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Discussion Starter · #1 ·
I synced the carbs two days ago. The bikes pulls better, I can now leave from a stop at a normal rpm where as before I had to rev it to 2800 or more. I can't believe the improvement in power and how smooth the power comes in.
But now it runs really rough when first started cold. I have to use full choke just to keep it running till it starts to warm up. Then it doesnt want to hold a steady idle and always runs the idle lower if I sit at a stop light. To the point to where I have rev it to keep it running. It also idle rougher then before and sounds like a mild harley at idle. Lastly when I decellerate the motor as an odd vibration and sounds little like a truck using it's motor brake.
Any advice ?
I'm happy with the new power but need it to idle and run cold too. :)
 

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1985 GL1200 A
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1200s are older and cold blooded by nature, but the idle problem when warm suggests an air/vacuum leak.

All the screws pulled for synch'ing tight and with washers or seals? Did you bump a vacuum line loose? Are the intake elbows tight to the head? Did you replace the large O-rings in the bottoms of them before you synch'd the carbs? One thing about synch'ing your carbs is that since they're more finely tuned they also show any imperfections in air/fuel more clearly than when the whole business is hinky.
 

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Discussion Starter · #3 ·
My bike sounds a little like this one, and shows some of the same symptoms.

 

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1985 GL1200 A
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The first test I'd do is for air/vacuum leaks. With the engine warmed up and idling enough that it's doesn't kill, go around the intake elbows where they join the heads one at a time with ether, Gumout, or whatever and test with a quick light spray. If it idles up/smooths out at any spot, that's where there is an air leak. If the bottoms of the elbows pass, then check the tops, where they hook to the carbs; then any vacuum lines hooked to the carbs, etc.

I assume your carbs,especially the slow idle jets are clean, clean, clean?
 

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It aint rocket science
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The guy in the video claims it revs good, sounds terrible to me with it missing.
And that the carbs are spotless yet no mention of ADJUSTING them through an idle drop per manual.

Ignition system check

Vacuum leak check (incl. pinching off hoses)

Compression test

Plug condition check

The shade tree mechanics way to engine diagnosis: 1500, others similar.

Will start off on the premise of an otherwise well running and properly maintained engine and has developed a driveability concern. Filters and plugs have been changed at regular intervals. This is a first check of basic systems that anyone can do.

First off is to check the ignition system. With the bike warmed up and idling remove and replace one plug wire at a time from the spark plugs. What we want to observe here is a uniform idle RPM drop when the wire is removed providing no ignition to that cylinder. At this same time we are also testing the plug wires. While slowly removing the wire from plug you will hear a distinct snapping noise of the spark jumping to the plug. It should jump from 1/2-1 inch, this tells us the wires are good with no excess resistance. In rare cases the plug will only fire with the wire partly disconnected adding RPM and is likely due to a fouled plug.

Next up is to remove the spark plugs noting their condition and location within the engine. Note the condition of the plugs below which were taken from my 1500 when acquired with no driveability concerns except a release start type of turning over. Plug gaps were on the wide side. These could have been original twenty two year old plugs as the bike had only thirty thousand on the odometer and I was still going through it with frequent starts and idling. Note the two inner plugs (cyls.3&4) look like they have been running slightly hotter or leaner than the others. This is due in part because inner cylinders on an engine will typically run hotter than the ones on the corners. Note these same cylinders are the ones the vacuum hoses are attached to in the runners which could also contribute to the slight difference. All white or all black plugs on one side usually indicate carburetor problems. If you suspect a bad plug you can swap it with another and repeat to see if the problem follows the plug.

When pulling plug wires and none to little RPM drop we must then find if it is lean, rich or compression related.

Next up

A good quality carb clean spray with straw is a valuable diagnostic aid and I keep a can in the bag.

Bike dies on the road: While cranking spray a little in air intake, if bike wants to fire, fuel problem confirmed.

Vacuum leaks?: Spray a little around suspect area, if idle increases or smooths out found source of leak.

Lean carbs?: Spray a little directly into its individual intake, idle increases or smooths out lean carb confirmed.

Rich carbs?: Spray a little directly into its individual intake, idle decreases
rich carb confirmed within reason.

Quick confirmation of pilot adjustments: Consistent idle (RPM) response when a little spray is introduced to each individual carb intake.

High speed miss?: Spray a little into intake stream, if smooths out and picks up lean condition confirmed.

Avoid overuse around composite parts. (1500 slides for sure) Starting fluid is too volatile and messes things up.
 
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