Left Cylinder Intermittent Firing Issue

[Peter Ronney]

Hi everyone! I recently got my 1978 XS400 running after sitting for 2-3 years. I've been driving it a little but I can't help but notice the left cylinder fires intermediately. It smells like a wayyy too rich mixture coming from the pipe, the plug is black with soot, and the exhaust pipe doesn't get as hot as the right one which fires fine. My buddy has been working on his XJ650 and recently needed to change his plugs out for 0 resistance ones, he actually had a similar problem where two of the cylinders were firing intermediately. He has electronic ignition on his bike whereas mine has points. Could this be a source of error? What resistance plugs and caps should I be using? Could the mixture be off and if so how could I adjust it? Could it be valve gaps? Or something else?

 

[amc49]

The points not set right can most definitely do that. I use zero resistance on everything myself. Of course if carbs not right then they will drive you nuts with issues like that.

 

[Peter Ronney]

The points not set right can most definitely do that. I use zero resistance on everything myself. Of course if carbs not right then they will drive you nuts with issues like that.

I tried redoing the points with a static timing light and it got a little better. I'll try some zero resistance plugs.

 

[amc49]

Your point DWELL (gap) is more important than the timing is, it is what determines how hot your coils fire. Set gap FIRST then timing.

 

[xschris]

Also make sure you sync the carbs with a manometer.

 

[Peter Ronney]

Update: I changed the plugs to zero resistance ones and no change. It’s gotta be the mixture. How do you adjust the mixture on these carbs? They’re the makuni ones with no brass screw on the side. On the engine side there are plastic screws but turning them only changes the idle speed.

 

[Peter Ronney]

Also, removing the right plug also revealed it is black with soot.

 

[xschris]

Post pics. Those on the top of the carbs with the plastic caps are the pilot mix screws. You need to remove the caps and the screws should come out. Check to make sure the tips of them are good and are not broken off into the bodies. This is very common. Make sure to lightly seat the screws when you reset them.

 

[Peter Ronney]

Take the caps off? I could give that a go. They look to be in good shape. All the way out (as far as they can go counter clockwise) and the left one still backfires. Turning it in just smells like I dumped the tank on the ground.

 

[xschris]

Where are the clamps for the carbs? You need them. Also if your intake boots are bad those need to be replaced.

 

[Peter Ronney]

Where are the clamps for the carbs? You need them. Also if your intake boots are bad those need to be replaced.

Working on that atm, they’re ordered. But the mixture is wayyy too rich, these would just make them leaner with more air right?

 

[xschris]

Not having any clamps will cause all sort of issues. I would set the mix screws to 1.5 turn out to start with. What size jets are in the bike? You should have 132.5 main and 42.5 pilot. The slide needles should be on the middle or third clip from the top. Make sure the floats are set to 26mm.

 

[Peter Ronney]

Not having any clamps will cause all sort of issues. I would set the mix screws to 1.5 turn out to start with. What size jets are in the bike? You should have 132.5 main and 42.5 pilot. The slide needles should be on the middle or third clip from the top. Make sure the floats are set to 26mm.

Thanks for the dimensions! I’m not sure, as soon as my new intake boots come I’ll be taking the carbs apart again and I’ll check.

 

[amc49]

(shaking head sadly)

You can't run the engine without clamps on the carbs.

Yes it leans out wildly but once you do combustion quits then any fuel that DOES come in does not burn and the missing you are seeing as well as artificially colored plugs due to no clamps. More airflow in the leak kills intake turbulence and then the gas does not vaporize and does not burn either, gets on plugs to wet them black. The CVs also quit opening slides to go rich as well, the lost airflow going past intake leak no clamp then would not lift slide.

You're asking for things that have nothing to do with your issues which were the clamps likely to begin with.

 

[Peter Ronney]

I put the clamps on it, didn’t change a thing.

 

[xschris]

Verify carb setting and jets sizes. When you put the new intake manifolds and gaskets on sync the carbs with a manometer. Did you sync before with the clamps off?

 

[Peter Ronney]

Verify carb setting and jets sizes. When you put the new intake manifolds and gaskets on sync the carbs with a manometer. Did you sync before with the clamps off?

Sync? Please elaborate

 

[xschris]

The carbs need to be vacuum synced to each other to make them work equally. Not doing this will cause popping missing and poor performance. You can make a manometer for a few dollars. There are plans all over the web and a few on this forum as well. The simple "U" type is the easiest. I use a set of old mercury ones which are very accurate but those are hard to come by now.

http://www.xs400.com/threads/my-experiences-with-carb-tuning-my-xs400-bobber.5241/#post-55739

 

[Peter Ronney]

So I went ahead and removed the caps off the adjustment screws and backed it out quite a bit. Better, but not much changed. Still pops and backfires when it’s reved. I noticed one thing since it was cold today. There’s a large rust hole in the bottom of the left exhaust which I thought wouldn’t be a problem however some smoke was coming from the right pipe directly out of the pipe that doesn’t have a hole in it. Could this be messing with back pressure or making a mixture in the pipe?

 

[amc49]

I for one am done here, this one is getting ridiculous. No clamps and it works fine. Adjusting mixture screws for range they have no effect over. Exhaust back pressure and 'mixture in the pipe', this one is lost and needs the 500 page book. Y'all enjoy doing that.