Quick XS400 question

I'm currently helping a friend of mine fix his 1980 XS400 which has been sitting in his garage for 3 years and I have a problem.

The bike will not push over 40 and it has a problem running, period.

-cleaned carbs, religiously cleaned them
-carb sync with vac gauge
-adjusted valves
-new plugs
-cleaned and reoiled old air filters

This guy is on a penny budget so that's all I've been able to do so far. It fires up on the first kick and idles great but it just will not go once it's on the road.

I'm thinking just replace to old air filters because they're probably 10 years old but he doesn't want to spend the $17 for new ones. :x

When sitting at idle it will rev past red-line but under weight it won't push. While running it also feels like it's being flooded or starving out, can't figure out which as he doesn't have a kill switch and I have not tried a plug chop yet.

Ideas?

 

Sounds like starving for fuel/air. Will he run it without filter just to see? Not long, but long enough to figure out if that is the issue?

Could be a fuel flow problem. Sounds like fuel tank low on fuel not swithced over to reserve. Perhaps there is something clogging the petcock or the fuel line (partially). Takes much less fuel (because of less need of power) to rev high in neutral than under load...

 

The '80 I worked on had points, If so make sure they are clean and free of oil. Check clearance, adjust as needed. Make sure the cam timing is correct. Then set the timing with a timing light. You might want to re-check your carb sync after.