Hello!
I have a (new to me) 1998 LS650 that was given to me by a friend who was frustrated because they couldn't get it to run past 3rd gear, no matter how many trips to the shop.
It is orange, but the paint on the sunny side has faded to salmon.
Long story short, I found a bad decompression module, but the most annoying thing was the float was set all the way down so it just filled the case with gas!
Got a replacement non-vacuum petcock, and set the float to the correct level. Then it wouldn't get any fuel in the bowl, no matter how I adjusted the float level.
Tuned out the seat was actually loose and dropping down with the needle (which was probably the issue that got it parked).
I removed the little fork that retains the seat, put it in the vice and put a slight bend in it give some preload.
This worked, fired right up!!!

My main question is this... the Mikuni is the first vacuum slide carb I have ever messed with. It feels really sluggish compared to a direct action cable float, although when the engine is under load it makes a crapload of torque.
But it falls on its face in the higher RPM's though.

Pretty sure it is running lean (I will drive it around and pull the plug for a read).

Are these carb's naturally lazy, or it is mixture the issue do you think?

Kinda wish I could find the same cfm carb in a mechanical version!

Thanks!
Mark

Also, understand that the throttle opens a butterfly valve and vacuum opens the slide.  So you need to make sure the slide and carb are clean, that the diaphragm is not damaged, and it is seated correctly.  Sticky slides lead to sluggish performance.

While you're at it, make sure the air jets in the top of the carb are correct (something like 230 and 45).  We have seen many where the jets were swapped from top to bottom.  Meaning the 52.5 pilot jet and 145 main were found in the top of the carb.

If your at 7000 ft you may need to move to smaller jets ,stock pilot jet is a 52.5 ,you may want to try a 50 ,main jet should be 145 which should be OK. The mixture screw should be set for best idle which means in your case you may be turning it in rather than out as the mix tends to be richer at your elevation.(the same reason to use the smaller pilot jet).

My house is at 7400'.  Since I removed the plug and adjusted the air/fuel mixture screw, my S40 has no problems with running or backfiring, etc. It is all stock, including carburetor, air filter and exhaust.

That's what I thought, but I am surprised at the number of
Savage/S40 owners who have carburation problems at even lower altitudes.