Airtex E8371 fuel pump 98 ST1100

tomspear · Jul 19, 2016

I replaced the in tank fuel pump last week and have noticed some interesting differences between it and the OEM pump.

Before replacing the pump the engine would start instantly when cold by applying full choke and no throttle. Now it will only start with no choke and no throttle. It starts quickly but struggles for several seconds at a lower rpm before it will maintain idle. When the engine is warm and the bike sits in the hot sun for a while (1 to 2 hours, 100 degree day) it can also be difficult to start. It used to start instantly when the engine was warm.

I did a 100 mile fuel mileage test at varying speeds (approx 45 to 70 mph) and it seemed to be about the same as before at 47 mpg. I also compared volume pumped (OEM 48 ounces in 1 minute vs. Airtex 8371 83 ounces). The current draw on the OEM pump was 1.1 amps vs. the Airtex 2.0 amps.

Other than the starting issues the only other thing I noticed was a higher idle rpm after putting in the new pump. I had to turn it down from 1200 to 1000. I have not removed or bypassed the vacuum shut off valve.

Thanks,
Tom Spear
STOC 2316

Oldbikefixr · Jul 19, 2016

The manual for the 1100 only specifies a minimum fuel flow...640cc or 21 ounces per minute.
The Airtex is putting out 4x that. Since your mileage is unchanged, it doesn't sound as though the pressure is excessive.
Any unburned gas odor after a ride?
Did you install the bypass section from the old pump?

tomspear · Jul 19, 2016

Yes I do smell some gas after a ride. Not very strong but it is there.

I couldn't figure out how to install the check valves / regulator from the oem pump so I hooked the new pump directly to the output tubing. I thought possibly I could install a regulator in line if I needed to up by the filter or vacuum cut off valve.

Another interesting fact about the failed oem pump was that the current draw was normal at 1 amp and the motor / impeller sounded and looked normal. Yet no fuel was being pumped. I assumed that might mean that it couldn't get thru the check valve that is located at the output of the pump. I also assume that the check valve acts as a pressure regulator. I took it apart by slicing off the plastic rivet heads and found nothing wrong or plugged up inside it. Nothing to it but a spring and a disk.

thanks,
Tom

Oldbikefixr · Jul 19, 2016

One of the threads on St1100 fuel pumps shows the original pressure regulator installed. It involvea placing a fitting into the input hole and connecting it to the new pump with a short section of tube.

It would be interesting to determine the failure mode. Normal current draw may indicate that the impeller failed rather than the brushes worn out.

My 1991 exhibited similar hot start symptoms as yours. It has just under 19000 miles. I was tracing down a raw gas odor. The only item in the fuel chain I hadn't rebuilt was the vacuum valve. The diaphragms weren't stiff or holed but they were extremely 'floppy'. Installed a K&S kit and the odor went away....so did my hot start problems. Consider this as a possible cause of your problem, brought on by increased fuel pressure. Other possible culprits are aged float valves in the carbs.

She's running, you have fixed the major problem with a successful pump change.

TerryS · Jul 19, 2016

I installed the Airtex 8371 in my ST1100 a few months back. I bought the bike as a non-runner as it had sat for nearly 10 years and everything was predictably clogged, the OEM pump was beyond saving. My Airtex is connected directly to the old fuel hose and I did not use any part of the old pump. I also removed the vacuum shut-off from the circuit. Current draw on my Airtex is around 1.6A. I can't comment on any difference in fuel use or idle with the old pump as the bike didn't run.

My bike needs full choke to start cold and is a bit on the cold-blooded side as it won't idle happily without choke until warmed up, but is perfectly happy to be ridden within a few seconds of starting, and once warm it starts instantly and idles like a metronome. I have tinkered with the pilot screws and these may not be in their optimum positions.

I'd suggest you may need to look at your fuel cap vent and maybe the carb float valves. The fuel tank will pressurise from engine heat when shut down, and you might be getting high fuel levels in the carbs leading to rich running when you re-start.

tomspear · Jul 20, 2016

I have a spare vacuum cut off so I might try that, or bypass it altogether. I'm reluctant to bypass because of the possibility of fuel flow into a carb with a float valve problem.

I did some experimenting with resistors in series with the fuel pump power feed this morning. The goal was to try to find out how low I can go on the applied voltage to the Airtex and still have fuel flow. I ended up having 4 ohms of resistor value to limit the current to 1.4 amps (down from 2 amps). Much more than that and the pump won't run. The voltage applied to the pump becomes 7.7 (with engine running and alternator output at 14V). This means the other 6.3 Volts is dropped across the 4 ohm resistor. The resistor gets too hot to touch but is rated at 100 Watts and I calculate only approx 10 Watts is required (6.5 V x 1.5 Amp). I did not perform a flow test but will take out today for a test drive. Probably won't make any difference but worth a try.

I still have a good OEM pump assy. I can put back in as soon as the level in the fuel tank goes down enough to get the pump out (just to make sure I don't have a new problem)

Edit: I forgot to mention the fuel smell might have been the 1 gallon container that was on the garage floor next to the bike with the cap on loosely... Also I'm pretty sure the fuel cap is good, at least in the "release pressure" direction. I can blow thru it and the hose to the canister. Not so sure about the "suck" direction for allowing air in to equalize pressure as fuel is used. Never could figure out how that worked. I also have a spare cap "somewhere" (probably in the same box as the spare vac cut off valves)

thanks,
Tom

tomspear · Jul 23, 2016

Update, 3 days later...

The test ride was a failure, ran out of fuel in approx 1/2 mile. The fuel pump sounded like it was running with 4 ohms of resistance in series with the power feed to the pump (7.7 Volts at the pump), but it was not pumping any fuel.

I decreased the resistor value to 2 ohms and tested the pump output. It now pumps 64 ounces per minute, down from 83 ounces per minute with no resistor. Fuel pump current draw became 1.75 amps down from 2.0. Voltage at the pump is now 10 Volts with the other 4 Volts dropped across the resistor. The resistor is rated at 50 Watts and the calculated power to it is 7 Watts but it still gets hotter than hell. I started with a crude heat sink constructed of a pair of super sized fender washers and the heat after 15 or 20 minutes of operation was measured at 200 degrees F. I made a better heat sink out of a heavy piece of steel and got the temp down to about 125 degrees.

The pump works well in this configuration but I'm not sure if I've really gained anything. I decided to go back to a good OEM pump and do some testing and come back to the Airtex later (if at all). I can post up a few pictures of my Airtex configuration and my resistor and heat sink if anybody is interested...

thanks,
Tom

Edit: I just went out to the garage and applied full choke (ambient air temp 75 Degrees), pressed the starter and the engine fired instantly just like it used to (oem pump is now back in). With the Airtex E8371 the engine would not start with any choke applied. It would always start (no choke) but the rpm's would be low (approx 500) until it warmed for a couple of minutes. The rpm's would gradually come up as it warmed to where I set it (1000). After reinstalling the oem pump I had to increase the idle rpm back up a little to get back to 1000 (where I had it with the Airtex).

Color me confused

Al st1100 · Jul 23, 2016

tomspear said:
Update, 3 days later...

The test ride was a failure, ran out of fuel in approx 1/2 mile. The fuel pump sounded like it was running with 4 ohms of resistance in series with the power feed to the pump (7.7 Volts at the pump), but it was not pumping any fuel.

I decreased the resistor value to 2 ohms and tested the pump output. It now pumps 64 ounces per minute, down from 83 ounces per minute with no resistor. Fuel pump current draw became 1.75 amps down from 2.0. Voltage at the pump is now 10 Volts with the other 4 Volts dropped across the resistor. The resistor is rated at 50 Watts and the calculated power to it is 7 Watts but it still gets hotter than hell. I started with a crude heat sink constructed of a pair of super sized fender washers and the heat after 15 or 20 minutes of operation was measured at 200 degrees F. I made a better heat sink out of a heavy piece of steel and got the temp down to about 125 degrees.

The pump works well in this configuration but I'm not sure if I've really gained anything. I decided to go back to a good OEM pump and do some testing and come back to the Airtex later (if at all). I can post up a few pictures of my Airtex configuration and my resistor and heat sink if anybody is interested...

thanks,
Tom

Edit: I just went out to the garage and applied full choke (ambient air temp 75 Degrees), pressed the starter and the engine fired instantly just like it used to (oem pump is now back in). With the Airtex E8371 the engine would not start with any choke applied. It would always start (no choke) but the rpm's would be low (approx 500) until it warmed for a couple of minutes. The rpm's would gradually come up as it warmed to where I set it (1000). After reinstalling the oem pump I had to increase the idle rpm back up a little to get back to 1000 (where I had it with the Airtex).

Color me confused

the oe pump has check valves and a built in pressure regulator. I used that portion of the old pump to regulate the pressure to factory specs. Also check the vacuum fuel cut of valve, if your not sure bypass it. there are lots of old fuel pump posts

tomspear · Jul 24, 2016

Thanks for that image Al. I saw that (or one like it) when I was researching the fuel pump problems. Do you have any pictures of the other side of the OEM regulator (or check valve)? I couldn't see any way to connect a hose to the inside of that piece.

I have decided to leave the OEM pump in place and carry the Airtex for emergency on trip. There seems to be a lack of factual data on the design of the OEM pump (regulator, output pressure, check valve, bypass, the "other" check valve in the end cap, the purpose of the large hose that connects to the big metal tube in the top of the tank opening, etc...). It's too bad one of the original ST1100 Honda engineers isn't on the forum to supply some technical data and answer some questions. It is obvious that the E8371 puts out too much pressure for our Mikuni carbs based on other's reports of lower than normal fuel mileage and my experience of abnormal choke operation. I proved positively (at least on this bike) that the fuel bowls are too full, causing the choke to be useless or problematic. I could "probably" solve that by installing an in line fuel pressure regulator but I'm not going to waste another $40.00 on this experiment.

thanks,
Tom