1200 rough idle

Information and questions on GL1200 Goldwings (1984-1987)
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Joined: Mon Aug 24, 2009 11:51 pm
Location: Las Vegas, Nevada
Motorcycle: 1994 1500SE

1200 rough idle

Post by racertim » Sun Mar 14, 2010 9:29 pm

Hi, I have an 1984 1200 Aspencade that has developed a rough idle. It was a problem that came on suddenly. The bike had been running fine before then. It will bounce between 800 and 1100 rpms at idle, gets worse after it warms up and I take the choke off. And the throttle response is sluggish like it is loading up. Once it reaches about 2500+ rpm it takes off and runs fine. Then when I slow it back down (like at a stop light) it runs rough and doesn't want to take off in first gear unless I rev it up. I thought maybe bad fuel, flushed the fuel and replaced the filter. Same results, then I ran some SeaFoam through it, still rough. Pulled out the carb assembly, rebuilt carbs, checked all gaskets, and replaced all the vacuum lines around the carb assembly. Same results. I then took some brake cleaner and sprayed it around the carb, couldn't really detect a difference. I was thinking that I had a leak but now I don't know what it could be. I pulled each plug and each one is firing. BTW, battery is new and fully charged.

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Motorcycle: 1996 FXSTSB Harley Badboy, 1980 GL100 Honda Goldwing

Re: 1200 rough idle

Post by hdriderbb » Mon Mar 15, 2010 7:08 am

I would check to see if you are dropping one cylinder during idle. When the bike is idling rough pull one plug cap off the spark plug at a time and then put it back on before moving to the next plug. If the bike changes tone or dies, then you know that plug is sparking. If the tone of the bike does not change, then that is the cylinder you are dropping.

If you find a cylinder is dropping, then I would double check the spark plug, plug cap (tight on wire and screw inside cap is tight), and wire end has enough wire to grap onto the cap. The spark plug caps screw onto the coil wires, so do not just pull them off the wire. You may also have an issue with the pulse generators. I would double check the spec on them too.

You may want to sync your carbs too, just to make sure the butterfly are all at the same point at idle. Did you verify the butterfly were at the same point when you had the carbs off? They should all cover the exact amount of one bypass holes in the carb throat. However, they would have to be way off to cut off one cylinder.

I would also adjust your fuel mixture screws too, just to make sure they are not turned in too much. Check the main, but I think the starting point is around 2.5 turns out.

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Re: 1200 rough idle

Post by WingAdmin » Mon Mar 15, 2010 9:42 am

A quick and easy way to check if one cylinder is the cause is to use a cheap infrared thermometer like this one. When the bike is cold and just started up, point it at each exhaust manifold, and if one cylinder is not doing its share of work, the exhaust will measure quite a bit colder than the others.

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