Cutting Out

[LKey]

Hi,
I ride a 1988 Bimota YB7 with a FZR400 ExUp engine.
Bike starts first hit on the button and idles perfectly around 1300/min.
Runs smoothly in all gears at all revs.
Needs quite high revs when starting uphill.
BUT nearly every time I stop it cuts out. Will start and idle again without any problems.
I can provoke that cutting out issue simply by compressing the front suspension a couple of times at stand still, that will make it cut out again.
I've taken the carbies apart, cleaned with ultrasound and they are tuned to the book. All gaskets and rubbers are good.
I tried out various float levels without any success.
Does anyone know an explanation or even better a solution for that problem?
Thanks
LKey

[Ben_OB1]

Hi,

Just to keep it simple there's ony compression, air, fuel and spark required to make the engine run.

I'd say you've got compression ok.

Sounds like you've tried all the carburation elements regarding fuel supply. That leaves air and spark.

Have you checked that there is no airflow obstruction (is there a rubber flap for example, that might be covering the inlet trumpet)?

Have you checked for ignition problems? This would be either a poor earth that reduces the voltage going through the coils (the bad earth acts as a resistor), or a short that effectively cuts the coils out of the circuit. The same issue on the pulse generator circuit would have a similar affect. Get busy with your multimeter.

To me it sounds like something is loose and is moving with the energy applied during breaking. Possibly a loose wire.

If the FZR400 engine has a fuel pump or any other electrical devices (exup valve?) then check power to them too. Power failure to the fuel pump won't make the engine stop straight away though since there's fuel in the float bowls.

Good luck, please let us all know what you find!

Ben.

[CMSMJ1]

That is a strange soundig problem.

What is it from compressing the forks that is making it cut out?

Is there any electrical connection being strained? Does the bike itself quit immediately like the kill switch is triggered?

We've another chap on here - Laminator - he is into his Bims..

[LKey]

Ben & CMSMJ1, thanks for your replies
by compressing the forks at a standstill I simulate the same forward moving/angle like when I brake to stop the bike. I mentioned that because I thought it might have to do something with the fuel moving forward when braking a little bit harder. (After so much testing and checking and still not having any positive result you get strange ideas). But without braking to a standstill the bike runs well even on bad roads where you have quite some fork compressing.
Multimeter was already running hot no electrical faults found
We've checked carbs, air filter, plugs, rubbers ...
We've also checked fuel pump and exup valves - all fine.
I think if it was some fuel or air issue the bike wouldn't run well as long as I do not stop
????
LKey

[CMSMJ1]

So when you compress the forks, either at standstill or when riding, it stops immediately?

runs fine..then dead?

Cannot be fuel then - it does not leave the carbs that fast and stops dead...

I reckon you have some loose connection and that the compressing of the forks is straining a connector block, or it is crushing something and causing the same effect as flicking the kill switch.

Any chance of filming this running and then show us how fast it dies?

[LKey]

It doesn't stop at all while riding it only cuts out when coming to standstill or when compressing the forks at standstill.
It doesn't die immediately it takes about 2 seconds to cut out. Symptoms are like fuel starvation or flooding.
Will try to film it (easy part) and post it (may be not so easy) over the weekend

[CMSMJ1]

Aye.. fuel starvation normally takes some time. the bike shouldn't drink all the fuel from the float bowls in 2 secs if it is linked to fork movement.

How sure are you that it is the fork movement?

It idles ok, you move it and within 2secs it shuts off?

random!

[LKey]

I agree it's weird
It might take 3 or 4 seconds to cut out which is still not long enough to empty the bowls.
Regarding the fork movement:
The bike rests on the side stand
You start it
You let it idle nicely for a minute
You take it off the side stand
You pull the front brake and compress the forks 3 or 4 times as hard as you can and it will die slowly

[willandrip]

That is a strange soundig problem.

What is it from compressing the forks that is making it cut out?

Is there any electrical connection being strained? Does the bike itself quit immediately like the kill switch is triggered?

We've another chap on here - Laminator - he is into his Bims..

Here now; dont you be forgetting our most knowlegable member; Evilchicken0 also has a Bimota.

[willandrip]

I agree it's weird
It might take 3 or 4 seconds to cut out which is still not long enough to empty the bowls.
Regarding the fork movement:
The bike rests on the side stand
You start it
You let it idle nicely for a minute
You take it off the side stand
You pull the front brake and compress the forks 3 or 4 times as hard as you can and it will die slowly

By that description " die slowly" you have ruled out electrical;it would have stopped immediately;but as a check ,instead of using the front brake (a brake switch fault may be overloading something electrical) compress the forks against a solid object = wall etc.

You have ruled out fuel starvation;what about fuel flooding the cylinders.
It would clear the excess fuel somewhat at higher revs but may be unable to do so at idle and will just choke .


Addition;
Immediately on stoppage at garage;
Pull out plugs;they should be soaked in fuel if its flooded.
Can you smell a rich mixture.?
Fuel level is pretty critical on small bore Mikunis but you do not say which engine ........ 3TJ???? derivative.
Boris will know this. !