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84 GL1200 standard Hesitation and backfire

3K views 21 replies 8 participants last post by  Doc Fisher 
#1 ·
So my 84 GL1200 suddenly started running rough like not on all cylinders. Since it is an old bike that I recently re-acquired and it showed some signs of neglect I decided to overhaul ignition and fuel systems.

I replaced both coils, all 4 plug wires and spark plugs with new OEM parts.

Then I pulled the carbs, inspected and cleaned everything I could. There were a couple of gummed up float bowl screens that could have starved their respective cylinders of fuel. I replaced fuel lines, filter and new fuel pump.

After reassembly, synchronizing and drop down setting of pilot screwsI (landed at 3 1/2 turns out), took it out and it ran so good I was grinning like an opossum eating bumble bees. On the way back as I was congratulating myself on the success of all that work about a mile from home it started running rough again. I pulled the plugs and they looked a bit lean so I backed the pilots out another 1/4 turn. Made no difference. It hesitates and sometimes backfires on acceleration.

I can smooth it out while riding by giving it a little choke. Since the choke makes the mixture richer, I wonder if I need larger main jets. It is a CA bike which leads me to also suspect the SAS. I did replace all vacuum lines while the carbs were out.

Any help here would be appreciated.

Meanwhile I am rocking Seafoam in the tank as a desperation shot.
 
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#7 ·
Thanks again for your help Dave,
What I can see is clean. and the new fuel filter should prevent crud from getting to the float screens?
When I drain fuel from the tank it comes out with no visible contamination. I suppose I could try to rig a long swab to drag around whatever bottom parts of the tank I can reach to see if anything lives there.
 
#9 ·
Thanks for the suggestion. I have not tried that and am not sure how to do it in a way that would identify a vacuum leak. It sounds like you are saying just spray all around and see if it changes. If it does, that means a vacuum leak? Ok how do I find the leak in that case?
And how would it run great and suddenly not?

The only thing that consistently changes how it runs is the Pilot screw adjustment so I'm thinking I did not get the circuit well cleaned when I had it apart. Looks Like I will do it again.
 
#10 ·
If you still have a plugged carb passage the best way to clean them is with an ultra sonic cleaner. It sounds expensive but they can be bought for under $100 and can be used for many things, such as cleaning jewelry which will make your wife happy. As far as finding a vacuum leak the prime suspect would be the intake rubber couplers, if you spray somewhere and the idle improved then by spraying through the straw of the can or using cardboard block off section by section until you can identify the vacuum leak. Hope this helps.
 
#11 ·
Another way to locate vacuum leaks without all the mess is to use an unlit propane torch to direct propane gas to the suspected areas. You will get the same change in idle speed but no liquid. And since it's a gas, it works even if you don't have a straight shot to the leak. I have one with a long thin vacuum hose on it, just for doing that on cars. By watching an exhaust analyzer you can even locate leaks that are too small to affect the idle.
 
#12 ·
There's a good chance something clogged up one or two of the pilot jets. I've had success removing the adjusting screw and giving a blast of carb cleaner into the passageway. If after doing this the engine runs better, you 'll know that you're on the right track. The "choke cure" does indicate a pilot circuit problem. Good luck.
 
#14 ·
YES! After I removed the pilot screws and blew carb cleaner in and followed with a blast from my air compressor it ran great for a short while then crappy again. So I figured there is still something in the circuit and did it again. This time I noticed one of the small o-rings was toast so I replaced all four and their washers with new ones from Randakk. afterwards it ran a bit better but still not right.
 
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