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#Post#: 180--------------------------------------------------
The suck part of the cycle
By: Fried Ape Date: November 20, 2014, 6:48 am
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(as in suck, squeeze, bang, blow)
Prior to selling the NF I had been toying with the idea of
replacing the carburettor. Not for fashion reasons, but to
improve the inlet shape as part of the grand plan to improve the
old nail.
The engine uses two shapes of inlet manifold, depending on the
source of the bike. The civilian ones seemed to have a
rearward-facing (downdraft) manifold and the military ones had a
sideways-facing (sidedraft) manifold. Neither of these is really
conducive to good gas flow.
I played with some designs to try and make a downdraft manifold,
but the frame, front wheel etc get in the way, and we do need
the front wheel. (Only Ogri can get home with a flat front tyre
by wheelying the whole way back)
One option I was considering was a downdraft carb. There isn't
much room under the tank but the Citroen 2CV has a very nice
twin-barrel carb that might do the job. Fit the top of it with a
low-profile bellmouth and an airbox leading out to the original
airfilter position, and it might just work.
The 2CV carb's barrels are timed to open in phase, one after the
other. This means that I could orient the carb so that the
barrel that opened first at small throttle openings was at the
top side of the inlet port. This would make the closed barrel
(potentially) provide a bit of anti-reversion in the slow-moving
side of the gas flow.
The complication is that I have no idea how to change the jets
in one of these carbs, or even if alternative jets are
available.
Plan B was to use an SU carb, modified according to the wizdom
of Mr Vizzard (author of 'tuning BL's A series engine' - the
bible of how to improve bad engine designs). This would hang out
in the standard position. Using a CV-type carb allows the use of
a much larger choke size without the complications of the engine
drowning if one opens the throttle too quickly.
The challenge would be the inlet manifold.
What I had in mind for this was to put the carb as far forward
as it would go and clear the frame tube, and the manifold back
to provide as much downdraft as possible, then bring the
manifold to a slightly smaller diameter than the inlet port in
the head. The 'mismatch' would be at the lower (with respect to
the head) side of the port, with the end of the manifold
finishing with a sharp lip.
The aim of this was to provide some anti-reversion - the valve
timings of the NF engine are fairly long, to allow the huge and
heavy valves to be moved back and forth. So the idea was to
deter any tendency to reverse flow back up the inlet at low
throttle openings.
I never did get around to it, although I do have the SU carb
lurking in the garage awaiting the attentions of a Dremel.
Anyone fancy getting their hands on a 2CV carb and having a go?
I think this is a diagam of the 2CV carb - the jets do look like
changeable items.
HTML http://www.theostry.com/NF/a_carb.jpg
#Post#: 186--------------------------------------------------
Re: The suck part of the cycle
By: banquo Date: November 20, 2014, 10:36 am
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Knowing nothing at all about carburettors, I always thought the
racing Aermacchi's downdraught carb looked the business...
HTML http://i17.servimg.com/u/f17/12/57/79/73/ricmet10.jpg
I did acquire a proper downdraught SU (off I know not what) but
the bore was far too small.
I assume 'reversion' is what happens when you get those
backfires through the carb when you try to add throttle opening
faster than the flywheel wants to catch up.. ???
#Post#: 188--------------------------------------------------
Re: The suck part of the cycle
By: Fried Ape Date: November 20, 2014, 10:47 am
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Nope - reverse flow.
It's possible for pressure pulses to go back up the inlet
towards the carb. If they reach the carb they go through, pick
up some more petrol, then come back on the next intake and pick
up yet more petrol.
This is one reason why lengthy valve timings (valves open for a
long time) can lead to poor slow-speed running.
Anti-reversion cones and similar measures are a way to stop the
pulses travelling back up the manifold - they also work for
exhausts.
Or you could go the Japanese route and use several small valves
with shorter timing and more rapid opening and closing
controlled by an overhead camshaft. But where's the fun in that?
#Post#: 189--------------------------------------------------
Re: The suck part of the cycle
By: banquo Date: November 20, 2014, 10:58 am
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Learn something new every day ;)
#Post#: 196--------------------------------------------------
Re: The suck part of the cycle
By: Fried Ape Date: November 21, 2014, 8:53 am
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I should have added - the reverse flow only applies to a
slow-moving flow of air (or exhaust). When the revs pick up, the
inlet charge moves faster and has more inertia so is more
resistant to reversing direction. This is why clever people like
Cosworth don't go for the largest ports possible (in non-racing
engines) but focus on getting good gas flow and speed.
This is why, if you do fit some form of anti-reversion device,
it's fitted in the slowest-moving part of the manifold.
It's also why you can usually improve an engine more by making
the exhaust longer rather than wider.
#Post#: 199--------------------------------------------------
Re: The suck part of the cycle
By: banquo Date: November 21, 2014, 9:19 am
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Thanx. I shall resist the temptation of ripping out my engine
and sending it by courier to Cosworth...
#Post#: 207--------------------------------------------------
Re: The suck part of the cycle
By: huub Date: November 23, 2014, 3:48 pm
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the 2cv carb has twin shokes , so you would end up having to
make a two into one inlet manifold.
probably more trouble than it is worth.
having said that , a downdraft carb would be nice....
#Post#: 214--------------------------------------------------
Re: The suck part of the cycle
By: Fried Ape Date: November 25, 2014, 8:36 am
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The 2CV mounts its carb directly onto a steel tube manifold -
there is no clever 2:1 piping, they just let both carb chokes
open into the same space.
What I had in mind was to obtain the inlet manifold and carb
together from a 2CV, saw through the inlet piping and weld the
carb flange to a short taper (with anti-reversion lip...) with a
flange to match the NF head. I planned to orient the carb so
that the first choke to open would be towards the high-speed
airflow side of the inlet port - nearest the top of the engine.
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