Clams Canino wrote:
wrote in message


Anybody know the point of intentionally running on only two out of
four cylinders?
Related question, how do the non-firing cylinders receive lubrication
without pumping raw fuel/oil out the exhaust at low rpms?


They are firing, and getting a fuel/oil mix. They are just not getting a
sufficiant amount of said mix to actually *combust*, so they remain
"passive" until the revs come up and they start sucking from the main jets.

Why? I've never gotten a totally straight answer. I know that unlike all
the 4 cylinder cross-flows Mercury did, this looper will *not* run correctly
at low RPM on all four. I gather it suffers from harmonics and bad
vibration. And from everything I've read, it's an inherant problem with no
work-around. I don't know what kind of spin Mercury Marketing puts on the
2+2 angle, but the fact is, it was the only way they could make it run right
at all.

It's just to slow them down at idle. By shutting cyls off the remaining
ones can run cooler piston temps because they can have a more normal
spark timing or in Yamaha's DFI case run a richer mix which is easier to
reliably ignite & doesn't generate as much lean mixture piston heat buildup.

All this stuff has been basic common knowledge about 2 strokes for 40
years & every OB mechanic will tell you how if a 2 stroke runs lean the
piston gets hot & bang. Yet they seem to have forgotten the basics when
DFI came along, or more likely they never understood anything to begin
with & just parrot what they're told at "tech" training.

The "reason" it's used at idle is that crankcase transferred 2 strokes
(the crankcase is the inlet manifold if you like) can't be brought to a
slow idle with throttle plates "alone" like say a car petrol 4 stroke is
(high manifold vacuum).

The reason "just" throttle plates aren't good enough are;

(i) Unlike a proper 4 stroke:-) (it's just me Clams, stay calm:-)) a 2
stoke has both the exhaust & transfer (inlet) ports open at the same
time, whereas at idle a 4 stroke has very little overlap & modern VCT
ones even less. Remember the old hot rodders??? well they're old now:-)
they put more overlap into their cam shafts to make more power at high
revs?? but that meant they wouldn't idle smoothly, because remaining
exhaust would flow into the high vacuum inlet manifold as soon as the
inlet valve opened, those that had a manifold pressure gauges could see
the gauge bouncing up & down at idle.

(ii) OBs are special 2 strokes in that the exhaust is submerged & even
with bleed holes in the leg the back pressure in the exhaust system is
constantly changing, depending on the type of boat, the load the boat
has in it, even as the boat rocks & sounds up & down in waves.

(iii) Crankcase transferred 2 stokes are very sensitive to exhaust back
pressure for the obvious reasons but even more so if there is back
pressure on the exhaust ports at idle. If there was a high vacuum in the
crankcase at idle, then as soon as the transfer port was uncovered,
instead of the new charge flowing up from the crankcase the exhaust
might flow back down into the crankcase. (as it is they spit a bit now
at idle when the occasional back flow)

(iv) Crankcase transferred 2 stokes "need" some back pressure in the
exhaust to run efficiently otherwise too much of the fresh charge will
just go out the exhaust ports, however they are usually setup so the
optimum back pressure occurs at the right part of the rev range, not at
idle, at idle any exhaust back pressure is a problem & a constantly
changing back pressure is fatal to a soft slow smooth idle.

"The Fix" you're well familiar with Clams, but can you believe it these
claimed 30 yr OMC dealers didn't know this bit till we told then in this
NG in 97:-) Honestly their so called tech training is nothing more than
product marketing brain washing, on my washing machine that would be
small, delicate & cold water only load;-)

(i) They allow lots of air/fuel mix through the engine at idle, this
means that there's plenty of pumping action by the engine at idle
because the engine is actually trying to rev up. This is why they "seem"
to be rich at idle, they're not "rich" (try leaning them & they stop
yes??) but just using lots of air & fuel to try & go.

(ii) The engine is then slowed by retarding the spark timing. Not just
a little either, it depends on the engine etc but firing the plug well
after TDC (say 16-20 degs atdc!!!) is not uncommon. With your old Mercs
do some measuring & you'll be astounded what the actual spark timing is
at idle.

(iii) Most 2 or 4 stroke petrol engines can be made to have a very
soft, smooth & stable idle simply by retarding the spark timing, but
there's a rub; there always is;-)

(iv) Because the piston is already traveling down the bore before the
spark fires the pressure is constantly reducing meaning the flame front
gets slower, meaning the chamber quickly gets overly hot. (if you have
an old car you'll remember one of the first checks against over heating
was to make sure the spark timing was right, if it's retarded then the
engine will over heat just idling in the drive).

(v) The reason they can use late spark timing in 2 stroke OBs is that
they run endless raw water cooling so they don't overheat as such,
however that totally uncooled piston does still get very hot during long
periods of idling.

(vi) Ever notice how often when you hear the 2 stroke engine blowup
story it includes a long period of trolling beforehand??? They say
something like we were trolling an hour or so then went to move to
another paddock opened it up, we accelerated, the power started to go
away, then we stopped. What happened was they were idling for hours on a
very retarded spark timing which got the piston very hot, the engine
itself was cool because as you know the rubber impeller pump is almost a
displacement pump at idle. They "suddenly" gave it WOT & the engine was
made to work, the spark timing suddenly shot forward to the max.
Combined with the hot piston if there was even one tiny spot in any
chamber over 250C then the mixture will auto ignite (detonate) this
sudden huge pressure rise creates huge amounts of more heat so the
offending piston gets even hotter, if the usr doesn't pull back very
quickly then the detonation becomes self sustaining & ..........it ends
like a Ficht:-)

(vii) OK Clams I better reconfirm one of the reasons "your" old Mercs
were so successful was the pistons were small, lots of them but they
were tiny. The surface area of the piston was small compared to the
total length of the rings, it's the rings that transfer the piston heat
over to the very cool bore. That's one of the reasons any 2 stroke over
500cc per cyl is considered suspect because as the diam of the bore goes
up the area of the piston top to absorb heat goes up X the sq of the
radius, but the amount of ring length available to transfer that heat
away, only goes up linearly. There's only one bunch of idiots I know of
who are using consumer funded testing to make cyls over 500cc in
consumer production, guess who??? ......... want to phone a
friend??..........give up??? OK; it's BS Bill & the Ficht
team............ again:-)

Conversely, thier three cylinder 90 (same motor just a three) runs fine at
idle on all three. I think that much like the 90 is kind of 1/2 of the V-6,
that Merc should make the mid-hp motors (100-125) half of the larger V-6's
and scrap that 4. I'll not forget to mention that they had a perfected
100-140hp powerhead untill 1989 when the 2+2 emerged.

-W




K

& the Krause lie for the day is an oldie but a goodie:-)

So this lying idiot sees himself as circulating in the upper circles,
honestly this lying idiot thinks this is believable, which is more proof
of his total stupidity:-)

As far as your other complaints, well, almost every president in
my memory, and I *remember* Truman, Eisenhower (who cheated on his
wife), Kennedy, Johnson, Nixon, Ford, Carter, Reagan and Bush, lied and
participated in deceit to one degree or another, and on issues far more
important than who was giving them blow jobs.

Good lord. I met *every* president in the damned group except
Bush, and I worked once for his father.