Operation of the FUEL PUMP

[MadDogMcQ]

Hi All,
Does anyone have a proper working knowledge of the fuel pump and relay on the CB-1?

The reason I ask, is that my bike seems to run perfectly well with a very full tank of petrol, but as it gets lower, there is a "hesitance" when trying to accellerate past 10,000rpm.  I changed the carbs completely and the problem still exists. I'm guessing that it is some form of fuel starvation.

I want to test the relay and pump but need to know how to. I have a multimeter and would appreciate any tips to make sure that all's well.

Regards,

Tom.

[sparkybloke]

Ive just replaced the fuel pump on my CB1.it was causing a fuse to blow about half a sec after starting the bike and i was searching and checking thro electrics on cdi ignition circuit and coils but noticed on wiring diagram that fuel pump was fed from cdi ( via relay as pump can pull a few amps). Ive replaced the pump with second hand unit from vfr 750 of same age and all is now well.the pump regulates itself by means of an end switch on the drive shaft, when energized from cdi/relay the coil pushes a diaphragm which forces fuel through to carbs and end switch opens if insufficient pressure in chamber the shaft returns and switch closes again.this continues until the pressure in fuel line to carbs stops the pump diaphragm returning until more fuel is required.
when i put the fuel tank back on and pressure was low the pump clunked a few times and then stopped and presumably now clunks when needed! the pump cost me £42 off ebay but i saw one after for £15! lots of 12v pumps are probably compatible esp Honda from same era. just one thing to watch when pulling off fuel lines to carbs ; wear some glasses cos petrol in your eyes really *****in stings!!

[sparkybloke]

forgot to mention that 12v supply to pump is only live when engine is running, suggest you try finding out if you have 12v at the connector that feeds the pump.it was located on lefthand side behind the rear plastic cowling, and the relay is around there too.if no supply to pump check all fuses failing that it could be relay or cdi unit. the  cdi is £125 from david silver but i saw 1 on e bay for 35 ish

[MadDogMcQ]

Thanks Sparky.  I ran the engine and with my finger on the fuel-pump, I could feel it "pulsing" regularly. These pulses increased with engine speed, so I'm guessing all's well in that department.

Any ideas what can be causing this hesitance when accellerating hard (bearing in mind that I've already changed the carbs).

[sparkybloke]

you say you get a hesitant respose to hard acceleration, does this mean that after the hesitation the bike then revs freely or still not responsive? my bike was restricted to 33bhp by previous owner because of his licence etc maybe you have something similar.the only other things i can think of are fuel filter or lines blocked.if the diaphragm in the pump is broken this might reduce pressure under high demand,also some fuel tanks used to have a strainer where the fuel tap connected to tank; is the fuel tank clean inside.

[MadDogMcQ]

The bike revs and pulls very freely and eagerly until it reaches around 9-10,000rpm. It is then reluctant to be pushed beyond that. If I twist the throttle back hard from 9000, the engine "bogs down".  I can cajole it to go beyond 9000 by gentle increase in throttle position.

Since it seems to perform better with a FULL tank, I am starting to think that there must be a blockage which is overcome by the pressure of a full tank but not by a low tank. I guess I should remove the filter and try it again.

And maybe I should check the strainer and diaphragm as you suggest. 

Dirty hands tonight then!