help required before i do something with matches , 1991 vfr400 nc30 , fresh battery , new rectifier , new plugs , wires checked , turning over but weak spark and wont start , carbs have been cleaned , fuel vac hose replaced , fuel tap and tank cleaned , still no go , but wait for it take 1 plug out doesnt matter which one so leaving 3 plugs in and ht leads connected and bike strts no problem and runs one 3 cylinders , what is going on here can someone help please ? oh and another point the battery was on fully charged went away turned everything of came back few hours later and the battery was absolutly flat so it draining while ignition off ?

Define fresh battery, new battery, old but fully charged battery? Have you load tested the battery. After charging, place battery in bike turn on high beam headlight and measure battery voltage. Should remain above 12.5 volts for at least 15-20 minutes.

How do you know the bike has weak spark?

To test for battery current leakage with ignition off, disconnect battery+ve lead at battery and multimeter between battery and lead, set to 10 amp range. Current reading should be zero. If zero current then bike electrical system is not the cause of the battery discharge and might be simply battery issue.

Sounds to me like you have an old (even if fully charged) battery which is way past its sell by date, and is poosibly a cause of the weak sparking. Then when the plugs are fitted the additonal strain of operating under load/comprsession is either not producing a spark, or not enough of a spark to fire up the bike.

Consiquently when you take out x1 plug you are reducing the load on the system and the combination of the remaining 'weak' spark and reduced compression is enough to allow the bike to fire and spin up.

Caveat - It is early and I havn't even finished my first coffee yet, but my offering would be a new battery and either a fresh set of plugs or ensure these ones are well cleaned and see where you go from there.

NGneer

P.S. Once it is up and running I would still do a full check of the electrical system to make sure everything is in order. Only takes about 30 mins with a multimeter and will either give you the peace of mind that everything is ok, or highlight potetntial problems before they kill your battery / reg rec / cdi etc.

thanks for the pointers , ive tried 3 different batterys although all of bikes that have ben sitting a while

Try hooking up a decent car battery on jump leads before you go spending out on a new battery. If the bike fires up, then you know your battery is at fault.

I've been in a similar situation before and it turned out to be the CDI unit.

scotjell wrote:I've been in a similar situation before and it turned out to be the CDI unit.

thats what im fearing a cdi , ive had a car battery on it too but still no joy , what about the pickup do they fail at all

Have you measured the voltage on the black/white wire that connects to the ignition coils when cranking, should be no less than battery voltage and above 12 volts with a fully charged battery.

Where abouts are you?

If its any help, I can lend you the cdi outta my bike.
I'm in Dromore so not too far away.