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Jet Needle size question

frosty5011

frosty5011

Forum Mentor
On the VM26 carbs for my '78 GS750, I am trying to figure out why the mixture is rich overall, especially in my #1 carb.

The needles originally in the carbs (when I purchased the bike) have the number 5F21 stamped on them, whereas the needle received in a carb rebuild kit (Keyster) has Y64 stamped on it.

The 5F21 needles have less of a taper, and the point has a diameter of 1.73mm. The Y64 needle has a 0.93 mm diameter. The clip notches are slightly different on the 2 needles, the notch spacing on the 5F21 needles being closer to each other than those on the other needle. The distance from middle notch to tip of needle are basically the same.

needlejet.jpg


Does this seem correct? Or is it possible that these 5F21 needles are from a jetting kit? It seems to me, however, that the Y64 needles would make the mixture even richer since they have the smaller diameter.
 
i have the big fat needles in my 750 carbs, i`d have to say they are stock. when i pulled the needles out to raise them i noticed that all bolts and nuts still had little unbroken paint marks which means never been touched. my bke runs a little on the lean side also.
 
I have a '79 1000 with VM26SS carbs. I am original owner. My stock needles say 5DL36 on them. And in front of the '5' is a small square with a smaller square in the lower left corner. I believe this is Mikuni's trademark. These also have much less of a taper than my Dyno-jet needles and 1 less clip position groove. Dyno-jet says their different shape improves the transition from pilot circuit to needle to main circuit. What some call the 'overlap' effect. I believe your 5F21 needles are stock. It is interesting that the Keysters are a different shape, since they are supposed to be a direct replacement and not meant to be part of a re-jetting kit, as the Dyno-jet's are. It's hard to guess what the new needles will do to your jetting.
 
Carb kits

Carb kits

I think lots of carb rebuild kits come with the wrong parts. I would get a hold of a schematic and try to use only Suzuki parts. They're expensive but taking everything apart is painful when things don't work right when you're done. :roll:
 
Thanks guys,

I figured it was probably the carb kit that was wrong. I've heard of quite a few negative comments, mainly concerning Keyster kits. After reviewing your feedback, I figure the Mikuni 5F21 needles are the right ones.

So, that means the next step will be to lower the needles 1 notch. Lets see if that gives me better all around plug color than what I'm getting now.

Thanks again!
 
Sorry for dragging up an old thread but I have exactly the same issue:

Just purchased some keyster kits for my ole '77 GS750 and immediately noted that the slide needle was visibly different.

My old needles (5F21's made from aluminium?) had very noticable wear on them and I sat both needles side by side and measured them.

Both needles are the same diameter where the old needle was worn so I am guessing that there shouldnt be too much effect from the skinnier bottom section on the new Y64 needle at idle and 1/4 throttle, but it will probably be noticable after 1/4 throttle as it will allow more fuel through.

My setup is shown below:

850 WISECO BigBore kit
K&N Pods
4-1 eggsauced
112.5 main jets
15 pilots
O-6 needle jets
Iridium plugs
Boyer Brandsen ignition



The way I see it is that if I return to the stock 102.5 main jet, it will allow for the new narrower tapered Y64 needle.

My old mixture screws were also buggered so i replaced them (and the factory set ones too)


I will run it with the new setup (standard main jets, Y64needles and screws), and will do some plug chops and monitor fuel consumption, and if its worse then I will attempt to order some new 5F21's.


I also noted that the GS850 and GS1000's with the same VM26 carb's show as having an O-4 needle jet but I couldnt find any specs on the needles but the pictures all show the narrower tapered needle similar to the Y64. Changing the needle jet on my GS750 to the O-4 (leaner) should overcome the differences in the needles if they have too much effect.
 
Coming down on the mains will surely affect WOT, I would be more inclined to lower the needle (down into the jet more) to compensate than mess with the mains.

The DJ needles always seem to be much skinnier with a bigger taper than the stock ones.

Dan :)
 
salty_monk said:
Coming down on the mains will surely affect WOT, I would be more inclined to lower the needle (down into the jet more) to compensate than mess with the mains.

The DJ needles always seem to be much skinnier with a bigger taper than the stock ones.

Dan :)
Click to expand...

Thanks for that Dan.

My thoughts were that since the needle was smaller that would mean that it could get more fuel through so it would help if I dropped the mains slightly to compensate.

I guess I can always run it and see how it goes, and then adjust later if needed.
 
It would at anything over than WOT. At WOT the needle is all the way out of the mains & the hole bore is what governs petrol flow. My point was that you don't want to cure 1 problem by causing another.
You may be able to go down a bit with no problems - who knows, you'd need to plug chop I guess.

Dan :)
 
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