3cv fuel pump - intermittent?

[shudnobetter]

My 1990 3cv stopped running and after much messing about I've discovered the fuel pump is priming the carbs for starting ok but seems to have a mind of its own as to whether it will pump fuel when the engine is running. Mostly it seems not to. I've pulled the fuel line of the out pipe on the pump several times when engine running and in 3 out of 4 times there is no fuel coming through. After a short time the engine stops.

Any inspiration will be welcome before I shell out on a new pump.

[FJ_Hooligan]

Get a brass plumbing connector (probably 5/16th inch) and connect the pump input line with the output line.  Make sure you have a full tank of fuel and see if that gets it running reliably again.

If it runs on gravity feed then that will tell you it's definitely the pump.

[shudnobetter]

Get a brass plumbing connector (probably 5/16th inch) and connect the pump input line with the output line.  Make sure you have a full tank of fuel and see if that gets it running reliably again.

If it runs on gravity feed then that will tell you it's definitely the pump.

I'll try that. I've heard it could possibly be the pump relay - do I need to exclude that somehow or just go for a new pump if the bypass test works?

[ribbert]

Get a brass plumbing connector (probably 5/16th inch) and connect the pump input line with the output line.  Make sure you have a full tank of fuel and see if that gets it running reliably again.

If it runs on gravity feed then that will tell you it's definitely the pump.

I'll try that. I've heard it could possibly be the pump relay - do I need to exclude that somehow or just go for a new pump if the bypass test works?

....and if it is the pump, use a multi meter or test light on the live wire to the fuel pump, that will tell you if the power supply is constant or intermittent, or, if you have neither of those, run a wire from the battery direct to the pump and see if it runs continuously.

IMO

Noel

[shudnobetter]

I've been doing some more tests. Cleaned points on pump. Runs fine for the priming 5 seconds ignition on, then stops when engine runs.
If I put a jump cable between the blue/black and red/white cables on the relay it runs constantly.

To a newbie it looks like the problem originates before the pump ie the relay. Thoughts?

[Pat Conlon]

If I put a jump cable between the blue/black and red/white cables on the relay it runs constantly.

To a newbie it looks like the problem originates before the pump ie the relay. Thoughts?

Even with bypassing the relay, The pump should not run constantly (like it does the first 5 seconds on start up).
You should just hear it tick every couple of seconds while the bike is at idle.

[shudnobetter]

If I put a jump cable between the blue/black and red/white cables on the relay it runs constantly.

To a newbie it looks like the problem originates before the pump ie the relay. Thoughts?

Even with bypassing the relay, The pump should not run constantly (like it does the first 5 seconds on start up).
You should just hear it tick every couple of seconds while the bike is at idle.

Understood.

OK. More messing about with the voltmeter and here's the latest.

When ignition on I have for 5 seconds:
Load wire to pump 12 v for 5 seconds. Good.
Trigger wire from CDI less than 1 volt for 5 seconds (corresponds with load wire). Good.
The other 2 wires 12 volts.

Ignition on after 5 seconds:
Load wire: 0. Good
Trigger wire: 0. Good
Other 2: 0. Good

With engine running:
Load wire: 0. Bad
Trigger wire: 0. Bad
Others 12: Good

UNLESS I start the engine during the 5 seconds before the pump stops clicking in which case all wires stay live as during the 5 second priming. In this case the pump stays running.

Seems to me that whatever signal is sent to the relay to restart the pump once the engine is running is not happening, but somehow it's fooled if the engine is started during the priming when all is on. I assume that signal comes from the CDI so there must be a fault there. Does that make sense?

[Pat Conlon]

Does your tank have at least 1 gallon in it? If not, you could be on the reserve circuit.

Try pushing the on/reserve rocker switch to "reserve" and see what happens.

[shudnobetter]

Does your tank have at least 1 gallon in it? If not, you could be on the reserve circuit.

Try pushing the on/reserve rocker switch to "reserve" and see what happens.

Tank is almost full.

[Pat Conlon]

Ok, assuming the fuel pump relay is good (you may still want to change it) check the tach signal and pick up circuits circuit to the DCI ignition box.
The DCI box has a safety circuit that shuts off the fuel pump when the tach/pick up signal is absent (other than the prime function the first 5 seconds at power up)

This prevents the fuel pump from pouring on fuel in the event of an accident. When the signals are absent (engine stall, etc), the DCI box shuts off the pump.

If the problem is within the DCI box, then there is a work around. You don't have to hunt for a new DCI box.
You could power the fuel pump directly from a key switched circuit and wire in one of these: https://tinyurl.com/zuydbcg


Hope this helps

Pat

[edit] You could run the pump directly from a switched 12v source but using the safety switch is highly recommended.

[shudnobetter]

OK - now for the really dumb stuff ... I cant locate the cdi unit. Anyone tell me where it is?

[Pat Conlon]

As you sit on the bike, under right side panel.

[shudnobetter]

As you sit on the bike, under right side panel.

Just not seeing it. I assume there has to be one!

[Pat Conlon]

Ok, my mistake, sorry.... I have a '84 and had a '92/93 and my recollection was at fault.
http://www.partzilla.com/parts/search/Yamaha/Motorcycle/1990/FJ1200A/ELECTRICAL+2/parts.html

Looks like it's under the fairing on the right side.....

[shudnobetter]

Ok, my mistake, sorry.... I have a '84 and '92/93 and the TCI/DCI's were in the same area, so I assumed (incorrectly) that your '90 would follow suit....
http://www.partzilla.com/parts/search/Yamaha/Motorcycle/1990/FJ1200A/ELECTRICAL+2/parts.html

Looks like it's under the fairing on the right side.....

No sweat - that's helpful. Now for a few more bits of plastic to come off!