as soon as the fan kicks in in traffic my engine revs die down and the engine stalls, what can i do to stop this happening

unplug the fan?

sorry
tom

Like it ha ha ha.

remove the fan??

try diconnecting the wiring from the sender and fan, and bridging the connections that were on the sender - check if it kills the engine that way. Do you have a multimeter to go through your loom as it sounds (to me as an electrical numpty) like a short or poor earth??

Does the power come from the battery through a relay if not it should

im not sure if it has a relay? ill check the wires, either way i think its the drop in volts that drops the revs and makes tick over had to maintain

Sounds like the fan is drawing too much current or the alternator and/or battery are suspect.

Does switching all the lights on (particularly main beam) and any other electrics (heater/wipers if fitted) have a similar effect? If so then I'd check the alternator is charging the battery properly and that the battery is holding charge properly.

Does the fan spin freely ?

when fan comes on, how quikcly does the entine die?

at least 2 possibilities:
- fan draws large current and voltage drops so the electroinc ignition fails
or dies more slowly due to alternator unable to supply current.

are you using an old battery or a teeny bike battery?

the battery should be able to supplly current for the fan and engine without alternator running but not for a long time.

also check the wiriing to the fan - any bad connections? you should use thick wires, as snort as possible to reduce losses.

is the alternator charging ok?

what is voltage on battery when ignition is off?
as above but when engine is running
as above but when fan is running