Fuel Starvation Problem ( ST1100 ) *

[KoTAOW]

Written by Rick Alvarez, STOC 7119:

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Fuel Starvation Problem

This is a follow-on to my earlier thread about my 1100 starving for fuel and the trouble I had diagnosing the problem.

The short story is that it was the OEM fuel filter and since replacing the filter with another OEM part I haven’t had another problem in over 2000 miles.

The longer story is kind of interesting:
I didn’t think that it could be the fuel filter because the problem was intermittent. Also I had changed the original filter at about 100,000 miles and the replacement filter (OEM Part) “only” had about 25,000 miles on it when I started having problems.
I also didn’t think that it could be the vacuum cut-off because I had replaced the diaphragm about 5,000 miles ago (and the problem was intermittent!)

This first time it shut down on me it was 28 deg F and many folks thought that I had carb. Ice. I thought that I may have had a vacuum building up in the tank so I checked the vent line and disassembled and cleaned out the fuel cap.
The problem didn’t re-occur for 1000 miles and then it was on a warm day (no ice). I left the cap loose and continued to have trouble so it wasn’t a vent problem.

I could not reproduce the problem near home. The bike would only shut down after 20+ miles of high-speed freeway riding. I tried to make it run bad by doing lots of full-throttle runs to redline and while lugging but it ran great and never missed a beat.

The ride home in the tow truck after another freeway shut-down was the last straw, and of course, once we got home the bike ran great

By now I was sure that either the fuel pump or the fuel control relay was the problem.
I decided to ‘instrument’ the bike to help find the problem. I added a Cycle Max voltmeter wired across the fuel pump and a fuel pressure meter on the handlebars so that I could see if the pump had voltage and pressure the next time it stalled. I also removed the vacuum shutoff and installed an auxiliary fuel pump (Facet ‘105) in place of the vacuum valve and wired up a switch that I could turn on when needed.

I jumped on the freeway and about 30 miles later the bike started to starve for fuel but the voltmeter and the fuel pressure looked normal! As soon as I turned on the Aux pump the bike smoothed out and ran great! I got the bike home, pulled the hose off of the filter, turned on the in-tank pump and fuel would barely trickle out. I turned on the Aux pump and had great fuel flow. I removed the filter and tried to blow through it and I could but I felt a little resistance. I cut the filter open and didn’t find any debris. But the paper was discolored. Maybe some type of varnish or Huh?

By the way, I usually run Arco fuel or whatever is handy (87 octane).

What I learned:
• Stock pump has low pressure but good volume. From testing my pump and reading about other 1100 pump problems it looks like our pumps are only good for about 1 psi of fuel pressure. They don’t do well with a restriction!

• Also tried applying a vacuum to the tank and the stock pump quit pumping immediately so make sure your vent line is open.

• The fuel control relay is more than just a dumb relay. It applies power for a second or 2 when the key is first turned on. Then it uses the ignition events (tach signal) from the ignition module to determine if the engine is running, if so it applies power to the pump.


I am going to leave the vacuum fuel cut-off valve off of the bike and leave the Aux pump on for a while. I was going to add a voltmeter to the bike to monitor the alternator output. so I may leave it connected to the fuel pump. It reads a little lower than battery voltage here but I can still tell that the alternator is charging. I’ll also remove the fuel pressure gauge and relocate the Aux pump switch.

Attached are a few pictures of my Aux pump, gauge and switches.