Carb Problem

[AussiXS11G]

will do Pete
the 7's are what Pitman recommends.....
B

[petejw1966]

bryan i see ur running 7's, thats a colder plug
compared to stock, if u have a look at the earth strap
there should be some colour half way up the strap,
on urs theres no colour,
run bp6es plugs and see if that helps.

a bit hard to tell by the pics but the plugs
 look  pretty good.
u want a nice even colour around the base circle.

[AussiXS11G]

plugs - 1 to 4 L to R





no - popping in the exhaust at most revs.....I am not too worried about that as the cams have quite an overlap and I can richen the idle stage (apparently this popping may be due to leaning of the idle stage)

[petejw1966]

what do the plugs look like,

is it backfiring on decel and bogging down on accelleration
around the same throttle position?
a little popping isnt to bad.

u could have different mixtures (lean/rich) in different circuits.

[pgnz]

If it's popping on the idle circuit and hesitating mid range you'd suspect too lean rather than too rich, I'd raise the needles one notch richer and see what happens

[AussiXS11G]

hello Pete
two problems....
1  "popping" in the exhaust when I back off....not a real backfire tho
        I have checked for air leaks and am confident that none exist.
2   bogging down in the midrange and then coming good - both with the throttle held wide open
       and during a steady roll on.
thanks
B

[petejw1966]

hey bryan,
what problems are u having?

ive used the factory pro as a guide before, works ok
the only problem is once the bike is warmed up,
u have to wait for it to cool down if u make any changes.

what ive done is marked the throttle like the third link u posted
but i use a roll off method,  for eg, checking main jet have the bike
in a higher gear as possible, have the revs at 6000 rpm then crack the throttle open
till it hits 8000rpm then quickly shut off the throttle but only a fraction, if u feel the engine
speed up ur running lean ( think of a lawn mower just before it runs out of fuel), if u can feel engine braking on decel then its jetted correctly.
if its running rich itl bog down on accel before picking up.

float heights, have the bike in 4th or fifth at 2000rpm, crack the throttle wide open
the bike should pull all the way to redline smoothly.

a lean mixture will feel smooth but flat on acceleration
a rich mixture will usely bog down before picking up upon accel, also
itll cause a bit of a vibration throughout the engine.


theres a cpl of different ways to check, i find it hard to articulate it in writing
so  i hope some of it makes sense.

[Eveready1100]

I'm not as carb savvy as Steptoe, but I'd definitely try the needles as they are what controls the fuel flow in that rev range. If it's not too rich at the top end, the mains should be alright.

Actually, as the carbs aren't the same as the XS's originals, maybe the needles that suit them have a different metering profile?

[AussiXS11G]

I like the idea of "calibrating" the throttle so one can see at what opening things are going sideways as detailed in the third link
Mine is over fuelling in the midrange....either needle position or float level.....
I will try dropping the needles first but I am not confident that it will be the answer...
I didnt muck around with the float levels when I fitted the carbs....
The BS36 units I have are not quite the same as the BS34 units in the needle and seat/float area

[steptoe]

the info in the first link is what I use to get mine close,, although I don't go to the enth degree as is written,,

I use it as a general guide to set mine up

[AussiXS11G]

I dont want to subvert this thread but I an using BS36 carbs on mine and have a midrange problem..
have you guys seen these links and do you agree with the procedure and general info?
http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

and this with reference to page 55 and page 77
http://www.mtsac.edu/~cliff/storage/gs/Mikuni_BS-CV_Carburetor_Rebuild_Tutorial.pdf

and this - the general info in particular....
http://www.nightrider.com/biketech/download/Mikuni_manuals/tune_mikuni_carbs.pdf

[steptoe]

all the info is correct and the photo's and diagrahms will get you sorted,, just make sure when the taps are "on", the fuel comes out the back pipe, then you'll know which side of the tank they belong
getting the hoses the right length is the only real hassel you'll have,
to long and they'll kink under the taps

[Eveready1100]

Ok Bert, I'll have a shot at getting you sorted. First off, being a sport, it should have two outlets from each petcock on the tank. Depending on whether the taps might have been removed and refitted on opposite sides (happens a lot!) either the front or rear outlet supplies fuel in both the ON and RESERVE positions. The other outlet is for the PRIME which, if you have the original octy valve present, will bypass the vacuum setup and supply fuel straight to the carbs.The Octy valve looks like this =


 The bare unit ( with bracket) which sits atop the carbs above #2. Bracket screws on between #1 & 2 carby tops.


The Octy valve with all the fuel lines in place.


And some diagrams showing where each line is supposed to go.


Just ignore the reference to the white striped lines as they are probably long gone by now.

Now the where it says in the yellow framed diagram the fuel lines for the left and right side carbys, run the lines to the tees between each pair of carbs just above the float bowls. I say it this way as earlier carbs also have a similar tee higher up that serve as a float bowl vent and it would be bad to hook the fuel lines up to them. If you do see lines coming from that region, they should be hooked up to the airbox spigots at the front top side facing the carbs.

Also the vacuum line from the Octy goes onto one of the spigots on the inlet rubbers of which you should be able to see 4 of. The thinner hose coming from the timing cover is for the vacuum advance unit and hooks onto the little spigot at the front LHS of the #2 carb body, Don't hook this one up to the bigger ones on the inlet rubbers or you'll upset the advance unit badly.

If you don't have the octopus valve, just run the fuel lines straight down and connect them to the bottom tees on the carbs and turn the taps off manually after each ride.


This pic shows the fuel lines connected to the proper tees on the carbs. Note the other lines coming out higher up? They are the breathers on the early carbs.


If you only have one outlet on each fuel tap, let us know, and also a picture is worth a thousand words, so if possible try to get some pics up so we know exactly what you're working with.

[Jonesy]

off each tap there should be a hose going to the inlet manifiold (vaccuum to open up the tap) and one to the carby. thats how it is on the standard models. Some of the sport Round Headlight models have an octy thing and from here on in you might need a fellow "roundy" owner to advise on hoses

[bert]

hi
we have a 1981 xs1100 sport - bought it in pieces and have just put it back together. We have got to the carbs and are not sure where all the hoses go? there is a fuel line coming off each petcock that we do not know where they connect up to- does this model have a vacuum valve? Any help appreciated
thanks
ian and susan