34mm Mikuni Upgrade

[CarbsandCylinders]

I apologize if these questions have been asked before. I searched and I could not find them in the forum.

I want to put 34mm Mikunis on my Titan. It already has expansion chambers and I will be having it ported. I already have a pair of 34's that were set up for a BMW R100. Can I use these on the Titan (after changing jets, needles, etc.)? I have read that there may be differences in 4-stroke and 2-stroke Mikunis that will make the 2-stroke carbs run rich at full throttle. Does this mean that the carbs I have will run lean at full throttle. From what I understand, the carbs can be properly set up for correct mixture except at full throttle.

If I can't use these carbs, does anyone know where I can get the proper 34mm's? I checked Sudco and Googled and cannot find a source for 34mm Mikunis that are set up for the Titan.

Thanks,
Brian

[tz375]

It's true that primary (2 stroke) type needle jets flow differently than Bleed (4 stroke) types, but those can be swapped out. Most aftermarket Mikunis come jetted with primary type needle jets and it's highly probable that the BMW carbs were like that.

The place to start is by stripping and cleaning the carbs you have and list all the jet numbers so you know what you have. That will give us a starting point to work out what needs to be changed. BTW, are we talking flat slides or regular round slide VM34 carbs? Please list the jets and their "type"
For example:

Main jet: #240 large Hex
Slow (pilot) jet: #45
Main air jet: #2.0
Needle jet: #159 P-8
Needle: #5DH3-3 (the last digit is clip position counted from the top)
Slide: #2.5
Float valve: #2.5

If in doubt take a picture.

[CarbsandCylinders]

Thanks, TZ. I will take them apart and let you know what jets, slides etc. are in them.

They are brand new carbs, I was going to put them on my BMW Cafe to replace the "fire starter" Bings but after I got them I decided that if I am going to change carbs on this bike I will go up to a bigger size, DelOrto's or Mikunis. Instead of trying to sell them I thought I would put them on my Titan.

Brian

[CarbsandCylinders]

Hi TZ,

This took longer than I expected. 10 days of flu and I can't read the small indentifications on the carb parts. I finally got a friends daughter with 16 yr old eyes to record them for me

Main jet: #260 Hex
Slow (pilot) jet: #35
Main air jet: #?
Needle jet: #159 Q-2
Needle: #6DH4-3 (the last digit is clip position counted from the top)
Slide: #2.5
Float valve: #?

This is what I have so far, I forgot to have her read the float valve and the main air jet.

How does this look?

Brian

[tz375]

Brian,

Well surprise, surprise, those are basically the settings that those carbs come with off the shelf.

The good news is that they are 159 series - two stroke type - primary needle jets. Ttitans with 34mm carbs came with richer N/Js and later models had smaller ones, so it's hard to predict where you will ned up. Good enough to get started though.

Stock pilot jets are a size smaller with stock exhausts and filter, so it's close enough to work with.

2.5 slide should be fine.

Air jet is 99% sure to be 2.0 and that's OK

Main jet will probably end up a size or two smaller, but it's better to start rich and move down. Some motors may need an even larger main, but I'd be surprised if yours does unless the pipes are just empty tubes and there's no air filters.

All in all, close enough to start with.

Eric( Zookie) and Pedro have more experience than I with 34mm carbs on that motor and probably have some good advice to share.

[CJF]

I can share my current setup. I have vm34's with jemco race chambers and velocity stacks...no porting (yet).

Main jet: 260 large Hex
Slow (pilot) jet: 45
Main air jet: 2.0
Needle jet: 159 Q-2
Needle: 6N01 3rd clip position from top
Slide: #2.5

The jetting is working great with the exception of a slight flat spot around 3/4 throttle when I open it up wide open quickly. I had a 250 main in and it was running lean for me...much better with 260.

[CarbsandCylinders]

That's great, thanks TZ and CJF too.

If I ever get over this stupid flu, the carbs go on right away.

I have an extra set of jugs and I plan to put Wiseco pistons and have them ported.

Brian

[CarbsandCylinders]

Hi TZ, or anyone else.

I put the 34mm Mikunis on and the bike starts on first kick but when I attempt to ride the bike it bogs a bit and then stalls. I have made no other changes to the bike. These carbs do not have a vacuum mounting for the petcock but I think that when set on prime, the carbs should be getting enough fuel although when it bogs and stalls it seems like it has run out of fuel. Is there something obvious that I might be missing? The only thing I can think of is that when on prime the fuel supply is too slow to let the engine run above idle.

[CarbsandCylinders]

I just thought of something, I assumed that the floats were set properly, from the factory, could they be off so much that they could cause this?

[tz375]

Yes, but it would be unusual. You have the same needle jet as CJF but #35 pilot jets. Try going up a couple of sizes to richen it up just off idle.

I don't have the details of that needle but 6N will have a much steeper taper and if it starts with the same root diameter it will be much richer further up. I don't know what the root diameter is, so I can't tell you much more than that.

[CarbsandCylinders]

Thanks Tz.

Is Sudko the best place to get needles and jets?