hello everyone,

I have another thread where i am rebuilding my bike and i just got it running for the first time yesterday, but i am having some issues with it running right. i suspect fuel pump to be my issue, but because i have never owned this bike when it was running properly, i am not sure how the fuel pump is supposed to behave. here are my observations. i start the bike and it runs ok, but gets worse and worse over the period of a minute or 2 until it just dies. if i then turn the key back and forth, i can hear the fuel pump priming, but as im cycling the key, the priming gets slower until it just doesnt prime anymore (normal?) then i can start the bike and it seems to run pretty good and rev pretty smooth, but then dies a minute or 2 later. while running, i can not hear or feel the fuel pump running (normal?). the bike sat for years in a garage and the whole fuel system turned to varnish. was able to clean it out, but have not replaced the fuel filter or fuel pump yet (both still had raw fuel in them when i took them off).

anyway, can anyone tell me if what i am seeing is normal behavior? i have the service manual, but it does not clearly indicate what normal behavior is.

 

@Connella08 I don't know the F4 as well as the F3's naturally since I don't have one. I've reviewed the wiring diagram and there is an extra relay as part of the Fuel Pump control circuit called the Fuel Pump Transfer Relay. Based on how it's connected, it appears to be an alternate control path for the power that is supplied to the Fuel Pump.

Fuel Pump Operation
Similar to the F3's the Fuel Pump is controlled by Ignition Coil signals that are also sent to the Fuel Cut Relay. These signals would only be sent when the engine is being started, or when the engine is actually running. However the difference is the added relay, the Fuel Pump Transfer Relay. The output voltage from the Fuel Cut relay would be a pulsing 12v signal which, without a Peak Voltage adapter on your meter, you most likely wouldn't see the full 12v. This output signal from the Fuel Cut Relay passes through the Fuel Pump Transfer Relay. This particular relay is controlled by the same signal that controls the Air Vent Solenoid Valve. This particular signal is active, meaning it is supplying 12v to the Fuel Pump only when the motorcycle is operating below 20 MPH.

So, how does this impact the Fuel Pump? When the bike is operating below 20 MPH, the Fuel Pump is supplied 12v pulsed signals in sync with the #1 Ignition Coil. When the motorcycle is moving faster than 20 MPH, the Fuel Pump Transfer Relay (along with the Air Vent Solenoid Valve) is turned off. This then supplies a continuous 12v to the Fuel Pump.

I'm not aware, and can not tell from the wiring diagram if there is maybe a timer circuit that allows the Fuel Pump to prime or not. I know that the F3's don't. But the F4's might. Maybe the owners manual says something about that.

I'm curious, is your Gauge Cluster plugged in?

 

my gauge cluster is plugged in. i have the bike setup so that when it finally starts to behave, i can take it for a test ride. i will see if i can prop the rear wheel up and get it above 20mph to see if the fuel pump starts running. if not, does this point to the transfer relay, or the fuel cut relay?

 

It's hard to say for sure. What you're describing is somewhat normal. Are you familiar with how a Solenoid works? If so, then that is essentially how these fuel pumps work. A magnetic coil that moves a plunger up/down pulling and pushing a diaphragm to pump the fuel to the carburetors. When the bowls are full and the Float Valves close, then the Fuel Pump diaphragm can't move anymore, and the pump stops pumping. As the fuel level goes down and pressure is relieved on the down stream side, the pump starts up again.

There is a Fuel Pump Volume Inspection procedure for bypassing the relays and providing power directly to the pump. Using that procedure while you're idling for 2 or 3 minutes should tell you if it's a relay issue, meaning a voltage supply issue, or a pump issue that once it stops, it just won't start again. Maybe something is binding on that plunger under the black cap on the end of the Fuel Pump.

 

i am quite versed with electro-mechanical valves and pumps, but i also know that carburetors have a maximum feed pressure before the float needle gets forced open. since i cant seem to find any form of fuel pressure sensors in the system, that must indicate the fuel pump is relatively weak if it is physically stopped from the amount of pressure it generates on its own. i did see the fuel pump volume procedure, but i dont have a graduated cylinder. i also saw a few posts online and videos on youtube taking the back of the pump off and inspecting the contacts. im going to do that as well. i am used to pumps typically being gerotor and not a literal plunger, so its just an odd design to me

 

Yes, this pump probably only generates 3 or 4 psi. It's only job is to fill the bowls and keep them at a certain level. The floats that operate the float needle can easily hold back the pressure due to the mechanical advantage of the float, float needle location and the pivot point. Surface area of the needle valve is about .05 sq in, so that's only going to amount to about .2 lbs. of pressure.

 

good to know. im probably just going to go through the whole fuel pump system (everything before the carbs) and verify that its operating as intended before i go much further. im sure some of my running problems could be due to the heavy pre-mix i put in the tank, but i want to verify operation first before i beat my head against the wall.

 

so i got the bike running finally over the weekend and got to take a nice evening ride with some friends. during my testing, i was able to confirm that the fuel pump does run all the time when it is above 20mph. however, it does not seem to run at idle aside of the initial prime at key on. following the troubleshooting flowchart in the manual, i was not able to identify anything wrong. fuel cut relay and transfer relay seem to test fine, but they only seem to get voltage at initial key on. i was not able to conduct the volume test on the pump because i cannot get it to run consistently into a container for 5 seconds straight.

 

Well, I think that unless it leaves you stranded or starts stumbling while riding, it all sounds normal. The exception being the volume test. The only reason I was having you perform it, was to verify that it would run continuously. But the output hose on the pump has to be open and free flowing. If it is still attached to the carburetors, pressure will build up and it will stop. Anyway, it seems to be working for you ok.

 

i would normally agree with that assessment, however i have to prime the key 5 or 6 times in order for it to build enough pressure to stop pumping. each time i turn the key on, the pump runs for about a second then stops. it does not appear to stop because it has built pressure but rather because it has lost its signal to continue pumping.