Loss of power at highway speed

I have a 2002 ST1100. I just bought it a few weeks ago. At highway speed (60 MPH+) it will run OK for about 5 miles and then slowly lose power. I can roll off of the throttle and coast down to about 40 MPH while occasionally downshifting until I am in second gear. At this point I am then able to accelerate back to the 60 MPH + range for a couple of more miles and then this repeats.

I have replaced all 4 spark plugs (correctly gapped) and the fuel filter. I have checked the gas cap vent hose and it is clear. I am able to move air through the gas cap vent valve after a little resistance.

To me this sounds like a fuel delivery problem but I don't know what steps to take next. Fuel pump, carburetors?

Also, in general, while the bike is pretty responsive in the lower gears it seems like the 1100 cc 4v engine should be more powerful that it is.

Can anybody offer me some insight on this?

Sounds like a leaky membrane in your vacuum fuelcock.

Martin.

Check the fuel filter, it could be getting blocked. It's under the tank lid on the right hand side.
Hope it helps.
Upt'North.

Sorry, just read you've changed the filter. Doh......

Portupan said:

Sounds like a leaky membrane in your vacuum fuelcock.

Martin.

Click to expand...

Easily bypassed... if you prefer, you can order the parts to fix it. There are several vendors, ma Honda wants to sell you the entire assembly for a small fortune.

I've been running without a vacuum petcock for 15000km with no issues. I'm sure that this will be the most likely cause for the loss of power.


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Portupan said:

Sounds like a leaky membrane in your vacuum fuelcock.

Martin.

Click to expand...

There is no listing for this in the service manual that I have. Is this the same part as the fuel tap?

The Red Menace said:

There is no listing for this in the service manual that I have. Is this the same part as the fuel tap?

Click to expand...

You could call it that, but it is the component connected to the fuel filter, then feeding into the carbs. It isn't in the service manual because Honda does not consider it a serviceable part. They intend you to replace it. The repair kits are not Honda parts, but aftermarket.

If you wish to keep the bike stock, check out the K&L repair kits on www.denniskirk.com. They are also on ebay but I got mine for $27 on the Dennis Kirk website. Otherwise, remove the unit. There is a sticky on this site...just search for it.
Your problem could also be a fuel tank venting issue or a fuel pump unable to keep up with demand.

Thank you all for the insights. I will likely bypass the petcock (at least to troubleshoot).

sounds like a fuel pump problem