Replacing the "Needle Jet" P5M (not the "jet needle" ;-)

[herennow]

Out of interest has anyone on here replaces the needle jet on later style carbs? its part number / size is P5M.

My bike has 40 k miles on it and has started showing symptoms of being too rich in the mid to high RPM, typical of worn needle jets.

Folks often forget about these jets but on many other bikes this is a standard wear / maintenance replacement item.

Anyone played with them?

Cheers.

[nurms]

Yeah, I have played with needle jets a lot! (If you mean the pipe where needle goes up n dow) If your carbs have never seen "full" rebuild on interior parts, go ahead and buy quality Keyster kit and rebuild interior parts.

It might also be that slide quide o-ring leaks air through it.(at least in older models)

You could try to compensate this richness by opening the air mixture screws symmetrically littlebit.

What are exactly the symptoms you feel for richness in middle throttle? Your needles might also be worn. My needle jets were little glogged and I had lean mixture at high rpm, cleaning and rejetting helped alot.

[herennow]

Good to hear experiences from someone who has played with that jet. In 50 k KM that thing gets worn, if you think about the amount of times the needle rattles against the jets with the vibration of the engine at 5k RPM+ (and the fact that the diaphragm is always moving up and down in response to engine pulses) it really should be a no brainer on high mileage bikes .

Long ago I tried a larger Main jet and a needle washer (why? Well everyone does it right;-)) Had a wretched stumble from 5 k up to 6 k near WOT. The only thing that made it go away was reverting to standard mains and removing the needle (steps done one at a time).

But now I'm starting to get a similar symptom creeping back in but higher up the rev range (7 to 8 k RPM). It goes away if I remove the air filter (more air = leaner mixture) Although I should check float level, as it might have drifted higher.

Unfortunately it seems as though the needle used in my bike is no longer available.... 5DH42 but the jet still is (09494-00881/P5M), as the needle is aluminium it might wear faster that the jet.

Nurms, was your jet made of steel or a lighter alloy? was it easy to get in and out?

[nurms]

Hi again!

I'm back and hopefully you already found the answers you needed but here is my experiences:

I am not that good in english and I do not know what wretched stumbling means.    But if the motorcycle is "lagging" like it would have electric restriction it is probably lean mixture and if it "rattles" and "buzz" it would be rich mixture. If you removed the airbox and it helped it is probably rich. You can also try to accelerate with choke on.

Needle jet is not shown that clear in original gs exploded view (1990), and the original needle jet in mikuni bst33 is made out of brass.
Keyster might make needle jets in their sets out of steel, you can look up keyster KS-0544 or KS-0602.

China carb sets tend to have correct needle jets but everything else in those poor china sets are messed up. (I have one china set and really, you can not use anything from it but possibly the needle jet if you are lucky to get the jet in decent quality) Removing the needle jet is not that difficult but I learned it from this video (https://www.youtube.com/watch?v=RqidhIpoXqk&feature=youtu.be). This video can be found on youtube with name: BMW F650 (169) Vergaser 2/3 Überholung (Mikuni BST33). You should be careful when removing the slide quide and the needle jet attached to it, in my case the needle jet was so dirty that is was hard to remove from the slide quide because there was some junk in the slide quide hole.

This video is for older gs did you have the newer one? I have the adjustable needle 5DH9, so I do not know about that. Also those kits I linked are for older mikuni bst33 carb.

And yes, about the float height. Measure the float height according to repair manual but I would not touch it. It is almost surely not the issue unless someone had been adjusting it(which should not be done at all) poorly. 

Cheers,