82 gs650 wierd power loss at higher RPM's

[spadefoot]

We rebuilt the carbs (mostly a friend of mine with a lot of mikuni experience did, I held stuff and dipped things and bought parts) a few months ago. It was a pretty complete rebuild, with new pilot jets, new(er) rubbers, full dip and pretty good sync. Also rebuilt the petcock. It starts fairly well, takes a little while to warm up. Most of the time, it runs just fine. From time to time, at higher RPM's, when I open the throttle, the sound of the engine changes, it loses power and doesn't speed up. It CAN happen in any gear. If I keep the RPM's under about 5500, it doesn't really happen.

Today, on a ride, I noticed something really odd: At highway speed (50-60 mph), if I extended my feet (pretending I had highway pegs, for instance), the problem would start to happen. If I tucked my legs back under, on the pegs, the problem would subside. I tried this about 10 times, coming and going, and it would happen every time.

In the course of working on this bike, we discarded the airbox (filter is still in place) and I'm thinking that, sometimes, the bike is either getting too much or not enough air, and that extending my legs causes some sort of change in the airflow that causes the problem to happen. It CAN happen without me extending my legs, it's just that the legs thing will make it happen reliably.

Weird as heck, any ideas or thoughts?

[GQROD]

We rebuilt the carbs (mostly a friend of mine with a lot of mikuni experience did, I held stuff and dipped things and bought parts) a few months ago. It was a pretty complete rebuild, with new pilot jets, new(er) rubbers, full dip and pretty good sync. Also rebuilt the petcock. It starts fairly well, takes a little while to warm up. Most of the time, it runs just fine. From time to time, at higher RPM's, when I open the throttle, the sound of the engine changes, it loses power and doesn't speed up. It CAN happen in any gear. If I keep the RPM's under about 5500, it doesn't really happen.

Today, on a ride, I noticed something really odd: At highway speed (50-60 mph), if I extended my feet (pretending I had highway pegs, for instance), the problem would start to happen. If I tucked my legs back under, on the pegs, the problem would subside. I tried this about 10 times, coming and going, and it would happen every time.

In the course of working on this bike, we discarded the airbox (filter is still in place) and I'm thinking that, sometimes, the bike is either getting too much or not enough air, and that extending my legs causes some sort of change in the airflow that causes the problem to happen. It CAN happen without me extending my legs, it's just that the legs thing will make it happen reliably.

Weird as heck, any ideas or thoughts?

Could be putting your legs up is causing turbulence, this lack of laminer flow ( over an airplane wing ) causes an imbalance of air pressure ( reducing lift on a wing ) and creates vortecies ( spiriling pockets of air rushing to fill vacuum left by object moving through the air ).

But it may be your mixture.

Plug chops will let you know how rich or lean it is running however at a given throttle opening so start there first.

Hope this helps.

[Nessism]

The GS bikes were on the lean edge from new. If you change the airbox in any way such as removing the lid, the engine will run lean which causes running problems.

Installing the airbox back is the fastest solution. You can rejet the carbs also but this involves trial and error.

Good luck.

[spadefoot]

I'm going to try some air restriction and see what happens... Man, that leg thing was strange...

[chef1366]

Do you have feet as big as Shaq?

[spadefoot]

Do you have feet as big as Shaq?

10.5 EEE, not nearly Shaq-sized...

[skidMarkNZ]

I would say without the airbox the air is getting up behind and effectively being rammed into the carbys, my gs used to do the same from 7k upwards...

and often ran on 3 cylinders...

Was too lean, once i richened it up she went like well...a cut kat!