Loss of power at high revs (Video)

[YoungEngineer]

I have a 94 K1100LT. Bike runs beautifully, I have fully serviced it inc, TB balance, TPS adjustment, filters, fluids and all the rest.

I seem to be having an issue with an intermittent loss of power at high RPM and when under load.

My theory is either the gas cap is blocked and not letting air in, (I do also smell fuel) or the fuel filter is clogged (I have yet to replace it)

I have a video here demonstrating the issue seen at around 2:00

[drut]

Fuel filter change would be good move as often neglected but also HT leads might be worth a look.If bike was mine I would check basics first: i.e. fuel pressure,compression,air leaks etc.

[YoungEngineer]

I agree on the fuel filter but surly this is not an ignition issue otherwise the issue would occur throughout the rev range and not just at the very top. This only seems to happen when I am giving it (Sh!t) for a prolonged period.  It seems to me like a fuel delivery issue. I do have a compression tester but for some strange reason none of the provided adapters seem to fit. Do K bike have a strange thread pitch? I have one adapter which seems almost identical in size to the plugs but it just wont go.

As for compression the bike has 92K but does not seem to smoke or drink much. Plenty of power for a brick. I have owned a k100 before witch half the mileage which smoked like a chimney. I can only assume I have a good example here from previous experience anyway.

[drut]

HT leads worth a check as they can break down under load(so can coils) & personally I distrust metal(radio suppression)shields on sight.Compression is  an unlikely suspect & 92K is nothing to a brick.When my fuel filter was blocking up strangely a mid-range flat spot@ 4-5K was the biggest symptom.All I was trying to say was try to rule things out one by one,when I first got my bike it had a bad connection to no1 injector:easy to sort but only once found!

[YoungEngineer]

Alright sounds like a plan. Il take my propane torch tomorrow and release some gas near the inlet tubes and TB boots to see if there is an increase in rpm. As for HT leads is there anything I can do to check them apart from inspecting them for cracking/breakdown of the casing? I replaced the plugs recently. I did add a bit of Silicone spray lube to the HT leads heads not sure if that could cause issues. I still think this is a gas cap issue just from an almost identical previous issue on another bike I had.

[Martin]

Run the bike on it's centre stand in a dark garage and look for arcing on the HT leads and caps. You will either have to cover the lights or pull the fuses in order to get darkness.
Regards Martin.

[stokester]

Not the most knowledgeable on K-bike here but it sounds like fuel starvation to me.

If it was ignition would we not hear backfire and popping as the spark came in and out?

[YoungEngineer]

I would assume you are right. Not included in the video was segments of power loss for more then 2-3 seconds. It felt like the engine had been killed as I would slow down and bring the revs down the engine would come back. The issue happened a few times, only when I was really pushing it. I had a similar issue on another bike which ended up being the fuel cap. The cap was blocked not allowing air into the tank which at high rpm and engine load would cause a vacuum in the tank and not allow the fuel pump to do its thing.

Does anyone know how air enters the tank? It it through a breather somewhere or is it a valve in the cap?

[kennybobby]

There is a breather tube that comes up thru the bottom of the tank to a small baffled chamber at the top of the tank--it also serves as a tank over-flow, maybe the tube is clogged.  You can take a test ride with the gas cap lid open and test if that is the problem, or get a wire coat hanger and router it out, blow or pull air thru it to verify it is open, etc.

Did you pull the gas cap and look in to see if there is water in the tank--that can also cause issue such as yours.

[YoungEngineer]

Alright well i've checked for airleaks all seems to be ok no change in rpm when exposed to propane. As for the fuel filter it states 1998. However there still seems to be a good flow in the tank when the pump is on. The return flow of fuel to the tank is good. I stripped the gas cap and cleaned and lubed all the O rings. As for the breather I blew it out with compressed air. There did seem to be some minor blockage.

Let me get this baby back on the motorway and see how it goes.

[bizzaro]

Still waiting to hear if you have determined what the cause of engine cutting out was, and how you resolved it? My bike does pretty much the same thing.  Very intermittent, and doesn't seem related to rpm or heat etc. Just quits ever so briefly on the run.  Sometimes it happens frequently, and others not much.  And it never lasts for any duration of time. Short seconds and goes away. 

And have a great trip.  I assume that you have resolved this issue  as part of your preparation for it?(the 10000 mile trip)

[YoungEngineer]

Still waiting to hear if you have determined what the cause of engine cutting out was, and how you resolved it? My bike does pretty much the same thing.  Very intermittent, and doesn't seem related to rpm or heat etc. Just quits ever so briefly on the run.  Sometimes it happens frequently, and others not much.  And it never lasts for any duration of time. Short seconds and goes away. 

And have a great trip.  I assume that you have resolved this issue  as part of your preparation for it?(the 10000 mile trip)

I rebuilt the GAS cap and blew out the breather and never had a problem again. Hope that helps.

[Laitch]

I rebuilt the GAS cap and blew out the breather and never had a problem again. Hope that helps.

Thanks for the update, YE. It's helpful and considerate of others when threads are closed out with the results of efforts, regardless of whether those efforts are successful.

[bizzaro]

Yes thanks. Sounds too simple for such a complicated machine but I will give it a go first thing in the Spring when I get her back on the road. 

Again, have a great trip! :riding:

[YoungEngineer]

Yes thanks. Sounds to simple for such a complicated machine but I will give it a go first thing in the Spring when I get her back on the road. 

Again, have a great trip! :riding:

If it does not seem to be linked to rpm then I don't believe it could be a blockage in the tank breather. The reason I suspected the issue was because it would only happen at high rpm's under heavy acceleration (When the most fuel is being used). I would also notice a strong smell of petrol when riding. There were no leaks anywhere so it could only be coming from the tank. Oh and I also noticed that fuel would disappear from my tank when the bike stood for a while. When I left the bike for three months in the summer about 2-3L of gas evaporated from the tank. In-fact the fuel gauge was still showing 1/5th and after many failed starting attempts I checked the tank and it was dry.

I had a small 100cc bike once that when doing anything over 30mph would do the same thing, it felt as though the fuel supply had been cut off. It ended up being a blocked breather which was built into the gas cap. There needs to be air allowed into the tank to replace the fuel which is used by the engine, without that a vacuum will be created and the engine will be starved of fuel.