Stihl FS400

[EngineMan]

My brothers fs400 runs well until it gets to about half a tank of fuel, then it stops, its had a new coil, new plug, fuel tank is clean, new fuel filter, new air filter, fuel pipe's are good new fuel bulb and the carb cleaned and fitted with new carb kit, again it will start OK and then die after half a tank of fuel (it seems) I have been working on engines for over 40 years but this one got the better of me, any help on this one would be welcome.
when it does stop if you remove the plug it is dry so its fuel related..!
Wiring from the switch to the coil is like new and find no problems, had meter on the kill switch no problems there.

I am happy with the job I done on the rebuild carb but I have ordered a new carb and fuel pipes to see...! its on its way and will be here next week, I'll keep you all posted.

 

[Lawnboy18]

Maybe a vent problem? Try removing the cap when it happens.

 

[EngineMan]

Maybe a vent problem? Try removing the cap when it happens.

Thank you Lawnboy18 but done that..!

 

[Rivets]

I have come accross this problem a couple of times on chainsaws. Each time I have found a small crack in the fuel pickup line, part number 4128 358 0800. Once fuel drops below this point only sucks air. First time i encountered this problem, it drove me to drink for a month, as the opening was very hard to see with my eyesight, about the size if a pin hole. Hope this is of some help.

 

[EngineMan]

Thank you Rivets, I have a new pipe on its way hopefully by the weekend if not Monday, we also change the fuel caps over and that made no difference. there are not that many parts left to renew, if I had hair on my head I'd be pulling it out by now..!

 

[EngineMan]

Update....took another look at it yesterday and would you adam and eve find that the fuel filter pipe was pig tailed and filter was not seating at the bottom of the tank, it run on a 1/3 of a tank of fuel and didn't stop, so filled the tank up and run again, all seems well at the moment, brother going to work it next week to give longer run, such a simple thing.


Time to retire again...!

 

[bertsmobile1]

If it is getting a bit on the stiff side then replace it just the same.
Even Tygon has a limited life in a fuel tank.

 

[cherami]

As a thread on this model has been started I thought it would be in order to ask about my own problem.

I have put a new zama kit into my carb and have made sure all gaskets etc are around the correct way as one can get things wrong. I have replaced the fuel lines as they were becoming a bit hard and the filter in the tank. I also made sure that the tank was completely clean, I also replaced the primer button as I broke the original one using an airline. A new spark plug just for luck.

Now the first thing was that the primer did not fill up with fuel and there was a lot of air being pumped up. However it did start and will run if the throttle is kept in one position, however when one tries to increase the throttle it stalls, almost as if the adjustments on the carb need correcting, however I have done some adjustments to little effect. Now as I understand it it the sump of the motor acts as a pump to push fuel through the to the carb, how do I find out if this part of the system is working?

 

[bertsmobile1]

If you can not fill the pimer bulb with fuel then it has a leak or the duck bill valve on the bottom is cactus.
It is either sucking air from outside the carb or just sucking up the outlet and blowing it back again.
Being that you blew it apart with high pressure compressed air could very well be both.
Now is it a remote primer or bolted to the carb ?
Did you replace the whole primer or just the bulb ?
Some bulbs have a thinner lip and wont seal around the base of the carb.

To check the pump you simply put an airline on it and blow /suck/blow/suck.
It should only blow or suck a tiny little bit ( around 1 - 2 cc).
If you can continuously blow or suck then the diaphram is fractured or the diaphram cover is leaking.
Generally you put the whole carb in a jar of water and pump it up to around 5-10 PSI and look for leaks.
No more than 10because the pop off pressure of the needles are generally 12 to 20 psi.

Go to Zama or Walbro and download the service information.
Cube carbs only come in 2 basic types and they all work exactly the same regardless of the name on the outside.
The nice coloured drawings will make what is happening blatantly obvious and much better than any string of words I can put together

 

[cherami]

Will investigate the things you mentioned.

carb is a C1Q S94 with a separate fuel primer.

Because I was not thinking I used my compressed air to clean the old primer and little did I realise that it was going to pop. I replaced the whole unit.

Could you explain about the duck bill valve?