>Now add a third carburettor. All we have done is shifted on cylinder from
>each >carburettor to another one. Now instead of seeing three intake pulses
>per two >revolutions, each carb will see only two. Instead of flowing for 540
>(crank) degrees >per every 720 for a 75% duty cycle, the carbs will now see
>only two pulses for 360 >>(crank) degrees per two revolutions for a 50% duty
>cycle.
>
>Gas flow through the jet is a function of instantaneous flow rates but piston
>location >is a function of average air flow so changing duty cycle from 75% to
>50% changes >everything.
You make a good point about the flow being cyclical rather than constant. I
suppose the carbs need to be sized to accomodate the peak air flow rather than
the average. Of course, the dampers on the carb pistons prevent peak air flow
demand from being achieved.
You don't say how a 50% duty cycle will help or hinder performance of a carb
compared to a 75% duty.
Just at first glance, I am not able to make a seat-of-the pants guess (and
maybe a wrong one, too) as to what effect the duty cycle will have.
What do you see as the net real-world effect?
I briefly communicated with Richard Good about his triple set-up. He
recommends that 3 stock carbs be used. I thought that he may have worked out a
new spring/needle combo for this new configuration, but he did not. I was not
sure how much effort he put into that side of the covnersion. I think that
each individual owner with their individual (probably non-stock) engine needs
to test the their own complete system.
That being said, one friend of mine, a lister here, simply used the stock ZS
carbs as recommended by Good, and loves the power and smoothness. It sure
sounds as if the carbs are about "right" for his suped-up engine. So Richard
Good seems to have been right about his recommendations. Or at least not
"wrong"!!!
This also supports my somewhat flawed conclusion that the springs as sized to
the carbs by the manufacturer are pretty close to optimal as far as air flow
speed across the bridge and atomization.. I >>am<< surprised that the needles
that were good for a 2 carb setup on a stock engine were anywhere close for a 3
carb setup on a non-stock engine.
For instance, on my TR7, I put UK-version TR7 carbs on my US version engine
(less compression than the UK) with TR4A oval carbs, and I had to change needle
profiles. They were very lean at maximum flow and caused a misfire! I
reprofiled them by hand and they seem fine now, even with K&N filters which
flow better. These changes were, IMHO, even less drastic than the triple carb
conversion, and I clearly needed different needles! I would have expected the
triple conversion to absolutely require new needles. Well, that just goes to
show what I know!
-Tony
=== This list supported in part by The Vintage Triumph Register
=== http://www.vtr.org
|