Ok this will take some working out. Its a road bike in standard trim and at tick over runs on 3ish cylinders with the worst of the missfire on No4. The missfire stops when you cover one of the air jets with your finger.

So far it has had the following swapped for testing purposes. carbs, cdi, plugs, coils, carb rubbers, airbox, generator, exhaust system, fuel pump and finally the engine!

Still the fault remains and goes away as soon as the revs are built up and the bike is moving. Once on the move the bike runs very well in all gears.

The only thing remaining unchanged are loom (but this has been tested for continuity), reg rec (but i cant see this really causing the fault), engine earth strap and the saftety circuits (side stand, clutch switch)

Any ideas??? The missfire sounds more of an echo in the exhaust system and is not enough to cause the engine to cut out, it will idle all day long!

Very fooking strange and annoying.

Are you sure it's a misfire ???
If it's running on 3 then it isn't going to idle for a minute let alone a day. Check the exhaust take the can off and see if it has a rattle, could the exup be loose or the cables not adjusted ???

Yup deff missfire but on 3ish...... it runs lumpy and picks up fully onto all 4 when on the move.

The Full system has been changed and the exup adjusted each time and has also been tried with the exup wired open..............

um - well that's sounds like a difficult problem you have there matey

If it goes after closing the airhole then it must be a mixture problem ... (but you changed the carbs). is it the same if you run it with no airbox, still covering the airhole ??? Could there be a hole in the carb rubber.
On the electrics side ... one cylider would mean the spark break down after the coils so swap 1 and 3 plug caps to see if it make a difference


I think this will have me stumped

Yup tried 3 sets of good carbs inc a set of my own race bike still rough at idle.

Since that have run bike with new loom, cdi and extra earth strap and still rough at idle.

Bike is now rebuilt as it was with the original carbs cleaned (again) and it is a lot better but still not 100%. It still has an RR motor in as it is being used at anglesy this wk end.

The bike is actually mattfzr400rrsp's little brothers so will keep you posted when the original sp engine is re-fitted.

I think the biggest problem is trying to chase perfection on a 17yr old bike.

Have you checked the compression on the offending cyl.

COMP WAS GOOD ON BOTH ENGINES ON THE OFFENDING CYLINDER.

maybe the airbox rubber, Valve clearance ???... but it's a guess

came across something like this before, it was the HT lead, on no. 1

Start the bike from cold, put your hand on the exhaust after 10 seconds, all 4 pipes should be equal temp.

The pipe will conduct head from the engine after a few mins, so it hard to tell after this, bike must be cold,

Check the ht lead, or it could be a duff spark plug!!

ht leads make sure they are pushed tight in to the coils, had a simular problem,looked in to it to deep valve clearances etc etc,when it was just a simple lead !