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1979 GS850 Ignition Coil Help

mpidsadny

Forum Apprentice
Hi All,

Fired my bike up yesterday after a summer of working on it. I've rebuilt the carbs, installed new airbox and intake boots/o-rings, checked for leaks while its running, re-shimmed the valves, all the usual GS rebuilding steps. It was put away in the first place because cylinder 3 was not firing and was running poorly in general.

The same issue came back where cylinder 3 would not fire. The [NODE="3"]Blogs[/NODE] plug cap can be removed while it's running without having an effect on the idle. Cylinder [NODE="2"]Forum[/NODE] is stumbling and the bike runs noticeably smoother when [NODE="2"]Forum[/NODE] plug cap is disconnected.

This is leading me to believe that the 2 - 3 coil is bad. When testing the [NODE="3"]Blogs[/NODE] spark on the block with a screwdriver, it looked very weak. When testing the coils, both primaries read 4.2 ohms and both secondary's read 12 kOhms without the plug caps installed. The plug caps (except for #3) are reading around 10 kOhms as well. I couldn't get a reading on it before I took it apart, but I'm assuming that it is similar as it still did deliver spark when tested. One thing I should note is that I am almost certain that the timing is off, but it is not far enough off for the bike not to run.

I am contemplating whether or not to order 2 new coils. The secondary reading is a bit low, but serviceable from what I've read. It could just be the plug caps, but I'm just not sure. If anyone could throw in their $0.02 I'd really appreciate it.
 
This 1979 gs850 still has points ignition ,right? Ignition coils are very durable, so I’d suspect plug caps before blaming coils. Try removing plug caps, snip coil wires back 10mm ..maybe also get new set of plug caps, as are inexpensive.
 
You should also run it in the dark & bend the plug wires around a lot whilst you do so. Sometimes there is a crack & the spark "escapes" but you can't see the crack or the spark escaping in daylight.

It would be odd for coils themselves to go.. I believe I have a known good set somewhere that I could part with if you're in need. I can check.

You could temporarily swap the coils over to see if the problem moves with it.... :)
 
You should also run it in the dark & bend the plug wires around a lot whilst you do so. Sometimes there is a crack & the spark "escapes" but you can't see the crack or the spark escaping in daylight.

It would be odd for coils themselves to go.. I believe I have a known good set somewhere that I could part with if you're in need. I can check.

You could temporarily swap the coils over to see if the problem moves with it.... :)

I just replied to another 850 owner with a similar issue instructing them to swap the coils... can't believe I didn't think of doing it to my own bike! I have a set of coils on the way just in case but I appreciate the offer. I also really need to figure out how to do the timing on this thing as I'm positive that it's out by a fair bit.
 
Static timing is pretty easy... The manual gives you the method :)
 
Could you try swapping the HT leads between 2&3. If the problem doesn't move the ignition system is off the hook.
 
Problem didn't move at all. But cylinder 3 appears to be firing now! Now my issue is a stumbling idle at at 1100rpm and the air idle screw adjustment have negligible effect on the idle. The tuning guide instructs to turn the screw out until the idle stops rising but the idle does not rise appreciably when I do this. If anyone has experience tuning VMs, please let me know how much of an effect the idle screw adjustment has on the idle. This bike is so close to running properly!
P.S. is it ok to lightly file/sand off the little bump that forms on the contact points? I think it's messing with my ignition point gap and therefore my timing.
Thanks for everyone's comments once again,
Mike
 
My initial thought is that you have an air leak somewhere. What are the lower screws set at?
 
Lower screws are set to 1 turn out on my vm26s on my 850. I sprayed carb cleaner around the boots and airbox and heard no difference in idle.
 
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