Surging at highway speeds - Where to start?

[Guest]

I went out for a ride today and couldn't help to notice that the bike was surging when I was steady-cruising at highway speeds (55-75mph). The surging wasn't really noticeable when I was accelerating though...

Where should I start looking? I've recently got the valves in spec, cleaned and synced the carbs, and roughly set the air screws. I just received a colortune, so I'll be doing that soon.

Judging by my search on the GSR it seems that there's many roots for surging, so I'm wondering where to start? There's a light rust inside the tank, so I suppose that could be related to it. I bought a back-up petcock during the Blvd Suzuki sale a while ago just in case, so I could throw that on...But, do you think that the colortune procedure will help out? Where else should I look?

thanks!

[Nessism]

Is the bike stock? If so, shimming the needles might help. First thing I'd do though is get that tank derusted. For light rust Evaporust works great.

Good luck

[7981GS]

Ed is correct.
Try to drop the needle one shim.

As the slide is open, it is drawing fuel from the main jet and as it draws too much full, the slide drops until it doesn't have enough, then it raises.
This process is being repeated as it floods itself then wants more after it shuts.
Basically, the slide is bouncing up and down repeatedly.
I hope that I explained that well enough to understand.
I almost recorded it when I saw it happening with the pod filters off during a tuning session.

Eric

[verde]

Make sure the vent tubes are routed properly and terminate in an area shielded from lots of air turbulence. Trim the ends at an angle.

[Suzuki_Don]

Kevin, you need to set your mixture screws up correctly. If as I suspect this surging is happening just off idle at highway speeds, with the throttle only slightly open then this is your problem. I had the same issue and found two mixture screws needed to be turned in 1/4 to 1/2 a turn each. Check the color of your plugs at idle.

[Dogma]

Make sure the vent tubes are routed properly and terminate in an area shielded from lots of air turbulence. Trim the ends at an angle.

This is the first and easiest thing to check. They should be routed over the stock air box and end a couple inches above the bottom of the box. My 850GL surged like crazy until I got that right. I haven't needed to cut the angle on the ends though.

[LoopGaroo]

I didn't realize how important the vent tubes are, and have them routed a little haphazardly. I'll start with rerouting the tubes and setting the mixture screws.


I couldn't really "get" the highest idle method, I guess I didn't have the ear for it, and just have them all set at 2.5 turns out.

[AJ]

No bad advice above, especially about the vent tubes.

Is this a new problem? Have you recently added some aftermarket components, like a new aftermarket air filter, aftermarket exhaust, pod air filters, or removing the airbox lid? If you did any of those things, you may have to adjust main jet or needle jet height. The fact that it seems to run fine wide-open throttle indicates that the main jet/needle height is probably ok.

If the bike is completely stock, then the main jet and needle are probably not the problem. The pilot jet system is most responsible for partial throttle response between 3 and 7,000 rpm (as in, cruise). Consider looking for partially clogged pilot jets or dirt in the air screws.

The idle mixture screws can compensate somewhat for problems with the pilot jet system, but their influence largely tapers off above 3,000 rpm, so I don't think this is the problem either.

[LoopGaroo]

The bike is stock, and yes, this is a pretty recent issue. but I haven't been do a whole lot of highway riding so far this year so it could have gone unnoticed for a while.

I checked the vent tubes, and two were venting off to the side of the airbox cover. I can see how this might cause a problem at high speeds.

[verde]

This:

I went out for a ride today and couldn't help to notice that the bike was surging when I was steady-cruising at highway speeds (55-75mph). The surging wasn't really noticeable when I was accelerating though...

Plus your follow-up puts all my money on the vent tubes. Careful changing more than one thing at a time. I'd go ahead and check the screw settings as others have suggested, but in general if your carbs were properly set-up for the stock configuration then leave them alone. It sucks to go on a wild goose chase with jets, screws, and needles just for a lousy vent tube

[AJ]

Plus your follow-up puts all my money on the vent tubes. Careful changing more than one thing at a time. I'd go ahead and check the screw settings as others have suggested, but in general if your carbs were properly set-up for the stock configuration then leave them alone. It sucks to go on a wild goose chase with jets, screws, and needles just for a lousy vent tube

Excellent advice. It's always a good idea to check the easy things first, especially when the diagnosis fits the evidence.

[LoopGaroo]

So just as a follow up, I rerouted the vent tubes and the surging stopped! Thanks everyone!