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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 !