1982 GS1100GK - not accelerating smoothly when engine is hot

[MtnManCO]

Hello,

My 1982 GS1100GK starts well, idles well, runs well, accelerates smoothly and strongly for about 10 miles (ambient temperature about 50 to 60 deg. F), but suddenly accelerates somewhat weaker and not smoothly anymore.

The best way to describe it might be that the engine slightly "shudders" during acceleration.

The effect is reproducible, the bike is still rideable. I was wondering if poor tank venting could be the cause, took of the gas cap. Did not hear any hissing and the bike did not run better afterwards.

I bought the bike about a year ago, did never go on a test ride with it because of wintry road conditions. Went through the bike front to back.

• carburetors were professionally cleaned and rebuilt
• valves adjusted and double-checked
• new air filter (K&N, slightly oiled), sealed air box
• new intake rubber parts
• tank is clean
• new petcock
• new stator and R/R. Battery gets charged
• new battery
• new spark plugs and caps. I cut the ends of the spark plug cables
• all electrical connections were cleaned, including ground connections
• did NOT replace the ignition coils
• have NOT checked the compression yet
• bike smokes slightly after cold start

What could cause this behavior?

[MtnManCO]

Here is a picture of the spark plugs

IMG_0047.jpg

[TxGSrider]

Those plugs look a little lean, but that doesn't mean much in diagnosing if it's a mixture problem unless you drove a little ways with the engine acting up. Then noted your throttle position while it was happening. Then, while it was acting up you immediately pulled in the clutch and turned off the motor. Then coast to the side and remove the spark plugs and take some good pics of them.

[MtnManCO]

Thank you, I will try this.

[Nessism]

Does the stutter happen with the choke actuated after warming up? Maybe try a test to see if the condition is due to either a lean or rich condition. Also, have the carbs been vacuum synced?

[MtnManCO]

No, the stutter happens without the choke actuated. It happens right after a period of a very smooth, strong running engine. The carbs have only been bench synced. Vacuum syncing is on my to do list.

I ASSUME the carbs are synced quite well as the idle is smooth and stable and the bike can run smoothly and accelerates strongly, at least until the engine is hot. I will definitely vacuum sync the carbs.

[Cipher]

Does the bike have to cool down for the symptoms to go away? After ten mins its just started to reach operating temps.
Of course you have no fuel filter in the fuel line yes? Fresh gas and battery and ignition tested using the method Posplayr notes in his sig line.

[MtnManCO]

Thank you, Cipher

Does the bike have to cool down for the symptoms to go away? I have not had a chance to try this yet. Once the snow is gone, I will go on a longer ride and try this.

No fuel filter in the line yet, will install one this weekend. Fresh gas and battery: yes.

Ignition tested using the method Posplayr notes in his sig line: I will look this up and perform the test.

[MtnManCO]

I only see charging/stator tests in Posplayr's signature line. Is there an ignition test method?

[Cipher]

Thank you, Cipher

Does the bike have to cool down for the symptoms to go away? I have not had a chance to try this yet. Once the snow is gone, I will go on a longer ride and try this.

No fuel filter in the line yet, will install one this weekend. Fresh gas and battery: yes.

Ignition tested using the method Posplayr notes in his sig line: I will look this up and perform the test.

Oh my no do not install a fuel filter it may starve the carbs. I had thought the existent of one a potential problem.
I meant Charging test on posplayrs sig sorry.

Did the bike get dosed with Stabil or any fuel stabilizer during storage?

[MtnManCO]

Got it, thank you. I performed the charging tests according to the Stator Pages before I replaced the stator and R/R.

I still have to do this after the replacement. All I tested is the battery voltage before (~12.8 V) and after a ride (~13.0 V).

The bike sat - when in my possession - for a couple of months with cleaned and dry carburetors. The tank was empty and dry and no particles came out when I emptied it.
No visible rust. When I filled the first tank I put in ~2 oz. of Stabil.

[Cipher]

Got it, thank you. I performed the charging tests according to the Stator Pages before I replaced the stator and R/R.

I still have to do this after the replacement. All I tested is the battery voltage before (~12.8 V) and after a ride (~13.0 V).

The bike sat - when in my possession - for a couple of months with cleaned and dry carburetors. The tank was empty and dry and no particles came out when I emptied it.
No visible rust. When I filled the first tank I put in ~2 oz. of Stabil.

I just mentioned the tests as a means of removing the charging system as being a part of the problem. 10 mins is about all my bike can do if not charging during a ride then the bike loses power acts bad but then eventually dies.

[MtnManCO]

Hello,
I did two test rides with different outcomes, trying to see a pattern, but I have a hard time seeing one.

1st test ride (40 miles, ~60 deg F): bike ran very well. Started, idled and accelerated strongly and smoothly. On a fast country road I revved the engine once to 7,000 rpms and it accelerated without any hesitation and it felt like it would have easily revved higher. The bike felt very healthy. At this point I thought the issue had gone away.

When I arrived at my neighborhood (after 39 miles of a smooth running engine) the dreaded hesitation and rough acceleration started again.

When I arrived at my garage I touched the plug wires to see if there was any arcing but I couldn't feel anything.


2nd test ride (20 miles, ~55 deg. F): the bike ran slightly rough the whole time, so the hesitation can even occur with a cold engine. Here is a picture of one of the plugs. The other ones looked the same. I took it in the garage after returning from the ride. Right before I turned off the engine the engine ran poorly.

Plug GS1100GK 042520.jpg

I noticed that sometimes, when the engine accelerates with hesitation, quickly opening the throttle wide leads to a sudden, jolt-like strong, smooth acceleration, like the engine was snapping back into normal acceleration mode.

I also did a cold/hot compression test and the results were within ~5 psi. I still have to vacuum sync the carburetors.

Any idea what causes this erratic performance? Carburetors or ignition, or something else? The fact that the bike - with cold and hot engine - CAN run and accelerate smoothly and minutes later doesn't, puzzles me.

[748cc]

Be sure your vacuum lines are clear (spiders build nests in the smallest places!).

[MtnManCO]

Be sure your vacuum lines are clear (spiders build nests in the smallest places!).

Thank you, I will check all lines with open ends. Do you mean vent lines?
I thought the only vacuum line is the one that actuates the petcock.