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Please help dropping centre cylinders 850

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Guest

Guest
Hi I'm tearing my hair out trying to find the fault and any help advice would be greatly appreciated.
I have restored a 1980 GS850G which hadn't run since 1989. As you can imagine the carbs needed lots of work and have been fully rebuilt and ultrasonically cleaned. Other than an oil change and filters I haven't touched the engine as it appears good with no rattles etc.
The bike runs perfectly idling at 1000 rpm and is very smooth and responsive with its CV carbs and electronic ignition. It revs and idles in the garage for 15 mins with no problems but as soon as I have done a mile on the road it drops the two centre cylinders with occasional backfiring and popping. Afterwards it won't idle and has no power. I have cleaned the carbs many times but the problem seems to reappear?
Could it be coils or the CDI even though they are fine in garage even when really hot?

Please help?

Ken
North East UK
 
I'm currently also trying to get my 850 to run properly and I can only provide you with knowledge I've read from other posters, not my own experience.

Since your centre cylinders (2-3) are dropping, it sounds like it is an electrical issue as the cylinders fire in pairs (1-4 and 2-3). It's possible that as your bike/engine comes up to temp, the coils heat up, causing a short. Since it's backfiring/popping, it sounds like those cylinders are firing intermittently, which could be caused by a bad ignition coil. You could try swapping your primary and secondary coil leads between the cylinder pairs so that the previous 2-3 coil is firing 1-4 and see if the issue repeats itself on cylinders 1 and 4. If the issue repeats on 1 and 4, I think the issue could be narrowed down to the coils. If the issue persists on 2 and 3 even after swapping the leads, then it might be worth looking into your electronic ignition system which could be failing as it comes up to temp.

For what it's worth, I haven't worked on an 850 with electronic ignition nor do I have my own bike running properly. Just trying my best to think through this issue logically. :D
 
Please check the Newbie Mistakes thread linked in my signature to see if anything applies.

You might want to check the plug caps for resistance. Replacing the coil wires is doable also, even though they look sealed.
 
The 1980 factory electronic ignition module wasn't terribly reliable, at least on the 850. It pegged out on mine at 20 years old, so it got replaced with a Piranha, which had already done ten years on the other bike. It's still working flawlessly.

http://autocar-electrical.co.uk/prod...uki-mc/ckt-su1
The 1979 version of the Piranha fits the 1980 engine with a simple swap of the nose of the ignition timing cam on the crankshaft end.

Or, you could try a Dyna copy...
https://www.aliexpress.com/item/1005004058151562.html
 
Last edited:
Might also be worth having a close look at the coil supply plugs. They can look okay from the outside but hide a lot of corrosion.
Another vote for the old Piranha system. The kill and ignition contacts are always worth a clean. If they are taking some voltage it wouldn't take much at the coil plugs to make misfires.
90% of carb trouble is electrical :)
 
Brendan W said:
Might also be worth having a close look at the coil supply plugs. They can look okay from the outside but hide a lot of corrosion.
Click to expand...
That's very true. When I was using the bikes every day I'd feel a difference if the 12v supply connections were getting dirty and corroded. Also, the trigger wires from the pick-ups at the crank, where they reached a connector.
 
Grimly, I have one of those Aliexpress dyna copies on my gs1000. Up to now it's fine, and running MP08 coils from a cbr.
 
Well I think this plea for help is dead. No reply from private message. I'm close by the poster, and willing to help. Never mind, I'll move on.
 
hi all
apologies I have been dealing with poorly parents. I am still having problems

Summary -
engine had new rings and honed
head had all valves reground and vac tested
new valve shims all correct
new coils
new ngk caps
new ngk plugs
carbs rebuilt with standard jetting
reg/rectifier replaced as was charging 18v now ok

bike runs well but when run for a while it starts misfiring? gradually gets worse not just middle cylinders random?
im thinking is the igniter dying when it gets hot?​
 
Grimly said:
The 1980 factory electronic ignition module wasn't terribly reliable, at least on the 850. It pegged out on mine at 20 years old, so it got replaced with a Piranha, which had already done ten years on the other bike. It's still working flawlessly.

http://autocar-electrical.co.uk/prod...uki-mc/ckt-su1
The 1979 version of the Piranha fits the 1980 engine with a simple swap of the nose of the ignition timing cam on the crankshaft end.

Or, you could try a Dyna copy...
https://www.aliexpress.com/item/1005004058151562.html
Click to expand...


This is one of the most likely possibilities. FWIW, it's not a "CDI", and I have no idea why everyone uses this term for electronic ignitions. But that's another rant for another day.

Anyway, if it's one of the early metal cased units, you can take the cover off and eyeball the circuit board. Even if you have no idea what you're looking at, you may be able to see heat damage beginning. When these things fail, you can definitely see the burned bits, but yours may not show this yet.
 
Hi, I agree with the other posters that the ignition is a likely culprit. I have had good luck with ignitech ignition and coils , they gave me good service. That been said, if it was my bike, I would double check the valve clearance. It costs nothing but a little time. I have seen many times, dirt bikes run fine when cold only to run like total crap when they get hot and the only problem was tight valves.
Good luck and keep us posted
 
Agree with him ^^^^ Tight valves can get expensive pretty quick, BTDT... Worth checking, even if showing no problems just to be sure.
 
He is close by me, just waiting on a reply to my PM. Sure we can sort it, I've been through plenty problematic GS bikes over the past years.
 
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