91 octane or 87 octane for 1972 XS2 with Mikuni carb conversion??

[Hanniusms]

This is my first XS2, I picked it up a few weeks ago. I took care of quite a few maintenance issues that needed to be repaired (rear brakes, sprockets, chain adjustments, new throttle cable and throttle tube assembly, etc) before I got it road worthy.

I got it on the road for the first time today. It has an extremely sluggish and jerky throttle response through the low RPMs to about 2500, (but it idles smooth?) and then it runs good and strong in the higher RPMs.

Running 91 octane in it currently and I don’t know if it’s the octane or the valves, I haven’t checked the valves yet. I know some guys with old bikes and they say you need to run 87 in them that the 91+ screws with performance.

The carburetors are completely clean. I don’t know if the fuel air screws are adjusted correctly and I don’t know the specs for these carburetors, they are not stock Mikuni’s. I’m gonna put some pictures up and see if any of you guys can help me identify what carbs they are, and what’s specs those carbs would have.

Thanks in advance!

 

[Signal]

The octane rating is a guide to how fast the fuel burns. Higher octane number fuels burn slower than lower number fuels. I think some members here work in the oil industry so will be able to give more in depth information.
I have found changing fuels when refilling has no impact on the way the carbs are set up
In the tech section of this site is the VM carb guide, it was a great help help to me.
Assuming your ignition is working in top order I think your carbs are not set correctly because your bike runs differently at different rev ranges.
The poor throttle response could be the carbs are not balanced. All this is covered in the guide.

 

[Hanniusms]

Thanks for the reply. I realize the carbs are most likely not tuned correctly. They are synced tho. I did that already.

But not knowing the model # of the Mikuni carbs I don’t have specs for their fuel air screws to get them balanced.

If anyone knows what they are (the previous owner told me they were the usual upgrade mod these bikes go through) that’s all he knew tho as the owner before him did the swap.

Regarding octane do you run 87 or 91in your bikes?

Thanks in advance,

 

[Hanniusms]

So I did some Research and I think they may be

Mikuni - VM34 Carbs​

Does this sound right?

 

[Sparkyduc]

So I did some Research and I think they may be

Mikuni - VM34 Carbs​

Does this sound right?

Bet they are

 

[Signal]

I run 91 or higher. My bike is not stock and has higher compression I prefer 95. Your carbs certainly look like VM34's the VM 36 carbs don't have the notch shaped bracket on the rear of the slide housing.
VM 34 is the most common VM XS 650 conversion.
If you are running pod filters check that they are not blocking the air jets at the rear of the carb. Cheap pod filters have a mounting flange that covers the air inlets on the rear of the carb.