81 gs850g Misfire Question

[Daven75]

Hey all, Newby here with an 81 Gs850g. Worked well then all of a sudden this started.

When cruising I have what sounds to be like a misfire. Not under easy or hard acceleration nor at idle. Only when at cruising speeds. New plugs, wires all good and it's getting good spark. Good fuel, filter is clean and I'm not getting and weird smoke/smells out of my exhaust.
Any suggestions on where I should start before I say screw it and rebuild the carbs?

[Gixer1460]

Symptom all the time or does it run ok and then problem develops? If latter them maybe look at coils breaking down as they get hot - they are likely 40+ years old now!

[wraith]

Also have a look for wear in the emulsion tubes in the carbs.

Had the same thing on one of my bikes.

Ps welcome to OSS 

Edited April 26, 2022 by wraith
Add a bit

[Daven75]

6 hours ago, Gixer1460 said:

Symptom all the time or does it run ok and then problem develops? If latter them maybe look at coils breaking down as they get hot - they are likely 40+ years old now!

So it's hot or cold, no real change that Ive noticed. 

 

I've narrowed it down to Cylinder #3 because it's much cooler than every other via measuring the header temp.

 

I'm thinking, at this point, it's gonna be me rebuilding the carbs. 

[Mike711]

Plug caps, try swapping them around & see if the issue moves with them.

[Daven75]

13 minutes ago, Mike711 said:

Plug caps, try swapping them around & see if the issue moves with them.

Just checking, should be as simple as this, correct?
https://gsarchive.bwringer.com/cbsaunders/gs/plug_cap_replacement.pdf

[Gixer1460]

12 minutes ago, Daven75 said:

Just checking, should be as simple as this, correct?
https://gsarchive.bwringer.com/cbsaunders/gs/plug_cap_replacement.pdf

First off - just swap plug lead from #3 with #2 and vice versa - if the problem moves its electrical, if it stays its carb / fuel related (probably!) 

[Daven75]

1 minute ago, Gixer1460 said:

First off - just swap plug lead from #3 with #2 and vice versa - if the problem moves its electrical, if it stays its carb / fuel related (probably!) 

oh, I didnt realize I could swap them, I wasnt sure of the firing order. I'll do that first and see if anything changes. Thanks! I should be able to do this at lunch and report back.

[Mike711]

If you can get it to misfire when rigged up with an auxilliary fuel tank you can gently pull the plug cap away from the plug (using plastic grips unless you want a shocking experience) slightly extending the gap between plug & cap increases resistance which in turn makes a fatter spark, if your misfire clears up you may well have a duff cap/HT lead but it can also be things like low compression or poor fueling. You can eliminate the plug cap by sticking a paper clip up the HT lead & twisting it round the thread of the plug.

[TonyGee]

ive got one of then crappy neon plug caps that the custom boys used to use, it was great for finding misfires, but I dont think you can get a long reach one like many suzuki's use ?????  the one ive got is the short one.  

[Daven75]

3 hours ago, Mike711 said:

If you can get it to misfire when rigged up with an auxilliary fuel tank you can gently pull the plug cap away from the plug (using plastic grips unless you want a shocking experience) slightly extending the gap between plug & cap increases resistance which in turn makes a fatter spark, if your misfire clears up you may well have a duff cap/HT lead but it can also be things like low compression or poor fueling. You can eliminate the plug cap by sticking a paper clip up the HT lead & twisting it round the thread of the plug.

Sadly I dont think I have the means to do this right now.

 

I did swap #2 and #3 to see if I see a dif and Ill test it tonight. 

[Daven75]

Small update, swapped plug wires and it did not follow the wire. Still on cylinder #3. Gonna try and spray some fluid to check for any kind of vacuum leak today otherwise carb rebuild it is!

 

[TonyGee]

what does #3 plug look like, is it wet/sooty ?  

[Daven75]

48 minutes ago, TonyGee said:

what does #3 plug look like, is it wet/sooty ?  

This is what they looked like with the old plugs, before I replaced them. But the misfire was happening with these plugs as well.

[TonyGee]

plugs look ok. it might be time to pull the carbs off for a clean/check.  have you got a compression tester ?  it wouldn't hurt to check the compression. 

[Daven75]

Just now, TonyGee said:

plugs look ok. it might be time to pull the carbs off for a clean/check.  have you got a compression tester ?  it wouldn't hurt to check the compression. 

Yea, I figured the plugs looks decent, MAYBE a little hot? But not enough to worry. I do not have a tester, I may snag one from the local parts store and check. But I feel like the carbs could use a good going through anyway lol

[TonyGee]

1 minute ago, Daven75 said:

Yea, I figured the plugs looks decent, MAYBE a little hot? But not enough to worry. I do not have a tester, I may snag one from the local parts store and check. But I feel like the carbs could use a good going through anyway lol

yeah they might be due a service 

[Daven75]

Just now, TonyGee said:

yeah they might be due a service 

Gonna be pullin em tonight if I get the time, hope to have them done by the end of the week!