Running a little rough - suggestions?

[scootertrash]

2008 Suzuki GS500F. Air cooled inline twin. Two carbs. Bought the bike a couple of months ago cheap, knowing it would need work. (seller was very open that it didn't run great, so it was priced cheap)

What I have done :
- Drained gas tank. Didn't find any water. Put in fresh gas. In-tank fuel filter appears very clean. Gas flows out of tank/filter well.
- Air filter looks good
- Removed carbs. Removed bowls. No dirt or crud. They were very clean. I removed jets and ran a thin wire through them. Didn't appear to be plugged.
- I did NOT check floats or float seat. (I wish I had)
- Removed valve cover and did valve gap check. They were all within spec. Some were on the spec border (high and low) but I figure I will do a shim change in a year or two.
- I visually checked all vacuum lines on carbs. All in good shape. Not dried out. I guess there could be a problem that is not visible, but the condition of the hoses are good.
- Carbs are firmly seated back in rubber boots. (carb-to-engine boot)
- New spark plugs. OEM spec plugs. (NGK brand). Plug wires look good. Visibly strong spark when I have the plugs out of cylinder.

Problem :
I gave it the first test run this past Saturday. Starts and idles good. First few miles ran good. A few misfires over the first few miles. I figured it was because it was cold. My son then drove it around the school parking lot for 20 minutes. First gear only, slow speed. It ran great, no apparent problems. I then jumped on it to ride it back home. Leaving the parking lot and accelerating to 50MPH it misfires on and off. Then it smoothed out for a few miles with only a few misfires randomly. I then had to go up a hill and take a 90* left turn. At that point, it misfired severely and totally died over a distance of 100 yards. At that point, the road was level again. I let it sit for 20 seconds and tried it. It fired right up. Drove home a couple of miles. Only a few misfires. Seems to misfire more when accelerating or going up a slope. But does some on flat. Idles great, no misfires at idle. I don't think it will misfire if sitting still and reving engine. (or at least not much)
Per the internet, GS500F have issues with the fuel shutoff, since it is vacuum actuated. Per the internet, a good test is to put the shutoff in Prime setting, which bypasses the vacuum. I tried the bike in all three settings (Run, Prime, Reserve) and had the same misfire results, so the fuel shutoff does not appear to be the problem?

What I plan to do next :
- Check coils with ohm meter. I don't think they are the problem, but I might as well check.
- I might run a fuel line directly from tank to carbs and bypass the fuel shutoff, just to completely verify the fuel shutoff is not an issue. (since it is a common problem with this bike)
- Pull carbs again and check floats and float seat.
- I do not have a special gauge to sync the carbs, but I am guessing that would not cause this sort of issue?

Any other suggestions?

I am thinking it is a fuel problem, not spark, but that is just my guess.

Thanks !

[Petorius]

We should start a club. I had problems getting my GS500 to run well last summer, eventually gave up but will be back at it soon.

A couple thoughts- check your float heights first. If it happens on hills or while leaning to turn, maybe they are not filling properly at certain orientations. Also, confirm that your issue is NOT related to throttle position. You said it worked fine tooling around in a parking lot in 1st, and had issues accelerating up to 50mph. Maybe it is not related to orientation or engine speed, but actually to throttle position. Tooling around in 1st, you wouldnt need to open the throttle much. Bringing it up to 50mph, you may have used more throttle. They are notorious for having issues in the needle range (~1/4 to 2/3 throttle). Lots of folks shim the needles up, or you may even have adjustable needles.

[sdog30]

Air/fuel mix gets clogged. A lot of people don't like to remove and clean them. I've cleaned carbs and not the air/fuel and have had problems.

[scottieducati]

Could also be the tank vent clogged and creating a vacuum.

[sdog30]

Also, check for any chewed wires.

[dontpanic]

When the carbs are off, check to see if the diaphragms are cracked.

[Kurlon]

On my GS with similar issues it turned out to be a petcock problem. At times the flow out of it would get aerated and starve the carbs. Let it sit for a moment to catch up and it'd run ok for a bit. I ultimately diagnosed it by pulling the tank, putting a clear fuel line on the petcock into a gas can and letting a full tank drain while I watched. It'd flow solid for a bit and then start bubbling, eventually the flow would stall for a moment, then resume with a gurgle. If that's the case with yours, OEM petcocks are cheap enough that I'd skip monkeying with it, just replace it.

[Dmitry]

One of the issues I've seen is a leaky fuel shut-off valve, it has a rubber diaphragm that cracks and floods one cylinder with fuel. Helps if you have a clear vacuum line so you can if any fuel is going though it.

[scootertrash]

Good ideas everyone. I made a list of things to check out. Thanks !

[scootertrash]

It's running better now. I went through it again on Saturday. Tank vent was ok. Coils ohm'ed out ok. No chewed wires. Fuel petcock seemed to flow fuel just fine. The Throttle Position Sensor ohm'ed out fine. I pulled the carbs again. This time I soaked the jets in carb cleaner and sprayed the carb with cleaner more thoroughly. I checked the floats and float valve. I had to slightly adjust one of the floats to get it to the exact spec in the manual, but not by much. Rechecked all vacuum lines. Carefully reassembled everything. Took it for a ride and no misfires at all this time.
I suspect it was the more thorough cleaning of the carbs. Or maybe last time I assembled it I had a vacuum line pinched or something. Getting the airbox and carbs and fuel lines and vacuum lines all in place is a real pain on the bike. Zero extra space.