Hello group, hopefully a simple question:
For a four-barrel carb, at what RPM setting should the 3&4 barrels be
set to open at? For example should they open when attempting to climb
onto plane, and then once on plane, throttle back and plane with only
1&2 open?

Mike Harrison wrote:

Hello group, hopefully a simple question:
For a four-barrel carb, at what RPM setting should the 3&4 barrels be
set to open at? For example should they open when attempting to climb
onto plane, and then once on plane, throttle back and plane with only
1&2 open?

Drag boat / jet boat? Snap the secondaries open off the line, these are
suitable applications for mechanical secondaries.

The vast majority of recreational boating applications are best served
by vacuum operated secondaries. The primaries are used at lower engine
speeds to keep the intake velocities up and provide good mixing &
atomization.

Draw the parallel: a boat is a big, heavy, sluggish vehicle with a WOT
top speed well below any comparably powered land vehicle, and with only
one gear to compound this!

The secondaries should be opened slowly and late, based on demand...
typical for a recreational boat would be starting to open around
3200-3500 rpm, and rarely fully open(probably about 1/2) as a top rpm of
4800-5200 is way too low to demand the entire 600-650 cfm from a small
block / up to 350ci engine.

Rob

Vacuum secondaries will come on when you demand enough to cause them to.
Mechanical secondaries will come on based on the the throttle position. RPM
has nothing to do with either.

"Mike Harrison" wrote in message
om...
Hello group, hopefully a simple question:
For a four-barrel carb, at what RPM setting should the 3&4 barrels be
set to open at? For example should they open when attempting to climb
onto plane, and then once on plane, throttle back and plane with only
1&2 open?

I dunno but here's about how mine (Rochester Q-jet) works which seems ok to
me. From a stop (alpha outdrive) with trim all the way down, I open the
throttle at say a count of 1000, 2000, 3000. With the trim down, the boat
comes up onto plane flat ... the bow does not point up. At about 2300 rpm I
am on plane and around about 2400 rpm (since the engine's under load) I hear
the secondaries opening (and if I go out tomorrow I'll check that) . If I'm
just going to cruise, I throttle back to about 2300 rpm while simultaneously
setting the trim for best speed/rpm which works out to engaging the trim
(up) button for about a second (count of say 1000). And from that point on,
I won't hear the secondaries open unless operating close to WOT which is
about 4000 rpm with the 23" prop and about 4300 rpm with the 21"prop. So,
for me I'd say it's about 2400 rpm coming onto plane but I'll check that
next time out.

"JamesgangNC" wrote in message
hlink.net...
Vacuum secondaries will come on when you demand enough to cause them to.
Mechanical secondaries will come on based on the the throttle position.
RPM
has nothing to do with either.

"Mike Harrison" wrote in message
om...
Hello group, hopefully a simple question:
For a four-barrel carb, at what RPM setting should the 3&4 barrels be
set to open at? For example should they open when attempting to climb
onto plane, and then once on plane, throttle back and plane with only
1&2 open?

Unless you are watching them you really don't know when 4 barrel secondaries
are open or how much. Quadrajets make a noise when you open the throttle
wide at lower rpm's and you hear the extra air flow on the secondaries. But
gently opening the throttle to increase speed can result in the secondaries
opening some and not making any noticable difference in sound.


"Bowgus" wrote in message
news
I dunno but here's about how mine (Rochester Q-jet) works which seems ok
to
me. From a stop (alpha outdrive) with trim all the way down, I open the
throttle at say a count of 1000, 2000, 3000. With the trim down, the boat
comes up onto plane flat ... the bow does not point up. At about 2300 rpm
I
am on plane and around about 2400 rpm (since the engine's under load) I
hear
the secondaries opening (and if I go out tomorrow I'll check that) . If
I'm
just going to cruise, I throttle back to about 2300 rpm while
simultaneously
setting the trim for best speed/rpm which works out to engaging the trim
(up) button for about a second (count of say 1000). And from that point
on,
I won't hear the secondaries open unless operating close to WOT which is
about 4000 rpm with the 23" prop and about 4300 rpm with the 21"prop. So,
for me I'd say it's about 2400 rpm coming onto plane but I'll check that
next time out.

"JamesgangNC" wrote in message
hlink.net...
Vacuum secondaries will come on when you demand enough to cause them to.
Mechanical secondaries will come on based on the the throttle position.
RPM
has nothing to do with either.

"Mike Harrison" wrote in message
om...
Hello group, hopefully a simple question:
For a four-barrel carb, at what RPM setting should the 3&4 barrels be
set to open at? For example should they open when attempting to climb
onto plane, and then once on plane, throttle back and plane with only
1&2 open?

I suppose a seat of the pants way to determine when the secondaries open
when coming onto plane would be to lock the secondaries shut ... do a run
.... then unlock the secondaries and do a run ... and compare. And I guess
the secondaries spring adjustment would be the best place to start playing
to get the opening point you want. My acceleration is smooth .. no bogging
.... all the way from idle to WOT so I'm leaving well enough alone.

OT: And so far, the float stuck open only once this season which was on
dewinterizing ... last season it stuck a few times ... I was not looking
particularly forward to replacing the needle and seat. No big deal ... it's
just that more often that not, one simple repair leads to another more
costly one, and so on.


"JamesgangNC" wrote in message
link.net...
Unless you are watching them you really don't know when 4 barrel
secondaries
are open or how much. Quadrajets make a noise when you open the throttle
wide at lower rpm's and you hear the extra air flow on the secondaries.
But
gently opening the throttle to increase speed can result in the
secondaries
opening some and not making any noticable difference in sound.


"Bowgus" wrote in message
news
I dunno but here's about how mine (Rochester Q-jet) works which seems ok
to
me. From a stop (alpha outdrive) with trim all the way down, I open the
throttle at say a count of 1000, 2000, 3000. With the trim down, the
boat
comes up onto plane flat ... the bow does not point up. At about 2300
rpm
I
am on plane and around about 2400 rpm (since the engine's under load) I
hear
the secondaries opening (and if I go out tomorrow I'll check that) . If
I'm
just going to cruise, I throttle back to about 2300 rpm while
simultaneously
setting the trim for best speed/rpm which works out to engaging the trim
(up) button for about a second (count of say 1000). And from that point
on,
I won't hear the secondaries open unless operating close to WOT which is
about 4000 rpm with the 23" prop and about 4300 rpm with the 21"prop.
So,
for me I'd say it's about 2400 rpm coming onto plane but I'll check that
next time out.

"JamesgangNC" wrote in message
hlink.net...
Vacuum secondaries will come on when you demand enough to cause them
to.
Mechanical secondaries will come on based on the the throttle
position.
RPM
has nothing to do with either.

"Mike Harrison" wrote in message
om...
Hello group, hopefully a simple question:
For a four-barrel carb, at what RPM setting should the 3&4 barrels
be
set to open at? For example should they open when attempting to
climb
onto plane, and then once on plane, throttle back and plane with
only
1&2 open?

"JamesgangNC" wrote in message
hlink.net...
Vacuum secondaries will come on when you demand enough to cause them
to.
Mechanical secondaries will come on based on the the throttle
position.
RPM
has nothing to do with either.

Not entirely correct...only if the weight, or drag changes on the boat.

Because water is the same thickness, and always flat, there is no load
change to effect vacuum secondaries...unless one of the previous mentioned
situations are present.

Vacuum secondaries (on a boat motor) will always open at the same RPM every
day, in relation to throttle position, unless there is more...or less
resistance.

This is also why boat engines don't need vacuum advances...the never go
"uphill", or operate in pea soup.


--
-Netsock

"It's just about going fast...that's all..."
http://home.insight.rr.com/cgreen/

Vacuum secondaries are not used on boat carburetors.
JIMinFL

"Gene Kearns" wrote in message
...
On Thu, 19 Aug 2004 16:06:39 -0400, "Netsock" wrote:


"JamesgangNC" wrote in message
hlink.net...
Vacuum secondaries will come on when you demand enough to cause
them
to.
Mechanical secondaries will come on based on the the throttle
position.
RPM
has nothing to do with either.

Not entirely correct...only if the weight, or drag changes on the boat.

Because water is the same thickness, and always flat, there is no load
change to effect vacuum secondaries...unless one of the previous
mentioned
situations are present.

Vacuum secondaries (on a boat motor) will always open at the same RPM
every
day, in relation to throttle position, unless there is more...or less
resistance.

This is also why boat engines don't need vacuum advances...the never go
"uphill", or operate in pea soup.

That is a somewhat untrue statement. Vacuum advance has limited
usefulness under conditions of constant speed and load applications.
Thus, at cruise, it would be of limited value. Under other
conditions, of varying speed, a vacuum advance would be useful.
--



Grady-White Gulfstream, out of Southport, NC.

http://myworkshop.idleplay.net/cavern/
Homepage
http://www.southharbourvillageinn.com/directions.asp Where
Southport,NC is located.
http://www.southharbourvillageinn.linksysnet.com Real Time
Pictures at My Marina
http://www.thebayguide.com/rec.boats
Rec.boats at Lee Yeaton's Bayguide



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..JIMinFL wrote:
Vacuum secondaries are not used on boat carburetors.
JIMinFL


That's quite an absolutely wrong assertion... Please tell us you're
joking...

http://froogle.google.com/froogle?q=...Search+Froogle

http://www.overtons.com/cgi-bin/over...il.cgi?22244++

http://www.overtons.com/cgi-bin/over...il.cgi?22245++

http://www.overtons.com/cgi-bin/over...il.cgi?29137++

Rob

Oops! I'm so embarrassed. I forgot about Holly style carbs. That
marinemechanic link was a good read. Thanks Gene.
JIMinFL

"Gene Kearns" wrote in message
...
On Thu, 19 Aug 2004 16:49:25 -0400, ".JIMinFL"
wrote:

Vacuum secondaries are not used on boat carburetors.
JIMinFL


Oh?


http://www.boatingmag.com/article.as... r=2&preview=

http://www.barrygrant.com/demon/default.aspx?page=21

http://boatpoint.ninemsn.com.au/Revi....asp?id=118833

http://marinemechanic.com/site/page120.html

...to just mention a few.....

--



Grady-White Gulfstream, out of Southport, NC.

http://myworkshop.idleplay.net/cavern/
Homepage
http://www.southharbourvillageinn.com/directions.asp Where
Southport,NC is located.
http://www.southharbourvillageinn.linksysnet.com Real Time
Pictures at My Marina
http://www.thebayguide.com/rec.boats
Rec.boats at Lee Yeaton's Bayguide



---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.736 / Virus Database: 490 - Release Date: 8/10/2004