Wot do pilot jets do...Exactly..?

[Steve3351]

Hi
I know that the pilot jets let a small amount of air into the engine side of the intake...but why...?
and when the manual says the pilot screws should be two turns out , is that two full (360º) 0R two half (180º) turns...? :book

[Frosties]

Pilot jets are there to allow a balance amount of fuel (not air) into the carbs. They are ther for idling and up to about 3-5k rpm on most carb b ikes........full throttle is taken care of by the main jets. Think of a 16valve petrol car, half the valves are for low revs the other half is high revs. Two turns = 2 x 360 deg.

[His Dudeness]

If the bike is idling it has no throttle input right? If the throttle is closed the butterfly valve in the carb is basically fully closed so no air fuel mix can get through the main circuit so to keep the bike idling with no throttle input there has to be a separate idle circuit that's independent of the throttle. The idle circuit is made up of the pilot air jet that you mentioned, air from the airbox is drawn through that at idle. The pilot fuel fuel jet, fuel is drawn up through that from the bowl and then the air from the pilot air jet and the fuel from the pilot fuel jet mixes and is drawn into the engine. The mixture screw is a restriction in the idle circuit. It sets how much air fuel mix can be drawn through the idle circuit so on the Fazer if you turn the screw in it blocks more of the pilot circuit so that leans the mixture, if you turn the screw out it lets more air fuel mix through so that richens the mixture. The two turns out is two full 360 degree turns out from tightened. You have to be very careful when you tighten the mixture screw because the tip of the screw is very thin and easily damaged.

If you search for "CV Carburetor " on youtube there's a very good animation showing how a carb works. It's about a 2 minute video

[YamFazFan]

Think of a 16valve petrol car, half the valves are for low revs the other half is high revs.

That's only on VVT/VTEC type engines though isn't it?. In most cases they're all operating constantly without variation aren't they?.

[Steve3351]

ARF...!  thanks for the help...I did look up some carbs videos and discovered that the pilot screw is for allowing fuel, not air into the idle circuit...

[Steve3351]

When I shut off the throttle the revs don't fall back to idle immediately...theres a delay. The throttle cables seem to snap back OK (I ordered a new set anyway)...but could this be down to ill adjusted pilot screws....?

[Trebus]

Sounds like an air leak. Check all the vacuum hoses and caps on the balancing spigots.

[daviee]

easiest way to find an air leak is spray the inlet rubbers etc with brake cleaner if the revs go  up  you found it

[Steve3351]

Thanks for the tips lads....I tested for air leak but all seems OK.
When I think about this it's unlikely that the pilot jets are causing this problem (revs not dropping smartly down when I shut off the throttle).....since pilot jets only supply tickover fuel...
I suppose I will fit new throttle cables...but I'm not all that opti :groan mistic that will solve the prob....?

[His Dudeness]

That's the video I was talking about

https://www.youtube.com/watch?v=wyspAHrMbb8

Your problem is most likely either vacuum leak as was suggested or the choke plungers aren't fully closing. It's not going to be mixture screws or pilot jets. The idle circuit doesn't just supply fuel at tick over it contributes fuel up to about half throttle but the % of total fuel that it contributes gets smaller the further you open the throttle. There's a graph showing it

[His Dudeness]

Is that youtube video working for everyone? None of the youtube videos on here are working for me

[YamFazFan]

Is that youtube video working for everyone? None of the youtube videos on here are working for me

No

[Steve3351]

Thanks for informations and thoughts. I now understand wot pilot screws do. That video with the Italian guy is good... My bike is STILL having throttle shut off delay...when I shut the throttle the revs don't go back to idle SMARTLY.
Changing the pilot adjustment screws didn't make any difference...so back to factory settings. Used WD40 and butane to check for air leaks...negative. Checked my sliders and diaphragms...all looks OK...still no difference. New throttle cables...STILL revs won't drop back after throttle shut off...
Anyone had similar issues......?? :wall

[Steve3351]

ARF...finally sorted my carb issues... :rollin
First I had a bogging down while tipping along at low revs...open the throttle, and nothing ... for a few secs...then start to accelerate away.... turns out to be a clogged pilot jet...makes sense, since pilot jet is helping at low revs, not just idling, and wasn't working properly, but once main jet kicks in off we go.
Second problem...revs won't fall back to idle when throttle is shut off...NOT pilot jet, although some experts suggest that running too rich causes this problem. I went down that road and tried adjusting pilot screws...no difference except lumpy idling.
Put pilot screws back to factory setting.
NOT throttle cable, i fitted a new one and made sure it was snapping back all the way. NOT a partly open choke circuit.
Other expert points out that shutting the throttle should drop all needles into main jets and close them SMARTLY. If one needle is not falling for some reason (diaphragm damaged/not seated correctly or some tiny obstruction) then there will be a delay in revs falling.
I took off the carbs, and without dismantling them completely opened the tops and made sure the diaphragms were seated correctly and the sliders were working smoothly....BINGO..! Blip the throttle and she revs up and falls back like a good FZS should...took it for a burn and experienced bliss of perfect throttle response....this video was a big help - https://www.youtube.com/watch?v=IBFq9nHVmks&t=62s
thanks for the comments and advice

[His Dudeness]

Nice one. So it was sticking slides? The videos don't seem to be working

[Steve3351]

Weird that the video isn't working...the link sems to have disappeared from previous post...
if u get onto youtube and search - [color=var(--ytd-video-primary-info-renderer-title-color, var(--yt-spec-text-primary))]How does a CV Carburetor works - Quick and simple explanation
should bring up Italian guy with excellent animated demo....[/color]