Bike surging at highway speeds?

[Karl with a K]

When riding around 70 to 80 mph I notice the bike will surge forward and back in a jerky fashion.  At first I told myself it was just the wind blowing, but after a long ride today I think it is the bike.  It feels like the bike momentarily loses power, then regains it, loses it etc, in rapid succession. 

Has anyone else noticed this?  I've never felt it on any of my other bikes.  I thought perhaps it was due to the lightweight, and  the wind blowing it, but it happens at times when there is no noticeable wind.

I installed an aftermarket airfilter.  I wonder if it is getting enough air.

Any thoughts?  I think the filter I installed is a PMI.

[Kijona]

What kind of shape is the chain in? If the chain is too tight or has tight spots in it it can generate this effect. Do you notice it ever at lower speeds?

Is it rhythmic or?

[The Buddha]

Surging under steady throttle = lean.

Soft when opening throttle = rich.

You're lean I wanna say.
Cool.
Buddha.

[craigs449]

Are you running out of gas?  Mine does that when I hit the 165ish mile mark and need to switch to reserve.  Also check for pinched fuel lines blocking flow to the carbs.

[Karl with a K]

not running out of gas.

Chain is new, sprockets are new. 

I adjusted the chain to spec which seemed tight to me.  When sitting on the bike the chain is snug, but still moves up and down a little.

[Karl with a K]

I don't notice it at lower speeds.  It usually starts at 75 to 80 mph sometimes at 65. 

[bombsquad83]

I think Buddha is on to your problem.  Probably running a little lean and starving it at those speeds.  What's your carb setup?  Main jet, washers, pilot, turns on mixture screw?

Just to be sure though, if you are concerned with the chain being too tight, loosen it just tad and see if it cures your problem.

[Big Rich]

I think what Craigs means is: even if you did re-jet to compensate for the new air filter, you can still be running lean because of a pinched line, clogged tank vent, slightly clogged fuel filter, etc.

I'm with the general consensus. Sounds lean, but I can only guess as to why.

[Karl with a K]

The carbs are stock.  I cleaned the jets, but never changed anything.  The lurching isn't happening all the time.  It comes and goes.  If this is due to a lean mix I was wondering if anyone else has experienced it.

[Big Rich]

Check the easy stuff first - fuel starvation from what's listed above. Or if you don't mind spending the cash, just get bigger main jets.

[twinrat]

if your bike is later than 04 it is lean  or water in petrol  use methalated spirits in tank (cupfull) and this will mix with petrol and absorb water and carry on riding ,no need to empty tank.i think budda is right .

[mister]

Karl,

If you removed the tank to do some work there is a good chance you have a pinched fuel line. So try these things to help narrow it down...

Leave the bike in 4th gear and do these speeds. The bike will do 87mph in 4th at 9,000rpm so it is quite capable.
Leave it in 4th and take it up to the same rmp as the problem has been showing - sounds like around 5,500 to 6,000 - and see if the problem exists.
Put the petcock in prime and ride as you have been and see if the problem still happens.

Also, if you had the tank off might pay to double check you have the undertank petcock in the correct position.

Michael

[Dizzledan]

I've had this problem and have cured it by fitting an aftermarket petcock. I'm not %100 on the reasons behind it, but there's something about the vacuum operated system, and the drain hose on the carb (the top one) and my bike will starve at higher speeds. You can just flick it over to 'prime' or get a vacuum-less petcock.

[Kijona]

I've had this problem and have cured it by fitting an aftermarket petcock. I'm not %100 on the reasons behind it, but there's something about the vacuum operated system, and the drain hose on the carb (the top one) and my bike will starve at higher speeds. You can just flick it over to 'prime' or get a vacuum-less petcock.

I think what happens is the rubber in the petcocks gets kind of tired and allows air to seep into it, which would give you bubbles in the fuel line which would cause what he is describing. Putting it on PRI may or may not handle it for him. It's hard to say, it just would depend on where it's leaking from.