missing and running on 3 but occasionally 4

[Rob_K]

Hi there, 

my gs750 has started running a bit strange, so first off I checked valve clearances and had to do a few changes to bring them into the right range, most were ok, but 3 were definitely on the tight side. Next I've looked a plugs after running to find left two pots seem to be running fine, but the on the right the outer one always pulls out a wet spark plug, and the inner one seems to be running ok, if a little lean. Checking spark, they are yellow not nice and blue, but it's a new old bike to me so not familiar, my old fireblade has rich blue sparks. Plugs do look ok to me, but will of course change them if advice says just get that out of the way. 

I still have points in this thing but I wanted to rule out based on expense really, so the next thing was to check the coils and this is where it's a little weird based on the fact that the bike does run, if a little rough. Both coils have approximately 4.5 Ohms across the terminals, I hear between 3 and 5 is what is required, so that's ok, but neither of the coils have a reading from either plug cap to either of the terminals when I would be expecting maybe 10k Ohms. But the bike runs,  is this me being crap at testing, or is this the cause of the yello spark and a new set of coils will definitely help the issue? Was kind of expecting that no reading between HT and LT would give not spark at the plug. Appreciate some understanding around this please

 

[Captain Chaos]

1 hour ago, Rob_K said:

neither of the coils have a reading from either plug cap to either of the terminals

AFAIK there should not be. The low tension (terminals) and high tension (plug caps) are separate coils, they should not make contact.

4,5 Ohms between the terminals is ok and between the plug caps should be around 10-15 K Ohms at least.

Surely an adult will correct me soon. On the beer now (Grimbergen Double Ambree).

[prutser]

+1 that there indeed shouldn't be any connection between primary side (the 12V terminals) and the secondary side (spark plug caps).

Secondary side is measured from cap to cap, and if i remember correct, should indeed be something in the 10k - 15k range. (Workshop manual should tell.)

Now  beware, as there are also plug caps out there with a built in 5k (or 10k) resistor for interference suppression. In this case, add these values to the expected reading. (Unless manual already assumes these caps in the specs of course.)

Other setups might not have this resistance in the caps, but use plugs with built-in resistors (those NGK's with the big 'R' on them, for example). Others might have none of the above and simply don't care about radio interference.

If in doubt, remove the caps from the cables and measures coil and caps seperately.

And i advise to wiggle the cables around once you got the test leads decently connected to the cable ends, to check for internal breaks.

[Rob_K]

Right then a new set of plugs seems to have solved the problem, just went on a run and it's now running smoother. Going to give new air filter, clean carbs out and balance them and I think I should have a nice running gs. I have some new points and condensors, but i'm going to keep them as spares now and just carry them with me, I've also got a new set of coils coming as they were cheap, but I think the old ones are still ok. 

Last bike I owned was a 94 fireblade, and I fixed up everything in the first year I bought it and serviced it regularly changed oil every 6k and it just worked. I'm having to go back into the learning world of the 18 year old with an older bike that needs a little tender care, but today when it started running nicely it felt so nice. I do have some work to do on this thing now, but I'm happy, this is it, I love how dated it looks