Jetting questions

[roach61]

Hello all! So I got my hands on what I believe to be a 73 xs650. It has a 2 into 1 with a baffle and uni pods. The carb is a mikuni VM?? I'm having trouble figuring that out. It has a 25 and 180 with the needle on the second clip from top going down. The airscrew is all the way seated. She runs and idles no problem. Even starts on first to second kick cold. But she has a bad stumble at wide open. Then shoots like a rocket. I can slowly ease on the throttle and get her running threw all the jets with no problem. It's when I pull hard it stumble at the top end.

Sorry for the ramble thanks for the read! Open to any ideas and suggestions.

 

[roach61]

Some pics so you know it's real

 

[gggGary]

Couple hints say it IS a 73
First off it's not a CV, If you ain't older'n dirt you may never have had to worry about throttle opening speed.
Can't just wack open a VM Use a smooth roll on, that make a difference?
Sync the two carbs. It's more art than science on a twin VM set up.
While it's running spray a bit of carb cleaner around those intakes, if the idle changes you may have an air leak.
then;
Raise the needle a notch, see if it's better? If it is better try another notch. Go til you've gone to far, then back down a notch.

 

[gggGary]

That bike still have vacuum petcocks?
Is that vacuum line on the RH intake going somewhere, or plugged off?

 

[Wingedwheel]

I see a compression release valve cover so chances are it’s a ‘73 unless some fool PO put that cover on there.

 

[jpdevol]

Appears to have a later model coil though, perhaps not stock ignition?

 

[Wingedwheel]

Def a ‘73 side cover also. I like the copper Union on the gas line also.

 

[gggGary]

Looks like it's sitting with one choke on, one off.................

 

[Wingedwheel]

And two left hand carbs

 

[5twins]

No, actually I think it's two right carbs.

 

[roach61]

Couple hints say it IS a 73
First off it's not a CV, If you ain't older'n dirt you may never have had to worry about throttle opening speed.
Can't just wack open a VM Use a smooth roll on, that make a difference?
Sync the two carbs. It's more art than science on a twin VM set up.
While it's running spray a bit of carb cleaner around those intakes, if the idle changes you may have an air leak.
then;
Raise the needle a notch, see if it's better? If it is better try another notch. Go til you've gone to far, then back down a notch.

Thank you! I spray carb cleaner around and no leaks to be found.

I synced the carbs using a vacuum gauge.

I synced the throttles by watching them open and close. Also measured the "freeplay" of them when I tug on the cable on top.

 

[Wingedwheel]

No, actually I think it's two right carbs.

Why is the choke lever on the left? I do see the open barb though.

 

[roach61]

That bike still have vacuum petcocks?
Is that vacuum line on the RH intake going somewhere, or plugged off?

It doesn't have a gas tank at this time. I am using auxiliary fuel tank atm. The lines are going to a vacuum gauge.

 

[roach61]

Appears to have a later model coil though, perhaps not stock ignition?

It doesn't appear to be stock when I opened it up to check timing. I used a timing light and it looks good. I'll take a pick of it tomorrow morning.

 

[roach61]

Looks like it's sitting with one choke on, one off.................

Pretty confident it is not but I'll keep that in the back of my head.

 

[roach61]

No, actually I think it's two right carbs.

Uuuhhhh hahha how do I tell haha also would that be a problem?

 

[roach61]

Why is the choke lever on the left? I do see the open barb though.

Open barb?

 

[jpdevol]

I'm having trouble figuring that out. It has a 25 and 180 with the needle on the second clip from top going down. The airscrew is all the way seated. She runs and idles no problem.

Maybe remove air screws and take a pic - looking for step wear - as it shouldn't idle well shut off. Also spray some contact cleaner into the air passages (I'd remove the float bowl to see if it's clear that direction too).

Your running symptoms indicate leaness (though they won't take huge abrupt throttle as Gary says). I'd first richen needle to #4 slot to try. Then maybe a 190 main if that doesn't do it. It's a process.

 

[gggGary]

Uuuhhhh hahha how do I tell haha also would that be a problem?

No. just makes the air screw kinda hard to get at on the LH carb.

I do see the open barb though.

that's a vent should have a tube headed down but not a big deal.

 

[jpdevol]

https://www.mikuni.com/pdf/vmmanual.pdf

http://www.amckayltd.com/VM34-36.pdf