Hey guys

I've been trying to chase down this issue and I'm not quite sure what's up. I'm occasionally losing power on my '85 K100RT. When I get up to 5,000 RPM, it feels like the bike is bogging down or I'm running out of gas. I notice it mostly in higher gears, but occasionally it happens just cruising stoplight to stoplight in 2nd or 3rd. If I pull in the clutch, I can rev however high I want, but as soon as I let the clutch out and try to put that power down on the road, it happens again. If I shift up, I can get a little more power but it will inevitably happen in the next gear as well, until I'm right-lane cruising at 55-60mph in 5th gear. It seems to only happen when I have under 1.5 gallons in the tank. If I pull over and put in a gallon or so, I'm back to normal.

It is extremely intermittent. I ride 50 miles a day round trip to and from work. It happens maybe twice a week. At first I thought it had to do with heat because the first 5 or 6 times it happened were during the heat of the day between 1-3pm. It has now happened a few times after 10pm when it's nice and cool out. I'm beginning to wonder if it is something with the ECU. I've never had an injected bike before, so it is something I'm still learning about. I did run Techron fuel system cleaner through the bike but the issue is still there. I've got a new fuel filter on the way, but it seems like the problem would be happening all the time if it were the filter. Once it shows up, I plan on running through a full service.

I've had the bike for about a month. I've ridden motorcycles my whole life, but this is the newest bike I've ever owned and the only vehicle I have that is not carbureted. I'd really like to start taking my bike up the coast and on longer trips, but I'm obviously hesitant while this is an issue.

Any thoughts?

Put the new filter in as it sounds to me that you are losing fuel pressure and that is the first place to start.
Don't be too fast to suspect the ECU as they give very little trouble even at near 30 years old.

RicK G wrote:Put the new filter in as it sounds to me that you are losing fuel pressure and that is the first place to start.
Don't be too fast to suspect the ECU as they give very little trouble even at near 30 years old.

Thanks for the input. That's why I went ahead and ordered the filter. It seems weird to me that I can pull the clutch in and have no issue, but I guess it's the difference of there being no load on the shaft. Hopefully it the filter does the trick

Yes. Unlikely to be the ECU.
The symptoms disappearing when you refuel could mean you are getting a vacuum developing in the tank as the fuel level goes down. Try riding for a while with the cap unlatched to see that does the trick. The tank vent is the pipe on the inside of the tank and exits at the right/rear under the tank to a plastic collection cup which drains to the rear.
There's also another drain which comes from the cap recess.

Also, worth checking valve clearances imo. Possibly intakes are a bit tight. If they are you can do damage so worth looking at even if to just rule them out. They use a shim but are easy to check. I had tight exhaust valves a while back and the symptoms were very intermittant like yours.

Kaptain Holister wrote:Yes. Unlikely to be the ECU.
The symptoms disappearing when you refuel could mean you are getting a vacuum developing in the tank as the fuel level goes down. Try riding for a while with the cap unlatched to see that does the trick. The tank vent is the pipe on the inside of the tank and exits at the right/rear under the tank to a plastic collection cup which drains to the rear.
There's also another drain which comes from the cap recess.

Also, worth checking valve clearances imo. Possibly intakes are a bit tight. If they are you can do damage so worth looking at even if to just rule them out. They use a shim but are easy to check. I had tight exhaust valves a while back and the symptoms were very intermittant like yours.

Thanks. I noticed some vacuum pressure previously and posted about it. Eventually I wrote it off because sometimes it would happen and when I opened the cap, I didn't hear a rush of air. Whe. The filter comes in I'm just going to go through everything in the tank. Hopefully tomorrow.

Could it be cavitation at the fuel pump filter? It would be commensurate with low fuel levels and would go away on re-filling the tank. 

Later models had an 'anti-cavitation' tube fitted to the filter, which would suggest that earlier bikes could suffer.

Paul.