Midrange Miss/Backfire - 2008 FE550

Hi guys,

Trying to troubleshoot a problem. Hope someone here can help me.

2008 FE 550

I have a midrange miss/bog that just suddenly starting occurring both when accelerating and when backing off the throttle while trying to hold the bike at a constant speed (like when in steady one lane traffic. Originally it came on in a very slow way as just a very slight hiccup or engine surge when holding the bike at a constant speed in traffic. It wasn’t serious enough to even do anything about. Then all of a sudden one day coming home it just jumped to serious status to the point that its barely even rideable.

I pulled the carb off and methodically cleaned and examined all jets and passages within the carb and noticed nothing that stood out (following all the guidelines in the service manual). Put it back together with all the adjustment settings exactly where they were before.

Now with the carb cleaned out, its very hard to start, but when I do finally get it to start (usually with a tiny twist of throttle which never used to be needed before), it backfires like crazy at low and at midrange and doesn't want to idle at all. On top of that, the mid range miss/bog is still there. Once it climbs up into the main jet it seems to clear up and there seems to be power in acceleration from my limited testing.

I thought I incorrectly installed something in the carb so I pulled it apart and re-cleaned everything to rule that out. But both problems are still there second time around.

Intake boot looks new with no visual cracks and clamps are confirmed tight. I removed air filter to rule that out. Fresh fuel and filter is clean with full flow from tank valve in inlet hose on carb.

I have not changed the spark plug which I admit could be the culprit but living in northern Canada (Whitehorse), I can’t find a plug wrench or socket that fits between the camshaft housing. So if you strongly believe that this could be the issue, hold back those itchy fingers of yours until I can get my hands on a wrench that fits.

But if you have seen this before and don’t think it is the plug issue, I am open to any suggestions of where to go after the plug. I'm no mechanic but have a decent aptitude of understanding about how engines work.

Thanks in advance.

can't say what the original problem was, but pulling apart the carb as a first effort was prob a mistake and may have caused another problem. there are a dozen things that cause a poor running bike. the fact that you can't get the plug out seems to indicate you should be using caution when making changes.
try getting one of these cheap wrenches from a hardware store. they are thin wall enough to get to the plug.

Sounds a little like it could be a broken exhaust valve spring.

when putting the carb back together owners often put a plate back on the slide the wrong way up. get the carb off to look at that plate again from the front. it should read: 'M' and not 'W'.

cheers!

Taffy