bike background:
NC30 VFR400 1990.
stock everything (exhaust, carbs).
M max box fitted (was there when I got the bike).
Kph -> Mph adaptor (blue one).
Rick Oliver (from Radtec) upper and lower radiators.
Overwride switch for the fan.

Until a few days ago the bike was running perfectly fine.

Now here's the issue, which is an odd one to be honest.
You can start the engine without any problems (hot or cold), you start driving and it's still fine, however as soon as you go over a small pot-hole or on a not so smooth road surface the engine will die (but not cut out) (ie: revs will drop a lot).
It feels very well like a fuel starvation, same symptoms
This almost only happens on a bumpy surface, I can ride the bike on a smooth road and it nearly won't happen.


This is what I've done so far to diagnose the problem:
1.
Removed the vacuum hose (fuel tap) and started the engine.
I do get a strong vacuum pulse. Vacuum hose is not kinked, nor cracked, nor stuck anywhere.

2.
Removed the tank and using a manual vacuum pump (the one you would use to bleed your brakes) connected to where the vacuum hose would go, the fuel is gushing out from the tap without any issues.

3.
Removed the air-box and checked for any debris, it's all clean inside.
Also managed to move the black slides inside the carburettors by gently blowing (or sucking, can't remember now) in the two medium sized hose at the back of the air-box.




I'm open to all suggestions and I hope it's not something too serious .



thanks for your time
Stephane

I might be electrical, loose connection shorting when you go over rough/bumpy roads.

Good call.

I've been doing some search on the forum all day today.
found as you said a loose connection, rec/reg, killswitch etc..

I will do some electrical fault finding this week-end.

is your battery secure? Could be jolting in the tray and not have the best earth connection.

It doesn't sound like fuel to me - if you turn the tap off you can ride for a minute or so with the fuel in the carbs....
Electrical issue...

Funny thing is, it only started after I installed my fan switch.
I know the switch wiring is fine, but I must have knocked off something, like a plug or a wire.

will double check this week-end

CMSMJ1 wrote:is your battery secure?

^^ What he said!

It is now fixed, and it was VERY silly

When you remove the seat (cowling), you need to disconnect the tail light plug.
Now this was fine, but beneath that plug there is another little black plug (4 wires) which was loose
Took me less than 5 min to figure it out.
I tried to start the engine with that plug disconnected and it wasn't going

Took the bike for a ride, same road as yesterday and the bike ran great


Thanks all for the help

dont ya just love a nice easy fix that dont cost anything!