Acceleration Issue - Bike Loses Power

[SgtBaker]

Hi. I've got an issue with my 1980 XS1100. The problem has gotten worse, and the solution continues to evade me as I work on my bike.

This occurs when the bike is at standard operating temperature, but more so while warming:

As I accelerate smoothly (with the throttle pulled back about a quarter inch) the bike will sometimes lose power but not stall. In fact, it feels as if I lose a cylinder or two upon smooth acceleration, due to the reduction in engine noise and the dramatic drop in torque.

The only thing which resolves this issue is to pull in the clutch and let the engine back down to idle speed. In order to avoid a repeat of the same issue, I must accelerate hard as soon as the clutch is back out and the bike in gear. Additionally, the bike has started to afterfire louder and more frequently than when I bought it a year ago.

What has been done to the bike in the last year:

• The previous owner, a bike mechanic, cleaned and synced the carburetors.
• I put 5,500 miles on it (current mileage: 17,500).
• Today I synced the carburetors to within plus or minus 0.5 psi of each other.
• Yesterday I cleaned and lightly reoiled the air filter, thinking this might help.
• A few days ago I inspected the carb diaphragms for tares (but did not look for tiny holes).
• Last week I patched a hole in the right side exhaust (toward the rear of the bike).
• Last week I ran a concentrated polyether amide based fuel system cleaner through the bike for 300 miles to clean the carbs/valves/combustion chamber.
• Last week I cleaned off the spark plugs and re-gapped them.
• Two months ago I identified a missing plug in the bottom of my air box and replaced it. (Item 42 of this diagram.)
• Replaced the oil 4000 miles ago (still looks and tastes fine).
  

I have not checked the timing, ignition, or dismantled the carburetors but I think one of those might be the next logical step.

After all this work geared toward eliminating the acceleration issue, the bike idles and cruises smoother and quieter than ever. When it does accelerate properly it is quieter, more responsive, and less hesitant as well.

....But the problem is becoming more frequent. As is the case, I've joined this site to hopefully gain some insight from more veteran mechanics. Any input on this issue would be greatly appreciated.

I'll update this thread until the problem is resolved.

[BA80]

It sounds like you may have a bad pickup coil wire.

Under the left engine cover is the eqivalent of a distribuator in an automobile. Ther are pickup coils there that send pulses to the TCI to tell it when to fire the coils. Since these bikes have a vacuum advance like a car those wires constantly move when the engine is running and eventually break inside the sheathing.

Look HERE for how to check and repair them.

That "plug" in the bottom of you airbox is actually a drain and there should be a small filter there.

[SgtBaker]

Thanks BA80, I'll try that later this week.

That "plug" in the bottom of you airbox is actually a drain and there should be a small filter there.

Though I know that area to be a drain, I had no idea that a small filter went there. My schematic in the Haynes manual identifies that part simply as "Plug" so I fashioned a plug out of thick neoprene rubber.

Any idea if this matters?

[BA80]

Mine was missing too. I just put one of those plastic push in things that they use in cars to hold plastic pieces together. You know, with the wide head and all the little ribs to hold it in. It doesn't seal so tight that the airbox can't drain but it keeps dirt and stuff out

[Bonz]

Yo, that is either the pick up wire having a break in it, OR it is simply the mechanism needs some silicon lube sprayed where the it rotates.

Went through this exact issue with another member...

See below...

http://www.xs11.com/forum/showthread...t=silicon+lube

[ViperRon]

I agree with the others here you possibly have a bad pickup wire or connection to it or the electronic ignition box. The fact that going back to idle position and also the rapid takeoff works would indicate a break in one spot of the wire under the case cover.

You can slightly move the advance or gently pull on the wires to test.

[Bonz]

YES, check the pick up wires, but if nothing is broken, then lubricate the darn thing with some silicon lube aimed where the mechanism rotates. Rotate it by hand as you put the shots of lube in there to help the lube get in there.

The engine runs flat and the mechanism needs a "bump" with deceleration to free it up, and it's simply becuase low/lack of lubrication is not letting the vacuum advance mechanism rotate as freely as possible.

It's been 30+ years since that was done at the factory and it WILL make a differnce regardless if it is your main culprit for the problem. The throttle response will be snappier fo' sho'.

Let us know if it was a pick up wire or if the lube took care of it.

[SgtBaker]

Update: I didn't find a bad pickup connection wire, so I lubricated the rotating mechanism with silicone spray. It appears to move smoother now. When I started the bike back up, I moved the advance unit forward by hand to see what would happen. The bike then stalled and will not start up again, even with starting fluid.

I'll let it sit for a while then try starting it again.

[BA80]

Sure sounds like pickup wires to me.

[SgtBaker]

Alright, I'll look closer then.

Edit: My brand of silicone spray has dielectric properties. Could liberal application of the spray have crossed connections?

[GLoweVA]

Update: I didn't find a bad pickup connection wire, so I lubricated the rotating mechanism with silicone spray. It appears to move smoother now. When I started the bike back up, I moved the advance unit forward by hand to see what would happen. The bike then stalled and will not start up again, even with starting fluid.
I'll let it sit for a while then try starting it again.

So let me see if I got you right, you rotated/advanced the timing while the bike was running at idle?
I'd say that then you probably had spark a little too early in the suck/compress/spark/exhaust cycle and maybe had an intake valve open when it sparked.
This could have 'back fired' into the carbs upset the delicate flow paths in there thus causing it to stop sucking fuel in.
I'd say, let it sit a bit to let some of the fuel evaporate out of the carbs throats and cylinder head and try again with the choke and see if it 'll restart then.

[GLoweVA]

Also, something out you can easily check, it to pull the plugs and see if they'll spark when cranking. (you got to make sure they're touching the head though)

[Gallaecus]

not sure where you are in Ct, but i have some items you can plug and play and see if it helps. carbs, tci's regs.coils, etc...?????

[natemoen]

Alright, I'll look closer then.

Edit: My brand of silicone spray has dielectric properties. Could liberal application of the spray have crossed connections?

no, dielecteic grease is non conducting.

[SgtBaker]

Pictures and Videos

Eureka! The bike runs perfectly now after fixing three out of four wires. Thank you for all your input and advice.

With these three wires fixed, I have noticed a few improvements in performance:

• The acceleration problem is gone.
• Cold start-up is smoother.
• Significantly reduced the occurrence of afterfire on downshift.
• Engine drops down smoothly to 1000 rpm when I pull in the clutch. It used to drop quickly, hesitate around 800 rpm, and then sputter back up to 1000 if it didn't stall.

I recorded my progress to to allow others to gain insight from my problem.

The first bad wire I found was the red one which went to the top of the unit.


It was in a fairly inaccessible spot so I lengthened the wire by half an inch.


Then I secured the new splice by running my soldering iron across the heat shrink tubing.


The blue wire leading to the top of the unit was kinked in two places so I had to replace an inch of wire.


Here it is soldered together.


The white wire leading to the bottom of the unit was also kinked in two places and needed half an inch replaced.


Here is the happy and healthy bike ready for a test drive.


Youtube video of vacuum advance unit during cold startup.


Video of vacuum advance unit after warmup.


Once again, thank you all for your help.