2-stroke vs 4-stroke carbs

On Wed, 26 Nov 2003 16:55:10 +0200, Jan wrote:

 > Hello guys
 >
 > Sorry if this has been asked before but if you take your typical 2-s carb
 > like a Keihin PWK and you take a 4-s carb like a Kehin PD which is the
 > same size eg 33mm and you fit them with the same size jets would you get
 > the same amount of fuel into the combustion chamber over any given time ?

I would not expect that to be true between any two different models from
the same manufacturer simply because the passages are different.

"Jets" are not the only thing going on. The needle is very important. Also
the throttle slide is a tunable component with different cutout shapes.

I don't know why, but 2-stroke carburetors use a pilot air screw for fine
adjustment of the idle mixture while 4-stroke carburetors use a pilot fuel
screw. Clockwise richens a 2-stroke and leans a 4-stroke.

 > The reason I'm asking is because my XR250 is fitted with a PWK carb
 > (previous owners conversion) and if possible I'm going to jet first
 > before I try and sell the PWK and get a PD. The bike runs fine and
 > starts easy but fuel consumption is heavy (20 km to a liter if I'm
 > carefull)

20 km * 0.6214 mi/km * 3.7854 l/gal = 47 MPG which is pretty good for off
road but not that great on-road.

 > and there is alot of carbon in the exhaust and in the exhaust
 > ports.

Its running rich. I would start by dropping the needle lower. Set the air
screw to 1.5 turns out. Lean it until you get a light popping in the
exhaust on deceleration. Then start playing with the other items such as
main jet and needle taper if necessary. On my Keihin FCR found the needle
selection makes a big difference in the pilot/idle mixture.

By starting with a foreign carburetor for your bike its likely none of the
current values are right. When you start from the factory baseline you
don't have to juggle as many variables. Before you get too far into it I'd
give Sudco and/or Carb Parts Warehouse a call. They know this stuff far
better than any one else will, and may just have notes on how to do
exactly what you are attempting.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

Yep Tiago is on the right track here. When he says the difference
is in atomization.
A 4S carb is different in it's air passages and uses an emulsifier
jet in the main jet circuit.
Im surprised that your bike even runs half way well with a 2S carb
installed. Someone either knew his/her stuff or just got lucky with
the jetting, in my opinion.
When I was trying to figure out why my XL350R was running like crap
at 1/4-2/3 throttle. It turned out to be a clogged air passage from
the carb intake mouth to the area just above the main jet. Where the
needle drops into. The main jet is screwed into another brass tube
with a bunch of holes in it. (emulsifier) jet tube.
So the gas was being sucked out of the main jet into the air stream
as a solid flow of raw gas. It was running WAY to rich at that 1/4-2/3
throttle opening. When I opened the throttle WFO. It ran great. Part of
that is because on this bike it has a second carb that opens up from
around 1/2-2/3 throttle. So I guess the second carb compensated for the
over rich primary carb.
My suggestion is to get the right carb for the application.
Give <a style='text-decoration: underline;' href="http://sudco.com/" target="_blank">http://sudco.com/</a> a try. They have exploded views of both 4S and 2S
carbs on their web site.
Good thumping to ya.
Jeff
**********************************************************************

Tiago Rocha <tiagocrspamtalho.RemoveThis@terra.com.br> wrote in message news:<4li9svss4vbko9s2nu7n1m12rn3s9s0lmo.RemoveThis@4ax.com>...
 > On Wed, 26 Nov 2003 16:55:10 +0200, "Jan" <nospam.RemoveThis@mail.com> wrote:
 >
  > >The bike runs fine and starts easy but fuel consumption
  > >is heavy (20 km to a liter if I'm carefull) and there is alot of carbon in
  > >the exhaust and in the exhaust ports.
 >
 > well, don't know about the difference, something about fuel
 > atomization, someone with more knowledge will jump in, but my XR250 -
 > note that it has the same name but it's another engine - makes 20km/l
 > or even worse, I did 14km/l at an enduro that was on a place that had
 > more rocks and hills than anything else. My bike isn't blackening the
 > exhausts...
 >
 > fwiw...<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

Tiago-

As I recall, four strokes have a higher intake velocity than two
strokes. Therefor the carbs are set up differently in order to give
the appropriate response. Since two strokes have a lower velocity they
have to be set up to deliver gas more easily with different venturis
size and placement of jets and probably a whole lot of other things.
This is also why four stroke carbs have an accelerator pump (squirts
fuel when you open up the throttle). To fix your problem you must
first get the correct carb on your bike.
DF
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

On 26 Nov 2003, Jeff Wallace wrote:

 > A 4S carb is different in it's air passages and uses an emulsifier
 > jet in the main jet circuit.

I know nothing about anything.

Now that that's cleared up... I recall emulsifier tubes in the stock carbs
of an RZ350 that I worked on once. It may have been a Canadian market RZ -
not sure. Anyway, my point is that there were definitely emulsifier tubes
in the stock carbs of a two-stroke. What that means is anybodies guess.

How useless was that?

Craig<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

Thanks Tiago. Another question I forgot is whether other Keihin carb types
use the same size and shape jets ? I looked at this site and tried to figure
out if the jets are interchangeble ?
<a style='text-decoration: underline;' href="http://www.motorcyclecarbs.com/keihin/keihin_jet_list.htm" target="_blank">http://www.motorcyclecarbs.com/keihin/keihin_jet_list.htm</a>
When I want to go leaner should I go for smaller or larger main and pilot
jets ? Or is that jet needle and needle jet !?

Tiago Rocha <tiagocrspamtalho.RemoveThis@terra.com.br> wrote in message
news:4li9svss4vbko9s2nu7n1m12rn3s9s0lmo@4ax.com...
 > On Wed, 26 Nov 2003 16:55:10 +0200, "Jan" <nospam.RemoveThis@mail.com> wrote:
 >
  > >The bike runs fine and starts easy but fuel consumption
  > >is heavy (20 km to a liter if I'm carefull) and there is alot of carbon
in
  > >the exhaust and in the exhaust ports.
 >
 > well, don't know about the difference, something about fuel
 > atomization, someone with more knowledge will jump in, but my XR250 -
 > note that it has the same name but it's another engine - makes 20km/l
 > or even worse, I did 14km/l at an enduro that was on a place that had
 > more rocks and hills than anything else. My bike isn't blackening the
 > exhausts...
 >
 > fwiw...
 >
 > --
 > Tiago Rocha
 > Recife - Brasil
<font color=purple> > <a style='text-decoration: underline;' href="http://www.diariodastrilhas.cjb.net</font" target="_blank">www.diariodastrilhas.cjb.net</font</a>><!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

On Wed, 26 Nov 2003 19:36:25 +0200, "Jan" <nospam DeleteThis @mail.com> wrote:

 >Thanks Tiago. Another question I forgot is whether other Keihin carb types
 >use the same size and shape jets ? I looked at this site and tried to figure
 >out if the jets are interchangeble ?

There are basically two types of keihin jets, the small jets and the
big jets... Small, not in the sense of the orifice being small, small
in the sense that the jet itself have a small lenght... I have a
NX200 with keihin PD carb and IIRC, my RM125 have a keihin PWK carb
(not exactly sure about the model, the carb is a keihin for sure).
Both share the same shape of jet, but the jets on the RM have much
bigger orifices... I would think that yes, the jets are
interchangeable, but you need to know the right size... If your
exhaust is getting black and with carbon build up, I'd guess you have
also black smoke and for sure you have too big jets. If someone took a
motocrosser carburetor and installed it on a XR and keep the original
jets of the motocrosser carb, for sure you have too rich jets. The
performance gets very poor with too rich jets...

Also, the 2T and 4T carbs work different. I would not expect perfect
jetting... I'd sell the 2T carb you have now, as is, and get a 4T
carb...

 >http://www.motorcyclecarbs.com/keihin/keihin_jet_list.htm
 >When I want to go leaner should I go for smaller or larger main and pilot
 >jets ? Or is that jet needle and needle jet !?

your jet is that 99101 jet. You'll need to find the right size.
You need smaller main jets and pilot jets. Perhaps a leaner needle...

to go leaner, install smaller jets. To go richer, install bigger jets.
big and small in the sense they have bigger and smaller orifices...

In theory, the pilot jet influence closed to 1/4 throttle. Main jet
influence 1/2 to full open, while the 1/4 to 1/2 area is affected by
the needle, but of course, if you have a too big pilot jet, the main
jet area would be also affected, as the parts work like a team...

I've been struggling with the jetting of my XR for quite a long time,
since I bought it and decided that a racing air filter and a racing
exhaust would give it a few more hp... After changing lots of jets,
the only thing I still need to replace is the needle. Back ordered...
These carb parts are really hard to get around here.

Good Luck!


--
Tiago Rocha
Recife - Brasil
<a style='text-decoration: underline;' href="http://www.diariodastrilhas.cjb.net" target="_blank">www.diariodastrilhas.cjb.net</a><!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

Thanks for the reply guys. I have already dropped the needle to sit low as
well as played with the air screw. And the bike does "pop" on deceleration !
If it was not for the carbon build up and below par fuel consumption the
carb would have been ideal. I opened the carb up a while ago and was quite
surprised to see a very small main jet in it, think it was something like a
60 -70 ! Everyone who has ridden the bike says it really goes well for a
250. Thing is the carb has some Japanese writing on it with some engraving
on the carb float which I have not seen other PWK's so maybe this was some
kind of mod avaliable for this bike as it is a XLR 250 Baja Jap spec bike.
But it's still the normal XR motor in there. If I was to go for the leanest
setting should I go for a 60 main, 35 pilot and 1.5 turns out as this is the
smallest jets avaliable for a PWK ? Just want to get my jet sizes right when
I order some.

Thanks again guys !

David Kelly <n4hhe.TakeThisOut@yahoo.com> wrote in message
news:pan.2003.11.26.17.39.32.296100@yahoo.com...
 > On Wed, 26 Nov 2003 16:55:10 +0200, Jan wrote:
 >
  > > Hello guys
  > >
  > > Sorry if this has been asked before but if you take your typical 2-s
carb
  > > like a Keihin PWK and you take a 4-s carb like a Kehin PD which is the
  > > same size eg 33mm and you fit them with the same size jets would you get
  > > the same amount of fuel into the combustion chamber over any given time
?
 >
 > I would not expect that to be true between any two different models from
 > the same manufacturer simply because the passages are different.
 >
 > "Jets" are not the only thing going on. The needle is very important. Also
 > the throttle slide is a tunable component with different cutout shapes.
 >
 > I don't know why, but 2-stroke carburetors use a pilot air screw for fine
 > adjustment of the idle mixture while 4-stroke carburetors use a pilot fuel
 > screw. Clockwise richens a 2-stroke and leans a 4-stroke.
 >
  > > The reason I'm asking is because my XR250 is fitted with a PWK carb
  > > (previous owners conversion) and if possible I'm going to jet first
  > > before I try and sell the PWK and get a PD. The bike runs fine and
  > > starts easy but fuel consumption is heavy (20 km to a liter if I'm
  > > carefull)
 >
 > 20 km * 0.6214 mi/km * 3.7854 l/gal = 47 MPG which is pretty good for off
 > road but not that great on-road.
 >
  > > and there is alot of carbon in the exhaust and in the exhaust
  > > ports.
 >
 > Its running rich. I would start by dropping the needle lower. Set the air
 > screw to 1.5 turns out. Lean it until you get a light popping in the
 > exhaust on deceleration. Then start playing with the other items such as
 > main jet and needle taper if necessary. On my Keihin FCR found the needle
 > selection makes a big difference in the pilot/idle mixture.
 >
 > By starting with a foreign carburetor for your bike its likely none of the
 > current values are right. When you start from the factory baseline you
 > don't have to juggle as many variables. Before you get too far into it I'd
 > give Sudco and/or Carb Parts Warehouse a call. They know this stuff far
 > better than any one else will, and may just have notes on how to do
 > exactly what you are attempting.
 ><!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

On Wed, 26 Nov 2003 22:11:31 +0200, Jan wrote:

 > Thanks for the reply guys. I have already dropped the needle to sit low as
 > well as played with the air screw. And the bike does "pop" on deceleration
 > ! If it was not for the carbon build up and below par fuel consumption the
 > carb would have been ideal.

Exhaust popping can also be due to a leak near the head. A vacuum
develops between bursts from the engine which sucks fresh air in. Exhaust
gasses during deceleration have more unburned fuel than normal. The
additional air causes it to burn in the exhaust pipe. Pop!

Lean running can raise the temperature of the pipe and cause unburned fuel
to burn late, in the pipe. As can hot spots of carbon deposits.

 > I opened the carb up a while ago and was quite
 > surprised to see a very small main jet in it, think it was something
 > like a 60 -70 !

You don't know much about the prior history of this bike? Some idiot/guru
(take your pick) tuners are known to drill jets when they want bigger.

Another thing to worry about is there should be two jets dipped in the
float bowl. One for the main jet, the other for the pilot jet. 60 is rich
for a pilot jet, very very lean for a main jet.

 > I was to go for the leanest setting should I go for a 60 main, 35 pilot
 > and 1.5 turns out as this is the smallest jets avaliable for a PWK ?
 > Just want to get my jet sizes right when I order some.

I think that is excessively lean. And repeating my prior thought, you are
focusing on only a few of the tunable components where the Big Picture may
be more appropriate. In addition to the fuel jets you mention, look for
*air* jets. The pilot air jet restricts the amount of air which is
allowed in to draw fuel thru the pilot fuel jet. The main air jet controls
air allowed into the emulsifier section of the main/needle assembly.

Before exploring the lean range of jets, replace that needle. Possibly
also the needle seat. These items wear to the richer side.

On my KTM's FCR carburetor I found a richer needle (not just moving the
clip) richened the pilot circuit (which everybody *says* isn't affected by
the needle) to the point the fuel screw did nothing. Bike started cold
without "choke" and with fuel screw at 0 turns fully lean.

Depending on what the bike is worth to you I think the notion of finding
another carburetor which is known to work on the bike is your best route.
You might spend 100 hours trying to make this one work. But then again
you'll learn a lot.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

"Jan" <nospam.RemoveThis@mail.com> schreef in bericht
news:bq31fs$jde$1@ctb-nnrp2.saix.net...
 > Thanks for the reply guys. I have already dropped the needle to sit low as
 > well as played with the air screw. And the bike does "pop" on deceleration
!
 > If it was not for the carbon build up and below par fuel consumption the
 > carb would have been ideal. I opened the carb up a while ago and was quite
 > surprised to see a very small main jet in it, think it was something like
a
 > 60 -70 ! Everyone who has ridden the bike says it really goes well for a
 > 250. Thing is the carb has some Japanese writing on it with some engraving
 > on the carb float which I have not seen other PWK's so maybe this was some
 > kind of mod avaliable for this bike as it is a XLR 250 Baja Jap spec bike.
 > But it's still the normal XR motor in there. If I was to go for the
leanest
 > setting should I go for a 60 main, 35 pilot and 1.5 turns out as this is
the
 > smallest jets avaliable for a PWK ? Just want to get my jet sizes right
when
 > I order some.

There is no way of knowing for sure, experience might allow you to take an
educated guess about jest sizes though.
plz help me out here...your bike is a 4stroke?
If the oil turns black early on a 2stroke there might be something fishy
with the cranckseals.
Heavy stroking on a 4smoke might have something ado with pistonrings or
valves.
Best advice (imho) is given in various replies...get the wright carb for
your bike.

Grtz, P1

 >
 > Thanks again guys !
 >
 > David Kelly <n4hhe.RemoveThis@yahoo.com> wrote in message
 > news:pan.2003.11.26.17.39.32.296100@yahoo.com...
  > > On Wed, 26 Nov 2003 16:55:10 +0200, Jan wrote:
  > >
   > > > Hello guys
   > > >
   > > > Sorry if this has been asked before but if you take your typical 2-s
 > carb
   > > > like a Keihin PWK and you take a 4-s carb like a Kehin PD which is the
   > > > same size eg 33mm and you fit them with the same size jets would you
get
   > > > the same amount of fuel into the combustion chamber over any given
time
 > ?
  > >
  > > I would not expect that to be true between any two different models from
  > > the same manufacturer simply because the passages are different.
  > >
  > > "Jets" are not the only thing going on. The needle is very important.
Also
  > > the throttle slide is a tunable component with different cutout shapes.
  > >
  > > I don't know why, but 2-stroke carburetors use a pilot air screw for
fine
  > > adjustment of the idle mixture while 4-stroke carburetors use a pilot
fuel
  > > screw. Clockwise richens a 2-stroke and leans a 4-stroke.
  > >
   > > > The reason I'm asking is because my XR250 is fitted with a PWK carb
   > > > (previous owners conversion) and if possible I'm going to jet first
   > > > before I try and sell the PWK and get a PD. The bike runs fine and
   > > > starts easy but fuel consumption is heavy (20 km to a liter if I'm
   > > > carefull)
  > >
  > > 20 km * 0.6214 mi/km * 3.7854 l/gal = 47 MPG which is pretty good for
off
  > > road but not that great on-road.
  > >
   > > > and there is alot of carbon in the exhaust and in the exhaust
   > > > ports.
  > >
  > > Its running rich. I would start by dropping the needle lower. Set the
air
  > > screw to 1.5 turns out. Lean it until you get a light popping in the
  > > exhaust on deceleration. Then start playing with the other items such as
  > > main jet and needle taper if necessary. On my Keihin FCR found the
needle
  > > selection makes a big difference in the pilot/idle mixture.
  > >
  > > By starting with a foreign carburetor for your bike its likely none of
the
  > > current values are right. When you start from the factory baseline you
  > > don't have to juggle as many variables. Before you get too far into it
I'd
  > > give Sudco and/or Carb Parts Warehouse a call. They know this stuff far
  > > better than any one else will, and may just have notes on how to do
  > > exactly what you are attempting.
  > >
 >
 ><!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

In article <bq2eum$c5e$1@ctb-nnrp2.saix.net>,
  "Jan" <nospam.TakeThisOut@mail.com> writes:
 > I'd apreciate any info anyone might have !

I just ran into this site today. Some good info on carbs including a
section titled "The Difference Between Two-Stroke and Four-Stroke Carbs".

bruno.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

Yeah I was thinking of getting a PD carb but where ? I live in South Africa
where for some reason second hand on/off road carbs are very hard to come
by. And there is no way I'm going to pay a small fortune for a brand new
carb from Honda. If a simple float bowl gasket is $15 guess what a carb will
cost you !

PymerOne <pymerone.TakeThisOut@ppllaanneett.nl> wrote in message
news:bq3fle$oli$1@reader08.wxs.nl...
 >
 > "Jan" <nospam.TakeThisOut@mail.com> schreef in bericht
 > news:bq31fs$jde$1@ctb-nnrp2.saix.net...
  > > Thanks for the reply guys. I have already dropped the needle to sit low
as
  > > well as played with the air screw. And the bike does "pop" on
deceleration
 > !
  > > If it was not for the carbon build up and below par fuel consumption the
  > > carb would have been ideal. I opened the carb up a while ago and was
quite
  > > surprised to see a very small main jet in it, think it was something
like
 > a
  > > 60 -70 ! Everyone who has ridden the bike says it really goes well for a
  > > 250. Thing is the carb has some Japanese writing on it with some
engraving
  > > on the carb float which I have not seen other PWK's so maybe this was
some
  > > kind of mod avaliable for this bike as it is a XLR 250 Baja Jap spec
bike.
  > > But it's still the normal XR motor in there. If I was to go for the
 > leanest
  > > setting should I go for a 60 main, 35 pilot and 1.5 turns out as this is
 > the
  > > smallest jets avaliable for a PWK ? Just want to get my jet sizes right
 > when
  > > I order some.
 >
 > There is no way of knowing for sure, experience might allow you to take an
 > educated guess about jest sizes though.
 > plz help me out here...your bike is a 4stroke?
 > If the oil turns black early on a 2stroke there might be something fishy
 > with the cranckseals.
 > Heavy stroking on a 4smoke might have something ado with pistonrings or
 > valves.
 > Best advice (imho) is given in various replies...get the wright carb for
 > your bike.
 >
 > Grtz, P1
 >
  > >
  > > Thanks again guys !
  > >
  > > David Kelly <n4hhe.TakeThisOut@yahoo.com> wrote in message
  > > news:pan.2003.11.26.17.39.32.296100@yahoo.com...
   > > > On Wed, 26 Nov 2003 16:55:10 +0200, Jan wrote:
   > > >
   > > > > Hello guys
   > > > >
   > > > > Sorry if this has been asked before but if you take your typical 2-s
  > > carb
   > > > > like a Keihin PWK and you take a 4-s carb like a Kehin PD which is
the
   > > > > same size eg 33mm and you fit them with the same size jets would you
 > get
   > > > > the same amount of fuel into the combustion chamber over any given
 > time
  > > ?
   > > >
   > > > I would not expect that to be true between any two different models
from
   > > > the same manufacturer simply because the passages are different.
   > > >
   > > > "Jets" are not the only thing going on. The needle is very important.
 > Also
   > > > the throttle slide is a tunable component with different cutout
shapes.
   > > >
   > > > I don't know why, but 2-stroke carburetors use a pilot air screw for
 > fine
   > > > adjustment of the idle mixture while 4-stroke carburetors use a pilot
 > fuel
   > > > screw. Clockwise richens a 2-stroke and leans a 4-stroke.
   > > >
   > > > > The reason I'm asking is because my XR250 is fitted with a PWK carb
   > > > > (previous owners conversion) and if possible I'm going to jet first
   > > > > before I try and sell the PWK and get a PD. The bike runs fine and
   > > > > starts easy but fuel consumption is heavy (20 km to a liter if I'm
   > > > > carefull)
   > > >
   > > > 20 km * 0.6214 mi/km * 3.7854 l/gal = 47 MPG which is pretty good for
 > off
   > > > road but not that great on-road.
   > > >
   > > > > and there is alot of carbon in the exhaust and in the exhaust
   > > > > ports.
   > > >
   > > > Its running rich. I would start by dropping the needle lower. Set the
 > air
   > > > screw to 1.5 turns out. Lean it until you get a light popping in the
   > > > exhaust on deceleration. Then start playing with the other items such
as
   > > > main jet and needle taper if necessary. On my Keihin FCR found the
 > needle
   > > > selection makes a big difference in the pilot/idle mixture.
   > > >
   > > > By starting with a foreign carburetor for your bike its likely none of
 > the
   > > > current values are right. When you start from the factory baseline you
   > > > don't have to juggle as many variables. Before you get too far into it
 > I'd
   > > > give Sudco and/or Carb Parts Warehouse a call. They know this stuff
far
   > > > better than any one else will, and may just have notes on how to do
   > > > exactly what you are attempting.
   > > >
  > >
  > >
 >
 ><!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

 >
 > I don't know why, but 2-stroke carburetors use a pilot air screw for fine
 > adjustment of the idle mixture while 4-stroke carburetors use a pilot fuel
 > screw. Clockwise richens a 2-stroke and leans a 4-stroke.

The above is in actual fact nonsense..........2-stroke bikes can be fitted
with carbs having either a fuel or air screw, for primary mixture adjustment
( I have owned 2-strokes with both types of adjustment).

LK<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: 2-stroke vs 4-stroke carbs 

the main reasons you will find it very difficult to get a PWK carb working
on a 4-stroke, is lack of emulsion tube in the main jet system, and the fact
that the needle jet for any 2-stoke carb, is likely to be shrouded, rather
than open (which will cause all sorts of problems, when trying to sort
jetting).

best thing to do is to get hold of a stock carb, or properly jetted
aftermarket unit. you could offset the cost of this by selling the PWK.

LK
"Jan" <nospam RemoveThis @mail.com> wrote in message
news:bq5b5j$spk$1@ctb-nnrp2.saix.net...
 > Yeah I was thinking of getting a PD carb but where ? I live in South
Africa
 > where for some reason second hand on/off road carbs are very hard to come
 > by. And there is no way I'm going to pay a small fortune for a brand new
 > carb from Honda. If a simple float bowl gasket is $15 guess what a carb
will
 > cost you !
 >
 > PymerOne <pymerone RemoveThis @ppllaanneett.nl> wrote in message
 > news:bq3fle$oli$1@reader08.wxs.nl...
  > >
  > > "Jan" <nospam RemoveThis @mail.com> schreef in bericht
  > > news:bq31fs$jde$1@ctb-nnrp2.saix.net...
   > > > Thanks for the reply guys. I have already dropped the needle to sit
low
 > as
   > > > well as played with the air screw. And the bike does "pop" on
 > deceleration
  > > !
   > > > If it was not for the carbon build up and below par fuel consumption
the
   > > > carb would have been ideal. I opened the carb up a while ago and was
 > quite
   > > > surprised to see a very small main jet in it, think it was something
 > like
  > > a
   > > > 60 -70 ! Everyone who has ridden the bike says it really goes well for
a
   > > > 250. Thing is the carb has some Japanese writing on it with some
 > engraving
   > > > on the carb float which I have not seen other PWK's so maybe this was
 > some
   > > > kind of mod avaliable for this bike as it is a XLR 250 Baja Jap spec
 > bike.
   > > > But it's still the normal XR motor in there. If I was to go for the
  > > leanest
   > > > setting should I go for a 60 main, 35 pilot and 1.5 turns out as this
is
  > > the
   > > > smallest jets avaliable for a PWK ? Just want to get my jet sizes
right
  > > when
   > > > I order some.
  > >
  > > There is no way of knowing for sure, experience might allow you to take
an
  > > educated guess about jest sizes though.
  > > plz help me out here...your bike is a 4stroke?
  > > If the oil turns black early on a 2stroke there might be something fishy
  > > with the cranckseals.
  > > Heavy stroking on a 4smoke might have something ado with pistonrings or
  > > valves.
  > > Best advice (imho) is given in various replies...get the wright carb for
  > > your bike.
  > >
  > > Grtz, P1
  > >
   > > >
   > > > Thanks again guys !
   > > >
   > > > David Kelly <n4hhe RemoveThis @yahoo.com> wrote in message
   > > > news:pan.2003.11.26.17.39.32.296100@yahoo.com...
   > > > > On Wed, 26 Nov 2003 16:55:10 +0200, Jan wrote:
   > > > >
   > > > > > Hello guys
   > > > > >
   > > > > > Sorry if this has been asked before but if you take your typical
2-s
   > > > carb
   > > > > > like a Keihin PWK and you take a 4-s carb like a Kehin PD which is
 > the
   > > > > > same size eg 33mm and you fit them with the same size jets would
you
  > > get
   > > > > > the same amount of fuel into the combustion chamber over any given
  > > time
   > > > ?
   > > > >
   > > > > I would not expect that to be true between any two different models
 > from
   > > > > the same manufacturer simply because the passages are different.
   > > > >
   > > > > "Jets" are not the only thing going on. The needle is very
important.
  > > Also
   > > > > the throttle slide is a tunable component with different cutout
 > shapes.
   > > > >
   > > > > I don't know why, but 2-stroke carburetors use a pilot air screw for
  > > fine
   > > > > adjustment of the idle mixture while 4-stroke carburetors use a
pilot
  > > fuel
   > > > > screw. Clockwise richens a 2-stroke and leans a 4-stroke.
   > > > >
   > > > > > The reason I'm asking is because my XR250 is fitted with a PWK
carb
   > > > > > (previous owners conversion) and if possible I'm going to jet
first
   > > > > > before I try and sell the PWK and get a PD. The bike runs fine
and
   > > > > > starts easy but fuel consumption is heavy (20 km to a liter if I'm
   > > > > > carefull)
   > > > >
   > > > > 20 km * 0.6214 mi/km * 3.7854 l/gal = 47 MPG which is pretty good
for
  > > off
   > > > > road but not that great on-road.
   > > > >
   > > > > > and there is alot of carbon in the exhaust and in the exhaust
   > > > > > ports.
   > > > >
   > > > > Its running rich. I would start by dropping the needle lower. Set
the
  > > air
   > > > > screw to 1.5 turns out. Lean it until you get a light popping in the
   > > > > exhaust on deceleration. Then start playing with the other items
such
 > as
   > > > > main jet and needle taper if necessary. On my Keihin FCR found the
  > > needle
   > > > > selection makes a big difference in the pilot/idle mixture.
   > > > >
   > > > > By starting with a foreign carburetor for your bike its likely none
of
  > > the
   > > > > current values are right. When you start from the factory baseline
you
   > > > > don't have to juggle as many variables. Before you get too far into
it
  > > I'd
   > > > > give Sudco and/or Carb Parts Warehouse a call. They know this stuff
 > far
   > > > > better than any one else will, and may just have notes on how to do
   > > > > exactly what you are attempting.
   > > > >
   > > >
   > > >
  > >
  > >
 >
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