Fueling question

[CrazyChris]

Hey guys. Took a drive to the mechanic the other day to check the progress of my bike and he showed me something that I can not understand. He cleaned the carbs I bought and set them up. The bike needs choke to start when cold (understandable) but this is where I get stupid. Without the airbox it will not rev above 4k. I can hear that she is too lean. If he puts the airbox and filter on it won't rev over 4k either but it sounds (and looks by the smoke out of the exhaust) that its running too rich but with the airfilter out and just the airbox on it runs nicely. Can anyone please explain to me how in the world that is possible. Thanks very much.

[ross46]

Fuelling is likely out, with the airbox off it is running too lean so it will not rev over 4k, with it on, its running too rich and you get it right with the filter out, are the gloats set right? Needles at the right height? Did the mechanic set the carbs up on the bike?

[CrazyChris]

According to him he did set it up. Adjusted floats and all the necesarry. I'm just starting to get a sneaky suspicion that he is trying to bullshit me.

[ross46]

How mechanically minded are you? If your competent it may be a good idea to check his work, whip of the float bowls check the floats are at 11mm, turn your mixture screws back to standard which should be 1 3/4 turns out from fully seated, check the condition of the needle jet that it doesn't have any dents. And check that the needles are not damaged. After that it should just be checking for air leaks, and adjusting where necessary, I think this website explains quite well
http://www.factorypro.com/tech/carbtune ... gines.html
Hope all this helps get you on your way to fuelling perfection

[CrazyChris]

I am quite competent but when it comes to carbs I am totally green. Got the bike back today and had a bit of a shock. Will idle fine and rev up fine so I took her for a ride. Got as far as 2km and then she died. Started her up again and I had to keep her full throttle to keep her going at 1500rpm. Riding was out of the question after that. Gonna have a look tomorrow. Any ideas?

[ross46]

Had that last week while setting up my fuelling, its a bitch!! Pilots running too rich, try turning the pilot screws out 1/4-1/2 a turn. The trick with getting it to catch when its like that is starting it, holsing the throttle so it doesnt die and very slowly roll of the throttle and it will normally catch. There are plenty of guides on the internet on how to check carbs, most difficult thing would probably getting the screws out technically you need a JIS screwdriver but it can be done with normal ones. Where about are you located?

[CaNsA]

Look underneath his name on the right hand side of his posts.

Booking a flight batman?

[ross46]

I've been before cansa, lovely place, he'd have to pay for mine and the airlines petrol I'll see if I can make a quick guide on how to set the carbs up if one hasn't been done already, have a set of carbs stripped already so should be easy

[banner001]

as a rule of thumb, if your pilot screws are located close to the butterfly (like the zxr):-
turning your pilot screws counterclockwise (screwing out) RICHENS the mix,
turning your pilot screws clockwise (screwing in) LEANS the mix.

if your pilot screws are located close to the open bellmouth of the carb:-
turning your pilot screws counterclockwise (screwing out) LEANS the mix,
turning your pilot screws clockwise (screwing in) RICHENS the mix.


on the zxr the pilot air jet delivers a constant amount of air to atomise the fuel, the pilot screw controls how much atomized fuel goes into the cylinder, so screwing out gives more fuel, and screwing in gives less.

if on startup you can smell fuel, its too rich, screw them all in 1/4 turn until the bike stops smelling of fuel, if you go too far you will lean it out too much and you will require choke to start it always.

[rmkd177]

[quote="banner001"]as a rule of thumb, if your pilot screws are located close to the butterfly (like the zxr):-
turning your pilot screws counterclockwise (screwing out) RICHENS the mix,
turning your pilot screws clockwise (screwing in) LEANS the mix.

Im afraid this isnt true...it is a 'pilot air screw' and screwing this OUT weakens the mixture allowing MORE air...as screwing it IN allows LESS air.
The same insnt true however for the Keihin FCR which is the opposite.
A good indication is on start up from cold...if no choke is needed its too rich at the bottom end.
If the 'pilot air screw' is more than 3.5 turns out it is becoming out of range and you should try a smaller pilot jet (located in the float chamber/carb body] Allen Performance at Nottingham are carb specialists and are always helpfull.

[Animammel]

Chris - Im having the same damn issues. exactly as you put it. Will play around tomorrow as I have some time and see if I strike lucky.

[ross46]

if your having the same issue as chris and what i had, turn the pilot screws out 1/4 of a turn or back to standard. that cured mine

[CrazyChris]

Had a look yesterday at everything. Couldn't get her to run properly. Tried adjusting the screws (mechanic had them at 1.5 turns) and it didn't make a lot of difference (I had it out at 2.5 turns at a stage) so I phone the mechanic and he said its my fuel pump. It apparently does not switch off when the float bowls are full so the pump pushes past the needles and seats which in turn floods my motor. Installed a little hose clamp to lessen the flow of fuel into the carb and played around with it. All this happened yesterday. Today she does not even want to start!! Getting fed up with this bike now.

[adamm1704]

The carbs have overflow pipes to stop them from getting flooded. If the pump is pushing to much fuel it will just spill out of the 2 little hoses.

[ross46]

sounds stupid, but is the fuel pump connected the right way round? my bike wouldnt work after i put new fuel pipe on and turns out i wasnt paying attention and got the pipes mixed up. check the simple things first, its normally something stupid