Engine stutters at 4K

[compasseagle89]

Hey guys, first post for me...I decided to get more in tune with my bike and do more of the general maintenance myself.  I thought the air filter would be a good place to start.  I removed the gas tank with a little bit of difficulty from the vent lines, but other than that no issues.  Checked the air filter, and it was really clean.  I blew out a little dirt, and put everything back.

But now, if I accelerate heavily, between 4K-5K the engine really struggles.  If I accelerate more slowly it will go through this band with absolutely no problems.  It feels almost like fuel starvation, but I've gone about 50 miles and I don't think it's that.  I took the tank off again and reseated the lines, but the same thing happens.  Thoughts?

[rider123]

Try driving on prime, maybe you forgot the vacuum petcock line. You could also open up the gas cap to see if you have vapour lock in the tank preventing fuel flow.

You didn't loosen the carb boots did you? Maybe one is loose.

Also what year and model are you driving? It helps as there are slightly different fixes for the years of bike.

[compasseagle89]

Pardon my manners! I was so tied up in the problem that I didn't think about the details :)

It's a 2003 GSF1200. The vacuum line is definitely seated, and I didn't touch the carbs at all, so I'll try the other 2 suggestions of yours.

[compasseagle89]

I did some further testing just now.  I tried it with the petcock on "prime" and the tank open, and it initially started struggling around 3800.  After a few miles, I turned the petcock to "on" and closed the tank.  It looked like it went away, but then came back above 5K.  Kept the petcock "on" and opened the tank again, and went a few more miles, then closed the tank.  Very little, if any stuttering.

Not sure if it's worked itself out, but I'll keep trying.  Thank you so much for the suggestions!

[rider123]

Check to see that the breather hole under the tank latch is free of obstructions. If it runs ok with the lid popped, it might be that the breather hole is a bit gummed up and there is not enough air to displace the gasoline causing vapour lock.

[compasseagle89]

Check to see that the breather hole under the tank latch is free of obstructions. If it runs ok with the lid popped, it might be that the breather hole is a bit gummed up and there is not enough air to displace the gasoline causing vapour lock.

I took a picture...is the orange gasket the breather hole?  it looked pretty clean.  i did have the tank on it's side when i took it off in order to avoid getting anything in the petcock.

[pmackie]

Maybe you've got a little water in your tank, and after swishing it around and reinstalling the tank, you might have sucked a small amount of water into the carbs.

I would drain the carb float bowls, if you can, into a glass jar. The water will settle into the bottom of the jar. If you DO find any water, then completely drain the tank by removing the petcock to ensure everything is water free.

I know it sounds bizzare, but I have seen this before, inluding on my buddy's VFR800, which is fuel injected, but suffered a small, intermittant stutter as the small amount of water got sucked up by the pump.

[compasseagle89]

I would drain the carb float bowls

I have the service manual, but it doesn't specifically say how to do that.  There is some work on the carbs I could do I'm sure, but I wouldn't know where to start.

[compasseagle89]

Fixed it!  Turns out I wasn't as careful as I thought when putting everything back.  The air box wasn't completely sealed.  Closed it up, put in new spark plugs, and now it runs like a champ again.