Proper Jetting and Fuel/Air Screw Position Mikuni VM-34 on a 1972 Yamaha XS2 (650)

[Hanniusms]

I’ve posted a few times about this bike. Bought it a few weeks ago. Did quite a bit of road worthy maintenance / repairs on it before I could take if for its first test ride.

Idles good, low throttle stutters and extremely sluggish, 3k and above seems to run good.

Pulled right carb tonight to check jetting and air/fuel screw and overall cleanliness of carb.

This is what I found:

Main Jet #175
Pilot Jet #25
Fuel/Air screw .5 turn from closed

Carb seemed mostly clean had some crystal residue in bowl area and the intake was filled with an oily black layer from filter side.

Have a sonic cleaner so will do 2-3 passes on each before I put them back in.

What is proper jetting for the carb bike combo??

I know the F/A screw is way off. Should be 1.25 to 1.5 turns out correct??

I live in Tucson and elevation is 2.5k feet give or take 500 around town.

 

[5twins]

If you've read the VM carb guide you were directed to in your other thread then you know your jetting is very much "in the ball park" for what works in these carbs on this bike. You will want/need to check the needles and needle jets to see if they are what is recommended, also needle clip setting.

The fuel/air screw you refer to is strictly an air screw on this carb. That means opening it up more adds more air and leans the idle circuit, closing it cuts the air supply and makes it richer. There is also an air jet mounted in the intake bell. Normally for 650 use, you remove it and run without an air jet installed .....

So, after verifying that the needle and needle jet are correct, noting needle clip position (the middle slot is recommended I think), and making sure the air jet is removed, I would reassemble the carbs and play with the air screw setting, opening it up to somewhere in that 1 to 1.5 turn range (make it the same on both carbs), and see what you've got. Next step would be a pilot jet size change, either one up or one down. I would base my choice on what effect leaning the air screw setting has.

 

[WideAWAKE]

Need to know your needle, Needle jet and slide size.

You’re probably a bit too big on the pilot.

 

[Hanniusms]

If you've read the VM carb guide you were directed to in your other thread then you know your jetting is very much "in the ball park" for what works in these carbs on this bike. You will want/need to check the needles and needle jets to see if they are what is recommended, also needle clip setting.

The fuel/air screw you refer to is strictly an air screw on this carb. That means opening it up more adds more air and leans the idle circuit, closing it cuts the air supply and makes it richer. There is also an air jet mounted in the intake bell. Normally for 650 use, you remove it and run without an air jet installed .....

View attachment 263292

So, after verifying that the needle and needle jet are correct, noting needle clip position (the middle slot is recommended I think), and making sure the air jet is removed, I would reassemble the carbs and play with the air screw setting, opening it up to somewhere in that 1 to 1.5 turn range (make it the same on both carbs), and see what you've got. Next step would be a pilot jet size change, either one up or one down. I would base my choice on what effect leaning the air screw setting has.

Need to know your needle, Needle jet and slide size.

You’re probably a bit too big on the pilot.

I think the needle clip was on the 1st or 2nd setting when I installed the new throttle cable.

I’ll move it to the middle and start there. Haven’t had a chance to work on the carbs these past few days. Will go through it tomorrow.

I bought the Sudco Mikuni carb manual but it seems 1/4 manual and 3/4 parts catalog to me.

Not much on jetting and altitude / temp charts. Or a chart on CC engine size vs jetting either.

Most of the carb work I’ve done in the past has been cleaning old carbs, rebuilding with Keyster kits specific to the carb / bike / ATV combination and going from there tuning them.

This is new for me.

I appreciate all the suggestions and input.

 

[5twins]

This is a better guide .....

https://www.xs650.com/attachments/vm34-36-pdf.248257/

 

[5twins]

And just so we're on the same page here, Mikuni needle clip slots are numbered from the top down .....

Also, be sure to record your needle and needle jet numbers when you're in there changing the needle clip position. Make sure they jive with what's recommended in the guide I linked you to above.

 

[Hanniusms]

This is a better guide .....

https://www.xs650.com/attachments/vm34-36-pdf.248257/

Thank you!

 

[Hanniusms]

I pulled the needles tonight. They are 6F09s so that’s a good start. They were clipped on #2 so I moved both to #3 slot.

Will test ride tomorrow like this and with air screw @1 turn to 1.25.

Let’s see how it goes

Picture 1 - How I found the needles
Picture 2 - Adjusted clip

 

[5twins]

You'll often find the needles leaned a step like that to fix stumbling and break-up in the upper midrange brought on by larger mains bleeding over into it and making it too rich. To test for this, in 2nd or 3rd gear from about 3 to 3.5K RPMs, grab a big handful of throttle and run the engine up through the upper midrange and onto the mains, say to around 5.5K RPMs. Watch for break-up or stumbling around 4 to 4.5K RPMs. If you get it, the needles will need to be changed back to that one step leaner #2 slot to fix it.

 

[Hanniusms]

Sonic Cleaned the carbs. Put everything back together. Sync’d them, the left was taking off slightly before the right.

175 main
25 pilot
Needle on 3rd position
Air screw 1 turn

Ran up and down the street to warm it up.

I can’t post the Idle video I took. Don’t know how to post short videos.

Idled between 1k and 1.2k up and down.

Pulled plugs both look like lean mixtures.

Going to take air screw down to 3/4 on both sides.

 

[Hanniusms]

UPDATE:

Upped the main jet to 180 and upped the pilot jet to 27.5 today.

Bike ran a bit smoother but back fired at times. Still has a dead spot between 1/4 & 1/2 throttle. Bucks a bit and then the power comes on 1/2 throttle and above.

Stumbles when off throttle and letting engine brake bike also bucks a bit.

Adjusted idle screws and air screws to stabilize idle around 1200. Initially bike would not idle unassisted.

Now it does and will idle fairly smoothly (goes up and down from 1200 about 100 rpm in each direction) not quickly just a slow cycle like its hunting for a stable position. But it will remain idling unattended (had it sitting there about 5-7 minutes just idling by itself).

Air screw is @ 3/4 - 7/8 out.

Pulled plugs and they still show lean mixture.

I don’t know if I should go bigger on jets or turn down the air slightly??

No fuel smell from exhaust so I guess I could go up on jets maybe to 190??

What do you guys suggest??

 

[WideAWAKE]

still never answered what needle jet and slide you have.

It all makes a difference.

 

[WideAWAKE]

Do you have filters on it?

 

[Hanniusms]

still never answered what needle jet and slide you have.

It all makes a difference.

Are you referring to the round slide??

I don’t know. Is there a model number on it? I was assuming all VM-34s had the same slide in them.

Have not checked needle jet. Don’t know answer to that question. Will look tomorrow.

 

[Hanniusms]

Do you have filters on it?

Yes they appear to be the barrel K&N ones that come in the usual kit.

They are clean and they do not have a lip on them blocking any ports.

 

[Hanniusms]

Do you have filters on it?

Yes have filters on it. They seem to be the K&N filters that come in a kit. They do not block the air passages on the filter side of carbs.

Where is the needle jet? I only see the pilot and main inside the bowl section?

Slide? As in throttle slide? I didn’t know there were options variants on those.

 

[WideAWAKE]

#11 is your needle jet.

Slides come in different cutaways.

It should be stamped. Probably a 2 or 2.5.

 

[Hanniusms]

#11 is your needle jet.

Slides come in different cutaways.

It should be stamped. Probably a 2 or 2.5.

I’ll get the info tomorrow and post.

 

[Hanniusms]

Ok pulled 1 carb apart tonight and this is what I have

Slide - 2.0
Screw Jet / Needle Jet - 159 P-6

Don’t know what it means

 

[Hanniusms]

Am I good with those or do I need to adjust??