I need suggestions - poorly running GS500F

[scootertrash]

2007 GS500F. 15k miles. Two cylinder inline air cooled engine. Purchased last winter so I don't know much history on it.
Over the winter I rebuilt carbs and checked valve clearance. Some were on the low side (small gap) but all were within tolerance.

My son got his cycle license a week ago and has been riding it non-stop for a week. He has put 500 miles on it. It was running fine until yesterday, then it started running poorly.

Question 1 is "How much power should I expect from this bike ?" I am an average size guy. When the bike was running well, it had little power. I assumed this was normal, but maybe not. On the highway it was a struggle to keep it at 65 mph. When trying to go up a hill on the highway, it would slow down to about 50mph. I would downshift and try to rev the crap out of the bike, but still couldn't get more than 50 on a hill. It seemed to run fine, just no power. Not enough power to get the RPMs up high enough under load, but on a downhill it would rev up fine. Is this typical for a GS500 ?

Now for the new problem : yesterday the bike started running poorly. It idles fine but when you try to rev it up it backfires and stumbles. I quickly discovered that the issue is with the right side cylinder. The bike runs the same with the right side cylinder spark plug connected or disconnected, so the right side cylinder is not doing much. With the left side spark plug disconnected, the bike barely runs, but does run very poorly. So it seems the left cylinder is strong, and the right is very weak, but not totally dead. It will run on just the right, but just barely.
I pulled the spark plug and the spark appears ok.
Using the unofficial thumb test, compression seems similar on both sides. (strong on both cylinders)
I pulled the carbs again last night. Very clean. I removed the jets and they looked fine. I re-cleaned them. Floats look ok. Float level ok. Float needle valve shows a very small groove visible with magnifying glass, but I would guess it is a normal amount of groove and probably not the problem. (?)
I removed the rubber manifold ring that connects the carbs to the cylinders. They are fine. No cracks or leaks.
There is fuel getting to the right carb. I can loosen the carb bowl drain screw and get gas. Since the left cylinder runs good, I have ruled out the infamous fuel petcock issue. The same line feeds both left and right carbs.
I don't think it is anything major like a bad valve or piston issue. It's too inconsistent. Seems more like an issue with fuel or spark.
Tonight's plan: I plan to try swapping ignition coils to see if the problem follows the coil. (weak coil ?). But the spark seems ok to me, so I keep going back to the carbs. But I have had the carbs apart twice and they are really clean and the jets are good. No blockage. So, I also wonder if the fact the bike never had power is a clue. Maybe it is a major mechanical issue I am overlooking. My last resort will be to remove the valve cover again and check valve clearances again, but I don't think that is the problem.

[Petorius]

Curious to see the outcome. I was troubleshooting a similar issue with mine last summer and eventually gave up and haven't looked at the bike since. Coils were my next item to test but I never got around to it. Also, the bike should have more power than what you described in question 1. Sounds like something was not quite right even before it shit the bed.

[Kurlon]

I'm thinking air leak. There is a tiny o-ring under the top carb cap that likes to break or disappear that will cause an air leak, the sync/vacuum ports on the intake boots are also potential air entry points.

Power wise 65mph should be no problem for that bike. They'll do 100 if given room. If the valves are on the tight side, re-shim them to the loose side of the spec, it'll make starting easier.

[Tekime]

Yep, GS should have no problem at highway speeds and will easily do over 100 with an average rider and 0-60 faster than most cars.

Power loss under load, she's starving for air/fuel -- one cylinder affected I'm also thinking air leak. Spray some carb cleaner/WD-40 on the intake boot and around the carb to look for micro leaks. Valve adjustment is not a bad idea either just to rule it out.

[stoinkythepig]

Pretty sure those have resistor cap plug wires and they can crap out enough to allow a spark through, but very weakly. Try swapping the plug wires and see if the problem follows one of them.

[butcher bergs]

If you find that the coils (or electrical) aren't the issue I'd be revisiting those carbs and ensuring the emulsion tube is correctly oriented on that one side.

What you describe is exactly what would happen if one of those tubes was 180° out but that would also mean the bike never ran correctly from the get go.

[Fitron]

Replace plugs, right off the bat. Often, a spark plug will spark just fine outside the engine, but under compression, it shorts around the base of the resistor. I've also seen the rubber carb boots leak vacuum from a small inner spot that that's not seated properly, and isn't easily seen visually. Also check float height. If the tab got bent and the floats are too low, that cylinder will starve for fuel.

[Stromper]

I am with ignition
Iridium Plugs , new caps , check connections and even battery health, its really a battery that supplies the surge for sparks

Yes it should do 65 everywhere

[MHenry600]

Even if the carbs "look" clean, there are plenty of little passages that can be plugged up.

It would be worth the time to soak the carbs (in kerosene or such), and use an air compressor to blow out every port and passage you can find. Even the itty bitty tiny ones.

[scootertrash]

Thanks for the ideas. Today's update - still struggling to fix this.
I have ruled out coils, plug wires, and spark plugs. I swapped Left/Right. The problem remains on the Right side. The Left side still runs strong.
I tried the suggestion of spraying carb cleaner around the carb and the intake boot. It didn't seem to have any effect on how the engine was running.
So I pulled the carbs again. I very carefully checked the float heights with a caliper. They were a little off so I adjusted. I also very carefully checked the big rubber boot inside the top cover of the carb. It was in good shape with no cracks visible using backlighting. The jets are all good. I did not soak the entire carb in cleaner as MHenry600 suggested this morning. I'll have to add that to my list of options.
One thing I didn't mention : I replaced the spark plugs when I bought the bike this past winter. I bought the NGK ones specified in the manual. There are less than 1000 miles on them. The one that was in the good side of the engine looks normal. The one on the bad side has a black coating over entire insulation. Not flaky, more like glossy black paint. So wet fouling. Which is why I keep thinking it is the carb on the right side. I have the old plugs that were in the bike when I bought it. Same thing, one was good and one wet fouled.
I am really sick of working on that stupid carb. I am trying to decide whether to pull it off AGAIN and take everything apart AGAIN and soak the entire thing in cleaner, or if I just buy a set of carbs off eBay and hope they work better. Grrrr.

[Petorius]

There is a guy on GStwins that used to overhaul carbs for a minimal fee. Not sure if he is still doing it. You could check on there before you resort to buying a new set.

[Kurlon]

Get a multimeter out and check the batt voltage when running. I'm wondering if the reg/rect is on it's way out and slow cooking the CDI? My GS suffered that fate and is ultimately how I ended up with it.

You said you swapped coils/etc, try swapping the coils without unplugging them. IE have the left coil fire the right cylinder, etc. This is a wasted spark 180 degree twin with a single pickup, it won't hurt the bike in the least. If the problem shifts side you've got a CDI / electrical issue. If not then it's either carb or head/cylinder, might be worth doing a compression check the next time you have the carbs off.

[scootertrash]

Get a multimeter out and check the batt voltage when running. I'm wondering if the reg/rect is on it's way out and slow cooking the CDI? My GS suffered that fate and is ultimately how I ended up with it.

You said you swapped coils/etc, try swapping the coils without unplugging them. IE have the left coil fire the right cylinder, etc. This is a wasted spark 180 degree twin with a single pickup, it won't hurt the bike in the least. If the problem shifts side you've got a CDI / electrical issue. If not then it's either carb or head/cylinder, might be worth doing a compression check the next time you have the carbs off.

I checked the battery last night. Just about 12volts with the bike off. Around 13volts with the bike running. I think that's typical ?
I will try to coil trick. I assume the easiest approach is to make two jumpers to run the small primary (coil input) wires to the opposite coil.
After reading some posts on the GSTWIN web site, I think I need to remove the valve cover and check valve clearance. I did check over the winter and the exhaust was in tolerance but just barely. Maybe I need to double check and see if it might be too tight.
I also need to find an adapter for my compression tester. The adapters I have are all for cars with larger spark plugs. I need to find one that will fit in the small plug hole. Or maybe borrow a tester from Advanced Auto.

[Kurlon]

If you can coax it up in the revs, check the voltage around 5k RPM, see if it climbs into the high 14s or more.

On the coils, if the plug leads are long enough, just swap the leads from side to side.

[MHenry600]

Still calling it on the carbs. The VT1100 I picked up in the fall sat for over 10 years. Got it running, but it was super rough. Seemed super rich (black fouled plugs) on both sides. "Cleaned" the carbs, tried leaning them out including different jetting. Figured it was rich, so in my head a plugged port didn't make sense as this would make it lean, right?

So I finally tore them down again, soaked them, and made sure compressed air went to absolutely every passage that exists. Include the vacuum ports also for good measure. Put it back together and it runs like a champ. I've got over 2k miles on it this year now and still runs great.


My money is still on the carbs. Do both of them too, don't just focus on one side.

[Fitron]

Question: have you ever found it hard to start after letting it sit for 5-15 minutes, like running in a store? Or does it always start the same way, everytime, regardless?

[breakdirt916]

Spray quick start fluid into dead cylinder and see if it runs better?

Them it's carbs

[scootertrash]

Bad news guys. After looking in 4 different auto parts stores I finally found the right adapter for my compression tester. I just did a test. The good cylinder is 120. The bad is 80. The manual calls for 114 minimum, and a delta of 28 PSI max.
Tomorrow morning I will be pulling off the valve cover and doing a valve clearance check.
Then doing the old "oil in the cylinder" trick to see if it is the piston rings. Then I guess also see if I might be lucky and find it is a head gasket or something less expensive then a motor rebuild.
Damn.
I do appreciate all the advice so far.
Time for a rum&coke.

[Stromper]

My suzuki's float valve was sealed with an oring that surrounded the body and you had to pull the float seat to see it

It was all dried and cracks letting gas pass by the float valve and flood the motor

Of course make sure the dribbler choke shuts off

[scootertrash]

HA !
HA HA !
So, I was reading my repair manual about the compression test and how best to proceed with troubleshooting. Then I realized I did the compression test wrong. The manual (and a youtube video I watched after) indicated that I should have held the throttle fully open. So I redid the test. Both cylinders now show 130 PSI. Woo hoo!. So I guess I don't have to panic about a bad cylinder.
Back to the carb...

[Kurlon]

With CV carbs, I'd pull them off and re-run as you still have the slides trying to choke the flow.

[Tekime]

Just be glad it doesn't have Goats. In fact, don't be like me and ignore any clunky starting issues, the magnets like to come off the magneto and explode all over her insides.

[breakdirt916]

You sure that cylinder is firing at all?

Bike warm - splash water on the header pipes

[scootertrash]

Success !!!
I guess 4th time is a charm. I cleaned the carbs again. I was extra cautious. I did find that the O ring in the needle valve seat had a small nick in it. Not sure if that made a difference but I replaced it. I also removed the wrap around the main electrical harness. The guy before me put LED directional on the bike and was not the best with his wiring work. I found one broken ground wire. I also did a valve check and all were within tolerance. Compression check with carb removed was a healthy 140 on both sides. Put it all back together and it runs like new! A ton more power than before. It used to struggle going up hills. Now it accelerates up hills. My son is shocked with the power. I just hope he doesn't end up with speeding tickets now.
Thanks everyone for the help. Lots of good advice.

[Kurlon]

Niiiice!