Bike Runs, but Cyl. 2 seems dead

Bike: 1981 KZ650CSR
Mileage: 20K
Compression: Checks all across the board
Carbs have been cleaned and bench synced (redone so I can adjust idle)

So I got my bike running, and it fires right up, but Cyl. 2 is dead, always cold to luke warm on the header. There is spark there when I test it (could be intermittent once running?) This probably explains my doggy low rpm. It also Idles high even with the idle turned down as far as it goes.
I don't know where to start to diagnose the dead no. 2 Cyl. Either fuel or ignition. Any ideas? Thank You

I had a simialr problem on my '78 650, #2 was not firing at low rpms, idle was week, but once I got up to speed, she ran better.
What happened in my case was the diaphragm in the fuel valve was torn, letting gas leek into the vacuum hose which connects to carb #2.
Carb #2 would flood at low rpms, but once I opened the throttle further, letting in more air, #2 would kick in.
I replaced the fuel valve.

To confirm a dead cylinder/cold exhaust pipe, use a spray bottle of water instead of your hand. The water will flash to steam(212 F) if the cylinder is firing. If the water runs off, start checking what the problems: 1.Compression,Fuel,Spark,or Air(dirty air filter. If any of the 4 are missing, the engine will not run.

Would perform the clear tube test to assure that the fuel level is within specs on the carb that's feeding the non-firing cylinder.

Good Fortune!

After a 5 minute ride, I can hold my hand on the header on #2, 100% sure its dead if thats happening. I have full compression in all cyl.'s, Spark is good, and the air filter has seen its better days, but is still clean. I will check the fuel to see if its good in #2 carb with the clear tube test as mention by Patton. Hopefully it shines some light on this problem.

I suspect the carb too. But as a test, you may want to swap the spark plugs between 2 & 3 and hook the # 2 lead from the coil to #3 and the #3 lead from the coil to #2. If cylinder 2 is still cold and cyl. 3 hot after running the engine you can definitely rule out anything related to the ignition.

One other thought; have you checked your valve clearances? Ed

Check for a split/leaking or badly seated intake boot on that cylinder.
This will cause the high idle and reluctance to fire fully on that cylinder as it will be drawing in mostly air and not a full fuel mixture.

One other thought - does your carb set use an accelerator pump? If so, you may want to read through this thread (link below); both bikes had cyl #2 not working but they found a fix. Ed

forums.kz650.info/index.php?topic=7557.0

Would pull #2 spark plug immediately after shutting off the engine, and see whether it's wet or dry.

Tip: Avoid burning fingers by grasping the hot plug with a snugly fitted short section of rubber hose.

Dry could indicate a carb issue (absence of fuel mixture in the combustion chamber)

Wet could indicate a carb (mixture too rich) or an ignition issue (weak spark).

Let us know.

Good Fortune!

Very late reply, but the plug is dry and I am certain it is a #2 carb problem. It has a new accelerator pump rebuild kit in it and I can visually see the pump spraying fuel on the bench, so that rules that out. Now, I think my air screw passages are blocked. I can completely remove the air screw while the engine is running, and it makes no changes. I can make #2 run very briefly by screwing in the air screw, and then unscrewing it slightly. It will rev up for a second then die back down.

Three very fast and easy things to check.

1) put a shiny new spark plug into #2 -- don't re-use an old plug, we want to start with a shiny metal appearance so we can tell if it's burning something, you cannot do that with an already-used plug. Use a new plug here.

2) put the new plug into the spark plug cap and lay it against the cylinder, turn on the ignition, crank the engine over and see if you have a spark on the #2 plug. I suspect you will see okay spark there.

3) start the bike (with the air screw in a normal position, like 1 1/2 to 2 turns out or so)
let it run for a couple minutes, blip the throttle to draw some gas into the carbs (vacuum line
on the petcock only lets gas go to the carbs when engine is running)

4) shut off the bike and immediately loosen the drain plug and verify the #2 carb bowl has gas in it at all. This is just a quick 'sanity check' to see yes/no is that carb getting gas.


5) and pull that shiny new #2 plug and see if it's still shiny.


#4 and #5 above, all those will confirm is that you have gas getting to the #2 carb and whether or not the #2 spark plug is trying to burn fuel (getting brown). I'm guessing this: the spark plug still looks shiny and brand new, ie. not burning much of anything.


When you screwed in your air screw pretty far and it sounded better, that tells you you're getting too much air in the #2 carb fuel/air mixture and/or not enough gas in the float bowl.

SO TRY THIS.

With the air screw in a normal position, start the bike and *turn on the choke* and if #2 cylinder runs, that confirms you have too much air that is running #2 too lean because the choke when it's on decreases air flow and thus makes the fuel/air mixture more rich with gas.


I don't have my 650SR in front of me so I don't remember, but some carbs have 2 gas lines from the petcock to the carbs -- one fuel line goes to a 'T' in between the #1 and #2 carbs that feeds those 2, and the other fuel line goes to a 'T' between #3 and #4 carbs to supply those 2.

If you do not have an air leak, and you do have two fuel lines from the petcock, it almost sounds like the fuel supply to #2 carb is blocked.


If you *do* find gas in #2, then use the clear plastic tube technique to see if the fuel LEVEL in the float bowl is correct.

I suspect when you turn on the choke the #2 cylinder will fire -- if it doesn't, then you must not be getting gas into the #2 carb. Check the 'T' and the fuel line for that carb.

Since it is absolutely freezing out (had yet another blizzard today), I decided to take the carbs off and carefully go through them once again. I have an ultrasonic cleaner at work, and I will clean the carb bodies ultrasonically this time. Should make them like new right? I really hope to get this thing going come riding season!