Cylinder not operating.

One of my cylinders (#4) is not firing on my 900LTD. I put my thumb over the spark plug hole and am getting compression. I am getting a spark at the plug, thanks to Patton et al. But the new spark plug I put in is shiny and dry and the #4 exhaust pipe is cold when the engine runs.

I am assuming there is a problem with the carb. Is there any way to check this? I reved it to 5000 rpm. I loosened the screw at the bottom of the float bowl and gas came out. I just cleaned the carbs, and all of the floats seemed to be at the same level before I put them in.

What do I do?

What color is the spark coming off of the #4 spark plug. You want a nice, deep blue color. A faded yellow/orange is a indication of a weak spark. Thats something simple to check.

No guarantee that it will cure the problem, but, try revving it up a bit and hold you hand over the intake on the carb. This will create a lot of the suction on the jets and if there is a slug of crap in one of them it may pull it out. Have you checked for a vacumm leak at the carb boot by spraying wd40 or carb cleaner to see if it affects the running any? Also, look across the carbs along the front edge to see that they are popped all of the way into the boots. don't ask me how I know this can be a problem,, :lol: :lol:

The spark is a deep blue. And I will try revving it. If I take the float bowl cover off of the bottom, can I just unscrew the jets and blow them out without removing the carbs? Would there be anything else to do with the bowl cover off?

Ray J wrote:

...(#4) is not firing on my 900LTD...getting compression...spark at the plug...new spark plug I put in is shiny and dry and the #4 exhaust pipe is cold when the engine runs... loosened the screw at the bottom of the float bowl and gas came out. I just cleaned the carbs, and all of the floats seemed to be at the same level before I put them in. What do I do?

Am assuming fuel will continuously flow from float bowl drain hole while petcock is open. This assures a continuous supply of fuel is at least getting into the float bowl.

Would suggest first performing the quick easy clear plastic tube test to determine the service fuel level in #4 carb float bowl. The fuel level might be too low for fuel to be drawn up into the carb circuits.

Have we ruled out the air leaks at the carb boots yet? You're concentrating on not enough gas, when too much air can do the same thing to your mixture.

Thanks for all of the input. Sorry it took me so long, but the bike was in a friends garage and I did not get the chance to work on it. I pulled the float bowl off of the #4 carb, blew everything out, though all of the orifices looked clean. I started it up, and #4 worked, but #1 did not. I took #1 float bowl off and no gas came out. It turns out valve needle that the float closes, sticks, and the bowl never filled. There were some nicks in the needle that came out with #600 grit sand ppr. Now everything works! Well... there is some problem with the mixture, but I am going to post that separately.

Anyways, thanks for all of the help. Now I have a better feel for yet another part of my bike.