1983 GS850 Intermiteent Misfire.. Argghh please help!!

[snowgoose]

Hey Gang

Great site, so a big well done to all, cheers. ..apologies in advance for such a long post

I Have got an 83 model GS850 and I am pullin my hair our trying to sort this intermittent misfire which appears to be on Cylinder 4, thats the one on the far right when sitting on the bike.

Ok here's the history:

When purchased, the guy said he thought it needed a new carb for cyclinder four. So anyways these are the steps taken so fare:

1. Rebuilt the carbs on there with new jets, float needles etc etc.
2. Replace intake oring seals as they were stuffed and were previously repaired with silicone (it was a bloody horrible repair).
3. Relaced Carb to Head boots.
4. Replaced Carb to Air Box boots.
5, Resealed air box

Whacked all back together, still a misfire and confirmed by checking heat output on cylinder 4 exhaust header.

Checked spark at plug, it was fine. So just for the hell of it:

1. Changed ALL Plugs
2. Changed ALL Leads
3. Changed ALL Endcaps (making sure I got the right OHM ones).

Still misfire, so startin to think valve problem. So did:

1. Full leakdown compression test on all Cylinders, all results fine around 180 - 190 psi.

So then I returned to the carbs and bought a refurbished set and fitted these. Still this damn intermittent misfire was there.. arghh ..so checked valve clearances, all within spec.

So then I thought coils, so forked out for a set of Dynatek ones and fitted and still this damn misfire is there. Its driving me mad as I cannot think of what else could be causing this.. could a leaky exhaust header do this? Its pretty much the only thing left.

When plodding along the highway, the miss seems to disappear and your really only notice when starting and when going WOT. She almost dies in the arse , then picks up eventually and then at higher RPM load, starts breaking down and starts popping and farting through the exhaust.

Back at workshop after engine is warm the temps measured per exhaust pipe are

Cylinder 1 - 70-75 degrees celsius
Cylinder 2 - 80-90 degrees celsius
Cylinder 3 - 68-72 degrees celsius
Cylinder 4 - 55-60 degrees celsius

Again the temp on exhaust tends to confirm a definite misfire on CYL 4, and I dunno, maybe some others too.

So I am hoping someone can throw me some ideas, leaky exhaust header port? Intermittent electrical fault to the coil? I guess if it was some wiring fault to the coil, I'd expect to see probs with Cylinder 1 as well ..hmm I dunno guys, I have been tinkering with bikes for ages now and been quite successful at solving such problems, but this one has me stumped. So hoping a fresh set of eyes may provoke some ideas.

Thanks in advance gang,

Cheers

Snowy

[mike10]

Maybe

Check igniter box. signal generator. Bikecliff site can show you how to test them, look through there. or do you have the Dyna S set up. Smarter guys than me will give you more ideas.

[tom203]

Do you have enough slack in plug wires to switch 1 and 4 to see if misfire moves there ?- this wasted spark system needs umph, maybe one lead isn't delivering. Are these dyna coils lower ohm than the stock 4 ohm ones ?

[snowgoose]

Do you have enough slack in plug wires to switch 1 and 4 to see if misfire moves there ?- this wasted spark system needs umph, maybe one lead isn't delivering. Are these dyna coils lower ohm than the stock 4 ohm ones ?

Yes have done so already Tom thanks.. and misfire continues on No4, thats whats driving me bloody mad, it just doesn't seem to make sense. Such results suggest then that this is not ignition related. Apart from the igniter, pretty much both carburetors and ignition have been done along with compression test etc.

The fault was there before I changed out the old coils to the 5ohm dynatek ones, so don't think there is an issue here as such.. and while at it I have changed ALL the leads and plug caps ensuring I got the right resistive type and not to uses the resistor type plugs.

The annoying thing is that its regularly intermittent rather than permanently existing. As said previously cruising along without much load, she seems very happy and there is NO mis-fire, but stomp it and after the standard bog down delay, she will wind up and then at WOT, start missing, farting and popping.

I do have a new set of Keihins I could whack on to completely rule out some crazy carb fault, but I have swapped these out before with another standard set and the fault remained.

I would suspect if igniter was dodgy, I'd see the fault across cylinder 1 as well, would that be right?

The only other thing that comes to mind is maybe some leak around the no4 header, what do you think? I dont think this would cause this, but I'm quickly running out of new ideas and hair lol

Thanks for your kind responses

[tom203]

Swap in a set of Keihins to test? wow, you are ambitious!
Yeah, I would think an ignitor problem would show up on #1 also, .... but sure would be nice to swap in another to test. Post your location, maybe there's a member close by. This ignitor has the advance curve built in, so it's possible that something is amiss. You likely checked the connections at signal generator, so the the ignitor is all that's untested. I doubt a fuel problem.

[snowgoose]

Thanks Tom, I am in Newcastle NSW, Australia so not sure whether many locals have same model bike, but definitely worth a shot, many thanks