Jet needle not moving

Hello , today I cleaned my carby on my 1981 xj750 seca and noticed all my jet needles were spring loaded except for one . Not sure if that's how it's supposed to be or weather that one is malfunctioning ? Any ideas.

 

As in needle and seat ? All mine are spring loaded, i spose the odd one has been replaced at some time. Maybe have a closer look at it with a magnifier of some kind.... or above the diaphragm, if thats what you mean yes should be 4 of them.

 

Not sure , it's the long needle that's on the end of the diaphram , I think it's called the jetting needle . I pulled apart the others and they compress in each sleeve but 1 won't fit in any of the sleeves enough to move with the spring .

 

This has a parts diagram for your carbs, do you mean the spring - part 29
http://www.cmsnl.com/yamaha-xj750rj-sec ... /B-03.html

 

It could have been replaced, and it's very odd to have one different jet needle that doesn't fit with the others. I'd also be looking for other "tuning" changes on that carb rack, such as pilot and main, fuel and air jets being different sizes or different than stock.

Your PO might have tried an "upgrade" and given up halfway through, or damaged a component and replaced it with something that looked "good enough" and will give you tuning headaches until it's remedied.

This is just speculation, but I believe the jet needle is spring loaded so that it can completely shut off flow from the needle jet (note word order difference) and balance out the force from the diaphragm spring. Otherwise, the jet needle would slam into the needle jet when the venturi pressure drops, and since we are talking about brass components, they wouldn't take that for long.

 

Thanks for the pic Bushy , no its the spring no.30 and the jet needle no.7 . Thanks manbot13 , it seemed a bit weird that only one wouldn't move but they all look the same . I think I will try and find a new needle . Any ideas on where I could find one?

 

So the one that's not spring loaded, what's different with it? Is it completely different design with no spring originally, or has someone maybe just lost the spring & jammed a bunch of washers or suchlike up into the plastic screw in part?

(when you unscrew the plastic bit out the middle of the piston, you should find a spring on top of the needle's plastic collar)

 

Oh my bad, definitely disassemble it and check. I thought from your first post that "not spring loaded" meant you had already confirmed that the spring was missing.

 

It is exactly the same as the rest . It has a spring and I have swapped parts to see if it was the diaphram assy or the spring but this one needle doesn't seem to compress in any . I thought it could be the plastic bit on the top of the needle but it looks exactly the same which is why I thought it might be meant to move but than what's the point of having a spring on it . Really confused but I will order another needle and go from there I think.

 

Somethings gotta be stopping it, it's only a bit of metal with plastic on, a spring & another bit of plastic lol don't let the thing beat you that easy...

Any chance of putting up a pic of the dismantled piston/plastic nut/spring & needle?

 

Yeah , there isn't much to it but I have just flown out to work and won't be back for 9 days . When I get home I will put up some pics . The tip of the needle is a bit damaged so Manbot was right . Thanks for the help and will post pics soon .

 

Bushy excellent link you provided! Great to see so many schematics for the XJ650J also. I've already bookmarked it. Thanks!

 

Ahh, well you're right to search for a new needle then if you've found damage. It could affect low to midrange fuelling on that cylinder depending on the damage.

Just be aware that there are different thicknesses & tapers to the needles (usually have their code stamped into the top near the plastic bit but I have seen them with it inked on & dissolving too!) They should all be the same needle code & I'm sure if nobody else does, Chacal (Len at XJ4Ever) will know the correct ones for your bike and he will 99.999recurring% have them in stock too

 

maxim82.. yep a handy bit of info. As someone said a picture speaks a thousand words.