1981 Yamaha XS400 Special - Intermittent Spark on Left Cyl at Low RPM

[XS4004lyfe]

First off thank you for clicking on my post to see if you can help, I really appreciate it. REALLY.

So, the bike has a misfire on the left cylinder when rpms are between 1k to 2k.

I have ruled out the following;
Spark Plugs, Ignition Coils, Sparkplug Boots.

What would you check next in respected order?

If I crank the throttle at 1k rpm it dies, but if I blip it enough to get it to 2k I can can rip on it all day, so riding around really ain't bad since I dont lug my engine ever.

Another weird thing I've noticed is when I start it on full choke it does a normal 2k rpm as youd expect then maybe 10 seconds later sky rockets to the moon around 4 to 5k rpm. If I start it on half choke it's now doing the same thing sometimes. It does not do this on no choke.

I am 90% sure it's a spark issue at low rpm, but only real way I've been able to test is by holding spark plug on motor head which in my opinion isnt a really great test. If I didn't test spark I'd probably assume it's a fuel carb issue.

Anybody have similar issues? Shit goes fast above 2k rpms.

Extra info: only 4k miles. Both exhausts get hot at idle but if I pull the left ignition coil off the plug it doesnt change, but if I pull the right side it shuts off immediately. Owner says it fell up against a fourwheeler on his trailer and dented the tank ( same side as the misfire ), blinkers act funny and are hard to use, everything works MINT except for the misfire. Horn didnt work when I got it, works now with new horn. Compression is the same on both sides, runs great at high rpms.

Open to any questions.

Here she is

 

[xschris]

Sounds like the ignition box. I had this with my 80 for a while and thought it was the carbs. I replaced the box and the problem was gone. Have you synced the carbs with a manometer? If they are off your one side may be closing too much. Just something to check that is easy to do.

 

[XS4004lyfe]

I will have to get a nanometer and check, thank you.

Also do you know the spec resistances and stuff for a good igniter box?

Sounds like the ignition box. I had this with my 80 for a while and thought it was the carbs. I replaced the box and the problem was gone. Have you synced the carbs with a manometer? If they are off your one side may be closing too much. Just something to check that is easy to do.

 

[XS4004lyfe]

Anybody else got ideas ?

 

[xschris]

The manual has the specs for testing the ignition pickup. That's at the end of the cam shaft under a small cover. The are threads on is also. There are no tests or specs given for the ignition box itself. You can open it and look for loose connection or and damage. There was a member a long time ago that re-built his bad one but I don't remember any more info than that. Use the search function and you may be able to find it.

 

[XS4004lyfe]

I had trouble getting the tamper caps off the pickup cover.

The manual has the specs for testing the ignition pickup. That's at the end of the cam shaft under a small cover. The are threads on is also. There are no tests or specs given for the ignition box itself. You can open it and look for loose connection or and damage. There was a member a long time ago that re-built his bad one but I don't remember any more info than that. Use the search function and you may be able to find it.

 

[xschris]

You can test the pickup without removing the cover. If it's bad of course you would then need to. There are threads on that.

 

[XS4004lyfe]

You can test the pickup without removing the cover. If it's bad of course you would then need to. There are threads on that.

It tested fine, I took it for a full range ride today and it seems to back fire like CRAZY out the misfiring cyl. I really am leaning towards carb issue because I also had it at 4k rpm and pulled the left plug and it went to a normal 2k idle youd expect out of full choke.

 

[XS4004lyfe]

I fixed it! Turns out it was the damned carbs.

So the throttle slide was basically stuck, it didnt have good action and since its driven by air coming in the slide has to be fairly easy to slide up and down. Mine must of had dirt or something stuck up in it because it really did not like to move OR return to a resting position. I pulled it out, cleaned it real well, oiled it up and put it back in! Fired her up and she died.. died.. didnt put idle screw back up! Fired up and she LIVESSSSS. great throttle response!

 

[xschris]

Awesome!! Might want to make sure the rest of the carbs are clean also.