Needle jets; Bleed type vs Primary type

Ok, I've seen this topic tossed around on a few boards in the past but I thought I'd bring it up again see if there are any fresh ideas or new attempts;

The Sudco tuning manual says that Bleed type Needle Jets are for 4 strokes and Primary Type are for 2 strokes.

I just got a VM36 for my XT500 :smirk: and (like many after market carbs) it comes with a primary (type 159) needle jet. The stock carb was bleed type but it turns out most people with after market VM carbs are running primaries in their 4 strokes...

Now in theory the design of a Bleed type needle jet would provide better fuel atomization and this is of interest to me... The 176 bleed jets fit VM carbs and Once I get this carb dialed in I may try to dial it in again with a bleed type needle jet.

Anybody ever try this?

Bleed type work better at smaller throttle openings, primary (non-bleed) work better at larger throttle openings.

Well I do spend more time under 1/2 throttle.

Do the Bleed go rich or lean at larger throttle openings?

Is it too dramatic for a different bottom taper on a multi taper needle to compensate for?

Bleed go leaner at higher speed compared to non bleed.

As the vacuum goes up as the engine speeds up, it tends to suck the easiest object, air instead of fuel. At slow speeds, the air entering sor tof 'bubbles up' the fuel.

I do not think a needle is going to change the effect, single, double or triple taper. The mere change in the shape of the needle makes direct comparisons unequal. Apples and oranges.

Hmm, I'm also noticing bleed NJ's only go to P8 and I am currently on a Q5. So unless bleeds typically run smaller I think I may stay primary.

Would slightly larger bleed holes reduce what you described? Kind of make a bleed/primary hybrid... or a larger main...

Lean up top makes me weary.

No, the size is the same as far as I ever noticed.

I do think you are over analyzing this though. Time to step back and let it settle some and decide your goal.

No, the size is the same as far as I ever noticed.

I do think you are over analyzing this though. Time to step back and let it settle some and decide your goal.

I do analyze things allot. And I get crazy Ideas allot. Only sometimes do I follow thru and do them.

How about anyone else? Try it? seen it tried?


I have put a VM36 on my SR500 and built a manifold and used a VM32 on my Virago 700. Both have the primary needle jet in them. The 500 is not much of a problem, think the smaller engine is not run much at just off idle. The 700 does however spend it's life at low amounts of throttle. 75-80 mph is 1/8 throttle at steady speed. The virago doesn't transition very well to the needle unless the needle is set rich. I have tried slide cutaways down to zero. It is acceptable but If I were building up the virago again I would try a bleed needle jet.

Most my commute is under 35mph So I spend allot of time at low throttle opening.

I also have a High comp piston, supertrapp and K&N Drop in filter. arround my neighborhood yesterday I was getting into 3rd without going past 1/4 throttle. (~2k RPM)

Frewayspeeds on the other hand do require a bit more throttle.

lump_tha_thump do you have pictures of your VM36 setup on your SR. I'm still getting my jetting dialed in on my XT but I like seeing other SR/XT/TT 500 setups

Hi Jhkj

I just used the correct size rubber mikuni carb holder and a large oval K&N. Almost too simple for closeup photo. I couldn't get it to run very well with the supertrapp (mine is a down pipe) installed, it's way too restrictive with the disks. so I just took off the end cap and disks. It has the perforations in it. The throttle cable had a tight bend get it to clear the frame tube.

As far as jetting I am running a 230 mj now. Also an o2 needle jet, Needle clip is in the top groove, raising the clip seems to hurt the mid range. I had an o5 needle in it and felt it was too rich. Probably something between the two would be a good place to start. I can't find the data on the needle It is what it came with it and was jetted for a 2 stroke.

6FJ6 is what comes with a new VM36.

Allot of SR/XT/TT riders run the 6F9 needle.

Dimensionally the 2 begin and end the same but the 6F9 is 2 taper and the 6FJ6 is a single taper. My be try the 6F9 in your SR.

I found some info on Dellorto carbs that my aid me in trying to go to a bleed type system in the future. Dellorto carbs don't just have different size NJ's they also have varying Bleeders locations in relation to the float level.(More NJ choices for them lol) Higher bleeders deliver leaner mixture up top and lower bleeder holes deliver richer mixture up top.

If you are interested in Mikuni jetting rather than delortto you might consider a small spreadsheet that I wrote a while ago to decipherer the jet area of the needle jet on two different needles.

It doesn't directly pertain to your question but you might find it interesting.

mikuni needle jet areas

Here is my old road bike.


Nice lookin bike :ride:

What I was getting at was modifying a mikuni bleed NJ to keep it from leaning out up top and my starting point will be the delorto guide.

It looks like (don't have my hands on one yet) the topmost bleeders will get filled with silver solder to accomplish this effect.

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