Cylinder 3 running rich

[Guest]

Hi folks, I've been fighting the good fight with my gs850 and need some guidance on where to go from here. I've had the bike for about 5 years and ran it as my daily commuter for the first 4 of those. Two years ago I noticed it started running rough and had trouble idling after hot. It got pretty bad so I parked it last year (didn't have time to wrench on it). This year I got back to it and am trying to trouble shoot the same issue.

Cylinder three runs rich, and consequently there's a pretty frequent pop that comes from the unspent fuel sparking in the exhaust (there's a hole in my muffler...). I started with cleaning the carbs first, and played with the fuel mixture screw, but even at only 1 turn out it still runs rich. The valve shims were adjusted about three years ago, but again I didn't run the bike last year. Intake boots are only a few years old as well and carb cleaner spray didn't reveal any leakage on the intake side. I'm guessing next step would be a compression test. Any thoughts?

[Big T]

The mixture screw only works from idle to just over 1/4 throttle.

More likely a worn needle jet/emulsion tube is worn

[LAB3]

Are you sure this is a fuel issue? It's true that being in storage could make some goop build up in the carbs but if you're confident your carbs are clean and the leaks have been eliminated, the next easiest thing to look over is spark. Pull the plug and check your spark and plug wire cap resistance while you're at it.

[thurley94]

I just replaced the spark plugs so spark should be good. I originally thought it was a fuel issue but since it is persisting I'm not so sure. The emulsion tube looked fine to the naked eye when I dipped and cleaned the carbs.. how would I check that?

[Big T]

Emulsion tubes are difficult to tell when they're worn out
The rich mixture is the giveaway

[Steve]

I just replaced the spark plugs so spark should be good.

The plugs will only spark if there is something getting TO them.

And, you are saying they "should be good", which makes it sound like you have not checked. Please pull the #3 plug, put its wire back on it, lay it on the engine block so it has a good ground connection and crank the engine. "Should" it spark? Of course. DOES it spark?

One other thing that comes to my mind, ... which cylinder are you calling #3?
The cylinders are numbered from left to right, as you sit on the bike. The #1 plug is under your clutch hand, the #4 cylinder is under your throttle hand. If you happen to be numbering them in the other direction, <your> #3 has the vacuum line to the petcock. If the petcock is leaking, it can dump fuel down the vacuum line, giving you a rich condition.

One last thought. While you have your spark plug out, look at it. What color is the ceramic insulator in the center? Also, is it wet or dry?

.

[thurley94]

I'll check spark and make sure all is well there. I took a picture of the old plug I pulled out of it next to a plug from a different cylinder for comparison. Cylinder three plug is on the left. (3rd cylinder from the clutch hand)

IMG_20190414_185950568.jpg

[thurley94]

Spark test was good for cylinder 3!

[LAB3]

OK, so if you're confident the carbs are clean and the spark is good you've got the three things needed to make fire covered (air, fuel and ignition) the next thing to look at is your valve clearances. The manual for my bike says it's regular maintenance item every 3,000 miles.

[thurley94]

Will a compression test show a valve clearance issue?

[LAB3]

Sees to me it would, if you already have the gauge it's a pretty easy thing to do.

[Steve]

Will a compression test show a valve clearance issue?

Yes. And NO.

If the valves are in-spec, the numbers should be good, but what if the rings are bad? What if there is a gasket leak?

If the valves are tight, but still seating, you will still get good numbers, but are in danger of burning the valves because they are not spending enough time on their seats, cooling off.

If the valves are too loose, it's possible that compression numbers will be "ok", but a bit low, because the valves are not open long enough to allow proper air flow.

Bottom line: If the compression numbers are good, you know that everything is at least close, but don't take that as a guarantee that the valve clearances don't need to be adjusted.

It's a simple as following the advice in Nessism's signature:
"To measure is to know."

.

[Rob S.]

Have to agree about the wiseness of checking valve clearances, particularly if you don't know when they were last checked/adjusted. A bike can seem to be running fine and still need the valves adjusted for it's long term health.

Whereas, if it's running good, and you're not prepared for a major time/money investment, compression numbers fall into the category of "what you don't know won't hurt you."

[thurley94]

Makes sense! I just ordered a feeler gauge off Amazon so in a couple days I'll give it a check and see. After that, on to compression!

[Steve]

Please be aware of what measurement system the feelers use.

Some are METRIC. The feelers will be marked 0.03, 0.04, 0.05, etc.

Some are INCH. The feelers will be marked 0.0015, 0.002, 0.0025, 0.003, 0.004, etc.

The INCH feelers commonly also have metric APPROXIMATIONS on them. They only serve to confuse.

Also, please feel free to take adavantage of the offer in my signature.