My '86 750F acted up a couple of times this past summer when the feed line to the fuel pump seemed to vapor lock, and each time while I was sitting by the side of the road waiting for things to cool down I wondered if the fuel pump should know to run continuously when the line is empty or not. It would give a click each time the ignition was turned on, but that was it. The only way I could get the fuel to start flowing again without just sitting there grinding on the starter was to turn the key on and off again a whole bunch of times in quick succession. At the time it didn't seem quite right, but each time when the bike started again everything was fine so I forgot about it.
Well, eventually I did sort out the vapor locking problem, but the "run dry" thing happened to me again last night when I coasted down a very long steep hill with the tank nearly empty. I flipped to reserve fairly quickly (the old bikes have one, y'know!), but by then the line had run dry and I had to go through the whole side of the road/flick the key routine to get gas back to the carbs. Is this normal behavior, or is there some sort of a glitch in my pump/relay set up that I should be concerned about?
[image: View Post]AndrewNS, on Sep 30 2007, 06:37 AM, said:
Hi guys,
My '86 750F acted up a couple of times this past summer when the feed line to the fuel pump seemed to vapor lock, and each time while I was sitting by the side of the road waiting for things to cool down I wondered if the fuel pump should know to run continuously when the line is empty or not. It would give a click each time the ignition was turned on, but that was it. The only way I could get the fuel to start flowing again without just sitting there grinding on the starter was to turn the key on and off again a whole bunch of times in quick succession.
I had similar questions re: my VF500. I did a quick search on the V4 Magna tech forum and this is what one of their guys had to say about it:
The fuel sender switch gets its signal from one of the coils. It only fires the fuel pump under the following conditions:
1. The bike is running, and there is no backpressure in the fuel lines
2. The starter button is pressed and their is no backpressure in the fuel lines
Well, something is quite wrong. I often run out of fuel and have to flip to the reserve while riding. There should just be a stumble while the carbs try to fill back up, and you should not be having to sit on the side of the road.
I would tend to think that the switch that shuts off the fuel pump when it experiences back-pressure has grown weak and is shutting off the pump far before the lines are pressurized. Or there is some obstruction on the input side of the pump.
VFOr, your observations do seem to jive with the way the pump behaves, i.e. only pumps when the engine is running. I never looked to see if it pumped while the engine cranked, but maybe it does. I don't think so, though. I'll have to check that out - if true, I guess you would just keep cranking until the system filled up and the the thing would fire again. Each time it quit, I tried to go easy on battery power for fear of killing it dead before I could sort the problem out.
And Chev, I agree something weird is going on. I've never had a problem switching to reserve before, either. But I don't think the pump can be shutting off due to a back pressure switch malfunction, 'cause the fuel line and filter are empty each time this happens. But as soon as I can get gas to flow back into the filter it fires right up again. It seems to me that a false pressure shutoff would happen with the line and filter full.
All of this did start right after I changed what seemed like the original, 1986 era fuel filter earlier this summer. There certainly was a lot of crud loosened up when I did this, and maybe all of the twisting and jiggling required to get the old filter out could have caused a problem in the pump. The filter was replaced with another OEM item, so that shouldn't be the problem. And after the first time it happened, I cleaned and flushed the tank so I know the pickups at the tank end are clear.
not vapor lock........too cool for that, but some crud is blocking flow down to pump.
pull fuel line going into pump and check for flow; if not flowing, open filler cap, turn selector valve to OFF, take in a big breath of air, apply mouth to fuel line, turn valve to ON, BLOW!!! quickly turn valve OFF, reconnect line.
you just blew some crud off fuel pick up tube inside tank.
[image: View Post]squirrelman, on Oct 9 2007, 05:18 PM, said:
i've had same prob too.
not vapor lock........too cool for that, but some crud is blocking flow down to pump.
pull fuel line going into pump and check for flow; if not flowing, open filler cap, turn selector valve to OFF, take in a big breath of air, apply mouth to fuel line, turn valve to ON, BLOW!!! quickly turn valve OFF, reconnect line.
you just blew some crud off fuel pick up tube inside tank.
also, maybe clogged (old) fuel filter.
GOOD LUCK!
If you have an air compressor just put some air through the hose up into the tank. Blow out both on and res settings. Be sure to have the cap open.(I didn't once and split the seams on the tank - never do that again.) Works better (and is easier) than trying to clear it with your lungs.
No, it's not plugged pickups...I cleaned the tank (and the pickups) thoroughly after the first time I had the problem. I'm thinking the first two times it was vapor lock, cause I had just rerouted the the feed line to the pump and it was making hard contact with the left rear spark plug wire. That transfered a lot of heat to the line. It seems better now.
I guess my original question was if the pump should know to run until it primes itself and the carbs when the ignition switch is turned on without the bike running. And I don't think it should, judging from the replies I've gotten and my read of the relay circuit in the service manual. If anybody knows for sure differently, I'd sure like to hear from them.
[image: View Post]AndrewNS, on Oct 15 2007, 12:57 PM, said:
No, it's not plugged pickups...I cleaned the tank (and the pickups) thoroughly after the first time I had the problem. I'm thinking the first two times it was vapor lock, cause I had just rerouted the the feed line to the pump and it was making hard contact with the left rear spark plug wire. That transfered a lot of heat to the line. It seems better now.
I guess my original question was if the pump should know to run until it primes itself and the carbs when the ignition switch is turned on without the bike running. And I don't think it should, judging from the replies I've gotten and my read of the relay circuit in the service manual. If anybody knows for sure differently, I'd sure like to hear from them.
Andrew
It will not run continuously. It will only pump once when you flick the run switch from on to off to on. Not sure why they did it this way, but they did. You can jump the relay to get it to run continuously to test it. Be careful of running power straight to it, if you get the leads reversed you can kiss the pump goodbye. Apparently gravity feed works as well. I ran into a bout of vapour lock on my 85 1000R this summer, bike just quit while coasting up to a set of lights (it was about 2 hours into a ride with ambient air temp at 36C). Took it apart on the side of the road, switched to gravity feed for the rest of the trip home. Got home and tested the pump, it worked fine. Hooked it back up, and have been on many rides since longer than 2 hours, with no problems. Have not tried it on real hots days though, I just take one of the other two bikes on those days.
Thanks - that's exactly what mine does. I was worried that the pump or relay had packed it in, but I guess it's just normal operating procedure.
It also goes a long way towards explaining why it takes quite a bit of cranking every spring to get it to fire for the first time. I always drain the float bowls in the fall, and it would take a while to get gas up into the carbs again without any sort of automatic priming feature.
Incidentally, my two vapor lock incidents were also both while coasting on really hot days, just like you describe. Odd, but I think things are sorted out now. But I guess I'll have to wait until next summer to know for sure.
i also drain carbs for storage but always jump the relay to prime carbs again when removing from storage instead of draining battery and overworking starter motor.
[image: View Post]squirrelman, on 16 October 2007 - 09:35 PM, said:
i also drain carbs for storage but always jump the relay to prime carbs again when removing from storage instead of draining battery and overworking starter motor.
Just wanted to say thanks for this old thread. Very informative to me for a stupid little problem...
End of thread · 12 posts recoveredWayback Machine snapshot 4 Jun 2011