Xs650 backfires at 2k-3500 rpm on left side

Okay another update for everyone. I completely disassembled the carbs gave them the best cleaning of their lives removed every single thing I could blasted with carb cleaner and compressed air. (I've taken photos this time lol) got them back on the bike she starts up and runs pretty good. Mix screws are at 2.25 - 2.5 turns out. Carbs seem to be synced I did a test with a flashlite upon reassembly to make sure they were both fully closed and opened the same amount when I moved the throttle. (I removed the vac barbs so I'm not fully sure how else I can sync them) took the bike for a 2 hr ride only a tiny bit of back fire upon deceleration (not every time) but she pulls hard with no flat spots. Tonight I'm going to check the valve spacing and make sure it's within spec but I think the problem is much closer to being solved. I'll attach the pics of the bike and the carbs.
 

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If you haven't replaced the butterfly shaft seals, you may never get the carbs to work perfectly. Due to their age, pretty much all these carb sets need them. New, they are very soft and pliable. They are a rather loose fit on the shaft and into the carb body and air rushing through the main bore sucks them in tight and seals them - if they're still soft.
 
Shooting in the exhaust can be an air leak in the Exhaust. A mixture on the " Rich " Side which I believe other things considered is good
( Dont call environmentalists )
If an air leak it can suck in more air via ejector functionality ( I Believe ) ignite in the ,, Exhaust which the extra Oxygen taken in at the leak
:Without that it cannot ignite.
 
Well the Neverending issues seem to be continuing. Bike was running better or so I thought... that us until it wouldn't kick over at all this past weekend. I pulled the plugs and they were not looking good. I am thinking I might be running a little too lean on the pilot jet but running too rich on the needle setting but I am honestly not really sure.
 

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OK, first thing - you're running the wrong plugs. You should be running 7's which are one step hotter than the 8's you currently have. They will burn cleaner and give a better color. Next, you have a resistor plug. If you also have resistor plug caps, that may be too much resistance in the line and could be choking off the spark. You only want one instance of resistance in each plug line, cap or plug but not both.

On your '78-'79 carb set, you must lean the needle setting a step if you increase the main jet size at all. Stock needle slot setting was the middle or #3 slot. Stock main size was 135. If your mains are larger than that, you'll need to set the needles in the #2 slot (from the top).

Needle Slots.jpg
 
Are your pilots stock now (27.5)? They now make a 32.5 (2 sizes up) and sometimes you need that on the '78-'79 carbs. 35's are usually too big. The 32.5 may not be easy to find. It's a relatively new size. They used to jump from 30 to 35, then 35 to 40, but Mikuni has started making 32.5 and 37.5 now.
 
Okay another update for everyone. I completely disassembled the carbs gave them the best cleaning of their lives removed every single thing I could blasted with carb cleaner and compressed air. (I've taken photos this time lol) got them back on the bike she starts up and runs pretty good. Mix screws are at 2.25 - 2.5 turns out. Carbs seem to be synced I did a test with a flashlite upon reassembly to make sure they were both fully closed and opened the same amount when I moved the throttle. (I removed the vac barbs so I'm not fully sure how else I can sync them) took the bike for a 2 hr ride only a tiny bit of back fire upon deceleration (not every time) but she pulls hard with no flat spots. Tonight I'm going to check the valve spacing and make sure it's within spec but I think the problem is much closer to being solved. I'll attach the pics of the bike and the carbs.
My ‘76 is running well (knock on wood), perhaps a bit on the rich side but that’s ok with me. I do get some popping in the exhaust on deceleration but I’m not going to mess with it anymore. I’m just happy it’s running well.
 
Are your pilots stock now (27.5)? They now make a 32.5 (2 sizes up) and sometimes you need that on the '78-'79 carbs. 35's are usually too big. The 32.5 may not be easy to find. It's a relatively new size. They used to jump from 30 to 35, then 35 to 40, but Mikuni has started making 32.5 and 37.5 now.
Right now I believe they are 30's I'm going to see if I can find the 32.5s
 
Okay so I'm trying to find the correct plugs and non resistor caps I have found some plugs available from xs650 direct I'll attach the image but for the non resistor caps I'm having a hard time finding any that don't require the threaded stud type of plugs. Does anyone know if they can be used on either kind.
 

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Okay so I'm trying to find the correct plugs and non resistor caps I have found some plugs available from xs650 direct I'll attach the image but for the non resistor caps I'm having a hard time finding any that don't require the threaded stud type of plugs. Does anyone know if they can be used on either kind.
Those caps will do just fine. The ferrules on the end of the plugs unscrew and there's your threaded stud to fit in the caps.
 
Before fitting the larger pilots, I'd change the plug caps, install the correct heat range plugs, and insure the needles are in the #2 slot. That may fix the issue for you without the need for that even bigger pilot.
 
Okay I've moved the needle into the number 2 spot (it was at the number 4 spot) on both carbs fitted the new plugs and correct caps and the bike barely even wants to start. I got it to fire up but it won't idle without the choke on anymore and dies. I kicked it over a few more times and have seemed to of flooded the engine. I'm going to let it sit for a while then try again. This bike will be the death of my it seems to just be one issue after another. It ran decently for 150km now I have no idea
 
Your needles were set very rich and maybe the idle was turned down to compensate. You may need to go through the basic carb tuning steps again, set idle speed screw and mix screws.
 
Your needles were set very rich and maybe the idle was turned down to compensate. You may need to go through the basic carb tuning steps again, set idle speed screw and mix screws.
I have the mix screws set at 2.25 turns and the idle screw I was setting when it died. I've tried to start it now a couple hours later (kick start only)and no luck won't fire up at all. I have a number 30 pilot jet and a 140 main.
 
Jetting sounds good, it should run OK with that. I'm running the same in my '78 and have been for years. Maybe you've fouled the plugs?
 
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