Carb tuning questions … and yes I tried the SEARCH lookup

[bgk]

I don’t post here often, because darn near every GS question has already been posted and answered on this forum, and I can usually find an answer to my question by using the SEARCH tool.

Plus, I have owned and wrenched on multiple GS bikes for the past 5 years and I like to believe that I know what I’m doing with GS maintenance by now.

But, now I’ve got a problem with a 1983 GS850G that I just can’t seem to figure out.

I believe that the engine is stock. The airbox is stock except that it has a UNI brand foam filter (lightly oiled). The exhaust is an aftermarket 4 into 1 pipe, brand unknown.

I also believe that the carbs are stock. I have very carefully cleaned and inspected them and have checked that the main jets are size 115 and the pilot jets are size 40, which I believe are the stock jet sizes. I have also checked float heights, and rubber boots at the engine side and air box side.

I have not yet split the carb rack and have not yet replaced any carb O-rings. I have manually “synched” the carbs off the bike, and then dynamically synched the carbs on the bike with a Motion Pro tool.

Now for the symptoms: The bike starts up perfectly on choke and idles nice and responds well when I “blip” the throttle in neutral, but after warming up it “bogs” at low RPM under load (from idle in 1st gear while leaving a stop sign, etc.) and it doesn’t develop power in the low RPM range very well, but then runs beautifully above about 4000 RPM.

Today I pulled the carbs and the spark plugs after riding for half an hour and then “chopping” the throttle/clutch/ignition from 3rd gear at 3000 RPM and coasting into my driveway, and this is what I found:

The carb bowls, jets, gaskets, etc. are still clean as a whistle.

The air/fuel mixture screws and corresponding cylinder spark plug “reads” are as follows:

#1 plug is dark tan/light brown (slight rich) and the #1 carb mixture screw is out 1 ¼ turns.

#2 plug is light tan (perfect) and the #2 carb mixture screw is out 2 ¼ turns.

#3 plug is barely slightly off-white (slight lean) and the #3 carb mixture screw is out 4 ¼ turns.

#4 plug is snow-white (way lean) and the #4 carb mixture screw is out 4 ¼ turns.

It seems to me that the plug colors and corresponding carb mixture screw settings are the OPPOSITE of what I would expect.

And before you wise guys ask: Yes, I’m sure that I’ve got the carbs/plugs numbered correctly.

What this tells me is that the carbs and plugs on right side of the bike (3 and 4) are getting more fuel than the left side of the bike (1 and 2), even though the mixture screws on the right side are turned way out. How could this be?

Have any of you guys seen this before? Anybody have any ideas or suggestions?

[chef1366]

I would replace the o-ring in between the intake boot and head. Sounds like an intake leak on #3 and #4. I would also do this
Have you done a valve adjustment? You will never get it right if they are off.
Do a compression test.

[bgk]

I have checked/changed valve shims (last fall/winter).

If 3 and 4 are rich, how could this be an air leak at the intake boots?

Wouldn't an air leak result in a lean condition?

[chef1366]

Wouldn't an air leak result in a lean condition?

You said earlier they were lean.

[bgk]

Oops!

You're right, they are lean, and I'm at the tail end of Labor day, with a few drinks in me!

I'll check the boots tomorrow.

[Steve]

...I'm at the tail end of Labor day, with a few drinks in me!

By all means, wait until you are sober to make sure you do the job right. :shock:


.

[Nessism]

By all means, wait until you are sober to make sure you do the job right. :shock:

Say what? I usually gauge the difficulty of the repair job by how many beers it will take me to complete.

Hate to say it bgk but you will be miles ahead if you just bite the bullet and order an o-ring kit for the carbs and the intake boot o-rings as well. There are just too darn many places for these carbs to suck air and unless you have everything as intended tuning is just guess work.

[bgk]

Update:

I pulled the intake boots off the cylinder head this evening.

The good news is that at least one Previous Owner (and/or repair shop) was on the ball sometime in the past. The original Philips screws have been replaced with Allen head screws (or did 1983 GS bikes come stock Allen screws, my only other maintenance experience with GS bikes is with 1979 and 1980 model years).

The intake boots all look good: nice and flexible, no apparent cracks or deformation or separation between metal and rubber, etc.

The O-rings at the base of the boots are all intact, no rips or tears; but they are certainly a little bit “squooshed” flat. The two left side O-rings don’t look any different from the two right side O-rings.

I’m going to buy and install new O-rings anyway because they are inexpensive and I’ve got everything pulled apart to replace them at this stage; but tonight’s intake boot and O ring disassembly and inspection did not reveal any “smoking gun” evidence to explain my running condition and spark plug colors.

Any ideas anybody?