Loss of power

Need help. My '81 550 starts and idles fine. I get it warmed up and in about 8-10 minutes of riding, it feels as though someone pulls some plug wires off. The thing loses power and engine starts to knock and I limp on back home. I have replaced the coils and rectifier. I just put pods on it but that's not the cause because it was doing it before now. Does anyone have an idea where to look next? One more thing, when it starts to drop off power, I can hear suction in the carbs. Could this mean sticking valves?

Could be a few things.
Since you have replaced the coils I would start with the obvious and check that your valve clearances are not too tight and that the fuel cap vent is not blocked.

Yeah, just got to thinking this evening that the loss of power really mimics running out of gas on the main tank and having to switch to the reserve. I have an aftermarket fuel cap that seems to be working ok, and though I have rebuilt the petcock, I have heard other people having issues with it and just replacing it with a non vacuum assist petcock. That may be my next step.

When was the last time the valves were checked?
Steve

I don't know what kind of cap you have but crack it open when you start to have the problem, see if it goes away.

Check the valves first. And you must rejet the carbs as well if you didn't already after installing pods. I wouldn't even try to ride the bike until it's rejetted because it will be running lean and hot.

The issue there is that it was doing it with the air box as well. And if the valves were faulty, wouldn't show up from the start, not just when it's been running for a short time?

Gman58 wrote: The issue there is that it was doing it with the air box as well. And if the valves were faulty, wouldn't show up from the start, not just when it's been running for a short time?

In regards to the valves being a issue . Remember that the tolerances change once warmed up so the motor may become out of spec once warm changing
how the motor performs. If the valves are close to being to tight to start with cold then heat can move them out of specs.
Dave

No. When hot they will leak. Engine damage will result. Bikes are sold with "carb issues" only to find the PO was too lazy or cheap to adjust them.
Steve

Gman58 wrote: if the valves were faulty, wouldn't show up from the start, not just when it's been running for a short time?

If the valve clearances are too small when cold they will close up even further when hot and this will result in the valves remaining open and not seating.
This will cause loss of power as well as risking burning of the valves and seats and blowback through the inlets.
Depending on the severity this will lead to one or more of the cylinders not doing any work and produce the symptoms you describe.
The common causes of your problem are tight valve clearances, fuel starvation , ignition components failing under load or even a partially blocked exhaust..
These need eliminating before digging deeper.

Would assure correct fuel levels via clear tube tests. Too high level can fuel-foul spark plug (especially at normal operating temp) causing both plugs from the same coil to misfire, same as removing a plug cap.

Good Fortune!

Valve adjustments aren't that big a deal on the larger fours. The cams have to be removed on the smaller ones and the directions must be followed to the letter or engine damage could be the result. That's why they are "overlooked". It is a part of maintenance that must be done before going to the carbs.
Steve