Topic: Low Power at higher RPM.

[Tree]

I went on a longer than normal ride today - a couple of hours at speeds of 50 to 65 MPH.  I noticed that at RPM's of 4K and up, the engine seems like it's not producing as much power. Kinds of makes more of a "brrrr" sound than a  normal "rrrrr" if you know what I mean.  It's still smooth, just no "omph", so I'm thinking it may not be tied to a specific cylinder.  So far I've checked the gas filter inside the tank which was very clean, made sure the fuel cap vented OK, and made sure the air filter was clean.    I was thinking about checking timing next, maybe it's firing too soon after the mechanical spark advancer kicks in?    It has a pamco electronic ignition unit. 

Any ideas where I might look next?

Thanks. 

[Johnny5]

Fuel starvation? I know when I need to switch to reserve, my bike will sound different as it's not getting enough gas.

[TwoTired]

How old is the air filter?
Does it have the expected power with the filter element removed for a test?

What do the spark plug deposits look like?

When was the last routine 3000 mile tune up check list completed?

[davesee]

i had this same sort of problem on my 650 today. at higher revs and speeds it just seemed to resist a bit (the bbbrrr sound is a good description).  i was getting a bit low on fuel (i'm always on reserve) so i pulled into a gas station and filled up. back on the road the problem seemed to have dissappeared.
i also have a suspect air filter in there, so doesn't really nail the diagnosis dow too much.

[pamcopete]

Tree,

I compiled a list of things that can restrict high speed operation:

1. Low battery / charging system. A low battery will produce a low ignition voltage at the plugs which is most evident at higher RPM's.
2. Timing. Did you use a timing light to set the timing?
3. Stuck or no advance.
4. Binding advance .
5. Fouled plugs. Did you install new plugs?
6. Plug wire not making contact in the coil. Measure the resistance from plug cap to plug cap to ensure that both plug wires are making contact in the coil.
7. Brakes dragging.
8. Speedo out of calibration. You are actually going faster than 60 MPH! Use your GPS to check the speedo.
9. Tire pressure. Pump up to 32 PSI for high speed.
10. Drive chain too tight. That will rob you of power .
11.
12. Choke left on or partially on.
13. Weak or loose advance springs. Weak or loose advance springs cause the advance to use up some of the available movement because you have to essentially retard the advance to get it on the idle advance mark, so there is less movement available at higher RPM's.
14. Timing chain needs adjustment.
15. Cheap or bad gas.
16. Low octane gas.
17. Water in the gas.
18. Partially blocked pet cock or fuel filter.
19. Paper filter that has low flow when the tank is not full.
20. Try a run with a full tank of high octane, fresh, expensive gas.
21. Blocked fuel cap vent.
22. Incorrect sprockets.
23. Rear tire rubbing on brake rod or swing arm.
24. High wind resistance. Are you a large person?
25. Throttle cable not fully opening the throttles.
26. Blockage in the muffler(s)
27. Dirty or blocked air filters
28. High altitude
29. Very hot air, like 95+F
30. Very cold air, like 32F
31. Low oil
32. Really dirty oil
33. Gas in the oil

[phil71]

sometimes, that hollow sound can be caused by jets too large, or needle height set too high, assuming they've ever been changed or messed with.
If you know it's stock, then it's definitely fuel starvation somewhere.