82 Seca 650 - Cold misfire / intermittent tach issues

So ... long time listener, first time caller.

I'm having an issue where when the bike is cold (and I mean like, a couple degrees above freezing cold) I'm having a hard time starting. When you try to turn it over it sounds like it catches on 2 cylinders and runs at about 1k RPM with the choke on. After a few seconds it seems like another cylinder kicks in and it goes to 2k RPM. 30 seconds mode it seems like everything is running at 3k RPM. Now I'm reasonably certain all cylinders are firing, just not 100% of the time. When you take off after a couple minutes idle, it seems to be misfiring quite a bit, but once you go about 10 - 12 blocks you can hear that all cylinders are firing properly and the bike runs like a top after that. I usually use my bike to commute back and forth to work. When I leave work, the bike starts up fine (it's usally 15 - 23 C out by this time) and the misfire only occurs for a few seconds before it finally goes away.

Now as far as the tach goes, I was on the highway, after about an hour, the tach went nuts, then finally died sitting at 0. Since then it's been intermittent. On the cold mornings, it runs 100% fine, however, when I start my bike after work, the tach is dead and reads 0.

I can't help but think these two issues may be related. I did a resistence test on both ignition coils and both the primary and secondary windings appear to be within spec (2.7/2.6 ohms primary, 11.15/11.35 Kohms secondary). I did notice when I replaced the spark plug boots after testing the secodary coil resistence it seemed to improve the misfire condition but a noticable degree. I've checked all the spark plugs and swapped them all around, but they are all tan and seem to be in good condition. Have not been able to test spark because I can seem to get the ground right, however with the engine running I pull then replace the plug wire from each cyclinder and each time you notice the engine bog when that cylinder is not firing and return to normal whe reattached (yes I know this is bad for the ICs and pickups, but it was only way to test for firing at the time.)

This my third season with this bike, and I haven't had trouble starting in the cold in past years (at least not after I cleaned out the carbs). These issue seemed to start when I had to replace the bulb in the tachometer. Now I can see how maybe that would cause the tach to stop working, but I don't know how that would cause a misfire unless the tach lead from the electronic tach to the IC is somehow shorting out. I've taken a look at this but I can't see any issues so I'm kinda stumped. Any suggestions would be helpful.

Thanks

 

When you had to replace the bulb, did the wiring get pulled on at all? Maybe a connector in the headlight housing got partially pulled apart?

 

I've checked the wiring behind the headlight and everything seems to be intact. Manipulating the harneses back there does not affect the tach at all, so I think the issue is further down the line. The tach does not behave like there is a loose connection, it either works, or it doesn't. It seems to be a temperature thing but I'm at a loss as to what may be expanding/contracting to cause it to quit.

 

Well after further investigation, the tach issue appears to be temperature related. I started it on a cold morning and the tach worked fine. I took a heat gun to it and started it again, and sure enough the tach was not working. After letting it sit and cool down for an hour, I started it and the tach is working again. I pulled the tach off and tore it apart, reflowed the solder on the circuit board and tested the continuity of all the lines from behind the headlight all the way to the circuit board and it all checked out ok. I even ran a continuity check on the spiral windings around the needle spindle and that was ok too. At this point I'm at a loss as to what it could be. All indications should be that it should work but obviously something is expanding as it warms up and the system breaks down.

 

have you cleaned it out? Old lube oil and re lubed?

 

about the tach my 82 seca has a tach cable, it is not operated electrically

 

You have an American 650 Seca. The later Canadian-spec '82 650RJC Secas had a YICS motor, an electronic tach, and an oil cooler.