High RPM problems 78 GS550

[lewisac]

First off, this place is awesome. Long time lurker, first time posting.

1978 GS 550, everything is completely stock. got about 3-4 years ago. ran great.

as of the last year, i just can't get the bike into the power band. It used to take off like a rocket at 6k rpms. now it struggles to get over 6k rpms. I say struggles because thats what it feels like, almost sputtering.

I just had the carbs cleaned and re-synced. and there is a noticeable difference in other ways (throttle response) but the problem still exists.

I replaced the spark plugs ( there where carbon fouled from running rich for so long), and that helped other things as well (starts strong, sounds stronger), but the issue still exists.

I remember awhile ago I cleaned the airfilter and i'm don't recall oiling the filter, could that be an issue?

I also replaced the inline fuel filter (previous one broke), could that be impeding fuel flow? perhaps the petcock?

perhaps exhaust flow? its standard exhaust with a standard air box?

any ideas?

Thank you guys.

Alex.

 

[lewisac]

I've personally never adjusted the valves, however I believe the PO did, (4 years ago, i say this because the gasket looked new and not stock)

i've also never done this procedure before, ever. i'm fairly knowledgeable, but i'll be the first to admit when I don't know how to do something.

Is there an easy procedure to checking?

 

[koolaid_kid]

It is in your service manual. If you do not have one, go here and download a pdf copy.

 

[lewisac]

could this be the issue?

I mean is this diagnosis related to the symptoms?

or rather, what are the symptoms of ill-adjusted valves?

 

[tkent02]

At first it's hard to start. Tight valves will sooner or later burn up. Once the valves burn, it gets worse. Poor/no compression, then everything turns to crap.

Running lean due to no oil on the filter makes the valves more likely to burn sooner.

Have you checked the compression? This will tell you if your valves have started to burn.
Really checking the valve clearances is easy, and it is the most important maintenance of them all. Don't even think about not checking it.

 

[lewisac]

At first it's hard to start. Tight valves will sooner or later burn up. Once the valves burn, it gets worse. Poor/no compression, then everything turns to crap.

Running lean due to no oil on the filter makes the valves more likely to burn sooner.

Have you checked the compression? This will tell you if your valves have started to burn.
Really checking the valve clearances is easy, and it is the most important maintenance of them all. Don't even think about not checking it.

I had the compression checked, two cylinders were at 100%, two where at 85-90%

 

[tkent02]

Adjust the valves, the two low ones will go up if you didn't wait too long. If they have already burnt much, there's no coming back up.

If you wait longer they will all get worse.