Trying to jet the carbs with an unhandled air leak of course would be futile. There are
several potential source of air leaks: the homemade manifold itself; between manifold
and engine; the holes where fuel injectors are normally mounted; the oil injection
ports. Of course a low compression engine would also show a lower idle  MAP?

Finn

 
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