1976 irregular firing when cold


Information and questions on GL1000 Goldwings (1975-1979)
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kndw
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Joined: Sat Jun 19, 2010 2:56 am
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Motorcycle: 1976 GL1000 Gran' Luxe

1976 irregular firing when cold

Post by kndw »



Hi all,

My '76 GL1000 seems to be misfiring when the engine is cold. This results in a distinct sneeze and occasional loss of power when I start a ride. So far, I have cleaned the carburetors and tried a different set (which I know is clean, but is from a '78), I have checked the ignition timing, and the valve clearances. These were all in spec. Last weekend, I investigated a bit further with a colortune spark plug. On all cylinders, except the front left, I saw a nice blueish hue. On the front left cylinder, I could clearly see sparks arcing from the outside of the colortune plug to the cylinder head. I then checked the resistance of the wires leading to the plugs (all about .5 Ohms), and the resistance of the plug caps (all between 4.6-4.8 kOhm). The plugs all have a lightbrown tan, also the one on said cylinder.

Consequently, it seems clear to me that something is not right with the front left cylinder or ignition there, but I don't really know what to look for next. Any ideas?

Thanks!

Aside: dyna S ingition, and dynatek coils (the green ones: 3Ohm(i believe) ). Otherwise, stock.


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WingAdmin
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Re: 1976 irregular firing when cold

Post by WingAdmin »

Could you see arcing between the plug to the cylinder head OUTSIDE the cylinder? Was the arcing occurring from the base of the plug (the part with the six-sided nut) to the cylinder head? Did you have the tube (with the mirror on the end) over top of the plug?
kndw
Posts: 132
Joined: Sat Jun 19, 2010 2:56 am
Location: Driebergen, Utrecht
Motorcycle: 1976 GL1000 Gran' Luxe

Re: 1976 irregular firing when cold

Post by kndw »

Hi, thanks for helping. I forgot to mention that the 'sneeze' is a backfire through the carburetors.

Here are the answers to your questions:
WingAdmin wrote:Could you see arcing between the plug to the cylinder head OUTSIDE the cylinder?

Yes, the arcing happened outside the cylinder. When I saw a such a spark, there was no light to be seen through the colortune, so no combustion.
WingAdmin wrote:Was the arcing occurring from the base of the plug (the part with the six-sided nut) to the cylinder head?

I have a 14mm colortune with a 12mm reducer part to fit the cylinderhead. The 12mm part goes into the cylinder head (bottom), the 14mm goes on top of that and connects to the cable extension. The sparks came from the 14mm part and the cable extension.
WingAdmin wrote:Did you have the tube (with the mirror on the end) over top of the plug?
I did not use the tube with the plug. What I did try, was placing some heat shrink tubing over the extension part. This did not make much of a difference.
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WingAdmin
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Re: 1976 irregular firing when cold

Post by WingAdmin »

Can you identify exactly where on the plug it is jumping to the cylinder head?

Is it arcing through the heat shrink (entirely possible - it's enough voltage there to be able to do this), from the terminal "business" part of the plug that the wire connects to?

I'm suspecting that you may have two issues - one being the colortune plug not insulating properly against the plug well wall, and the other is the missing. Is the missing/sneezing happening in sync with the spark you are seeing, or are they otherwise unrelated?
kndw
Posts: 132
Joined: Sat Jun 19, 2010 2:56 am
Location: Driebergen, Utrecht
Motorcycle: 1976 GL1000 Gran' Luxe

Re: 1976 irregular firing when cold

Post by kndw »

WingAdmin wrote:Can you identify exactly where on the plug it is jumping to the cylinder head?

Is it arcing through the heat shrink (entirely possible - it's enough voltage there to be able to do this), from the terminal "business" part of the plug that the wire connects to?

I'm suspecting that you may have two issues - one being the colortune plug not insulating properly against the plug well wall, and the other is the missing. Is the missing/sneezing happening in sync with the spark you are seeing, or are they otherwise unrelated?
The arcing is happening in both places you mention, and indeed even through the heat shrink; the location varies randomly. Whenever I see such an arc, it stays dark on the inside of the cylinder, and there seems to be simultaneously a pop in the exhaust or a backfire through the carbs: I would say that the missing/sneezing is in sync with sparks not going inside the cylinder, but finding a more convenient path outside the cylinder.

The main issue, however, is that the missing/sneezing also happens with normal spark-plugs, when the engine is still cold. I now found on Randakk's that these symptoms are consistent with a lean mixture. The plugs were indeed a bit pale, so maybe there is something wrong with the mixture? :?:
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WingAdmin
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Re: 1976 irregular firing when cold

Post by WingAdmin »

It definitely could be. I would first eliminate the arcing issue - wrap your heat shrink in several layers of dielectric tape, something to insulate that conductor and force the spark down to the plug tip, instead of the plug well. Once you have the plug sparking consistently, you can then properly diagnose the missing: if you see the plug tip spark, but the cylinder doesn't light off, you know that you have a fuel/air issue.

Just out of interest, have you also checked the compression of your cylinders?
kndw
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Location: Driebergen, Utrecht
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Re: 1976 irregular firing when cold

Post by kndw »

I'll take some time to try wrapping the cables/colortune next week. Th le compression has not been measured, because I don't have a compression tester, and don't know anyone who has :(

Anyway, it may be that the fuel jets are not the right size: the bike has 755A carbs (on a '76 engine), and during the rebuild I may have installed jets for the '76. These are smaller than the '75 jets, so I will also check this, and place the proper jets if necessary...
kndw
Posts: 132
Joined: Sat Jun 19, 2010 2:56 am
Location: Driebergen, Utrecht
Motorcycle: 1976 GL1000 Gran' Luxe

Re: 1976 irregular firing when cold

Post by kndw »

Yesterday, I took the carburetors off, opened them up, and took a look at the jet sizes. These were 120/60, whereas 125/65 are the sizes that belong in the 755A type. After having them replaced, and the carburetors tuned, the sneezing has disappeared. So, most likely the mixture was too lean.

As an added bonus, the bike seems to be taking off faster. :)


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