Help Diagnosing a Misfire?


Information and questions on GL1000 Goldwings (1975-1979)
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skythepig
Posts: 13
Joined: Fri Aug 08, 2014 6:31 pm
Location: Bude Cornwall
Motorcycle: 1975 GL1000 Trike

Help Diagnosing a Misfire?

Postby skythepig » Thu Aug 27, 2015 3:12 am



HI I have a 1975 GL1000 which has been running fine until yesterday when it started misfiring on a left side. It is more noticable on deceleration when you throtle off there if a definite loud phut phut sound.
What is the basic process to go through to try and diagnose and fix the problem?
I have a really basic knowledge of bike workings and would like to try and expand my knowledge and fix it myself rather than spend a fortune at the local bike shop.
Any help gratefully received.



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virgilmobile
Posts: 7650
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Location: Denham Springs,La.
Motorcycle: 1988 GL1500 I
Previously owned
78 GL1000
81 GL1100
82 GL1100 I
83 GL1100 I
83 GL1100 standard
84 GL 1200 I

Re: Help Diagnosing a Misfire?

Postby virgilmobile » Thu Aug 27, 2015 8:04 am

Is this noise in the exhaust or intake?
How's the battery volts while running.
Have you set the valves and ignition timing.Is it points or electronic ignition.?Is the spark robust or weakly?

I'd be leaning toward ignition issues first.Its the easiest to work on and check.

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skythepig
Posts: 13
Joined: Fri Aug 08, 2014 6:31 pm
Location: Bude Cornwall
Motorcycle: 1975 GL1000 Trike

Re: Help Diagnosing a Misfire?

Postby skythepig » Fri Aug 28, 2015 11:42 am

May have gone around this the wrong way but...so far replaced all four plugs and drained each carb to see if any debris in them?
On taking out the plugs 3 and 2 were a nice dry even brown colour, but 1 and 4 were both blacker and appeared wet?
On running bike to the normal temperature, on idle right side sounds nice and even beat to engine but left side has the odd missed beat sound (misfire?)
Battery is new and charging OK.
Normal points still fitted so over weekend will check out the points.
Could it be that the mixture is too rich on cylinders 1 and 4 and needs some carb adjustment? If so is this only possible with a set of vacuum gauges?

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virgilmobile
Posts: 7650
Joined: Sun Sep 19, 2010 5:39 pm
Location: Denham Springs,La.
Motorcycle: 1988 GL1500 I
Previously owned
78 GL1000
81 GL1100
82 GL1100 I
83 GL1100 I
83 GL1100 standard
84 GL 1200 I

Re: Help Diagnosing a Misfire?

Postby virgilmobile » Fri Aug 28, 2015 1:30 pm

If the spark,timing,valve adjustments and compression are normal...the carbs are the only thing left to mess with.
It is possible that the right pair are supplying more fuel than the engine can burn.
Obviously I'm gonna say that you need to sync the carbs to the engine and adjust the idle mixture screws by the book..including using some vacuum gauge or device method for accuracy.
However...one can get it close if your good.
Sense it sounds like the left bank is normal and the right bank is rich,you can "trim"the adjustment that links them together.
Its on the left rear carb.When turned,it either increases or decreases the throttle of the left bank.The right bank stays steady by the idle screw.keep it adjusted for around 8-900 rpm.
In your case,I'd idle it down to around 800rpm.Turn the adjustment to open the left bank a bit.This will increase the idle...Adjust the idle down again..Move it a bit more.
As you play with it,you'll get the feel of how each side is firing,and if its lean or loading up.
At some point you will notice that the right side smooths out and the left starts to go funny.This will be the range.Set it half way and run it.

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Ncscooter
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Location: North Carolina
Motorcycle: 77 GL1000
76 CB400F

Re: Help Diagnosing a Misfire?

Postby Ncscooter » Sun Sep 06, 2015 4:19 pm

Just so we're all on the same page, 1 is the front right and 3 is the rear right. 2 and 4 would make up the left side. One coil fires 1 and 2 and is timed by the left (outside points) 3&4 are serviced by the right set of points. If the phut phut is a backfire coming from one side or the other, I'd start by setting my idle mix screws 1 3/8 turns out for the '75, synching @ 950 rpm, and then slightly adjusting the idle screws one at a time until you get the best idle. Point gap and timing should be checked and correct before doing this. Great running bike when it's dialed in.
Keep your expectations tiny and you won't end up so whiny.

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skythepig
Posts: 13
Joined: Fri Aug 08, 2014 6:31 pm
Location: Bude Cornwall
Motorcycle: 1975 GL1000 Trike

Re: Help Diagnosing a Misfire?

Postby skythepig » Mon Sep 07, 2015 5:50 am

Thanks for the info on cylinder numbers. I was going by the numbers written on the ht leads which I now think have been numbered wrong on my bike and means that its 1 and 3 (front left and rear right) that are showing signs of oily residue.
Will take out new plugs and recheck condition later in week. Check points again.
Haven't noticed a lot of oil going so hopefully its not oil getting into the cylinders due to wear and tear which someone else suggested.
Just going on a trip to the Isle of Man so will have a go at tweaking carbs when I get back.
Typical speedo just packed up so ordered new cable!!!!!!
Will post info after carbs adjusted.

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Ncscooter
Posts: 53
Joined: Wed Jun 17, 2015 11:20 am
Location: North Carolina
Motorcycle: 77 GL1000
76 CB400F

Re: Help Diagnosing a Misfire?

Postby Ncscooter » Mon Sep 07, 2015 8:46 am

Sitting on the bike, looking down, 1 is the front right, 2 is front left, 3 is rear right, 4 is rear left. 1 & 2 are fired by one coil (left I think, my bike is fully dressed, I would have to take some parts off to be sure) 3 and 4 by the other coil. If the fouled plugs are both on the same side, it's a carb issue most likely. If they're both in the front (or back) it could be an ignition problem. Or a carb problem. Flipping wires around will help diagnosis in this case.


Keep your expectations tiny and you won't end up so whiny.


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