Stihl FS 400 problem

[Steve_Ranger]

Hi,

 

Sorry for posting this in the chainsaw forum, but I didn't know where else to put it.

 

I have a problem with my Stihl FS 400 Brushcutter as it is refusing to start.

 

It will fire on the choke, but with the choke off there are no signs of life despite having a good spark.

 

So far I have:

-replaced fuel line and filter

-new spark plug

-stripped carb, cleaned and rebuilt with a carb kit

 

I have looked through the intake and exhaust ports and the cylinder walls and piston don't show any scoring. I tried to get the cylinder head bolts off, but 2 of the 4 are on very tight and I've already sheared 4 T25 bits trying to budge them (after giving them a tap and squirting a bit of GT85 down there). It feels like the engine still has good compression, but I do not own a compression tester so I can not rule this out.

 

Does anyone have any idea what may be causing this/experienced it before? From reading other posts about faulty FS400's some people suggested it could be due to another problem with a jet in the carb although I have blasted carb cleaner down all the jets and they do not appear to be blocked.

 

Also, the carb only has a single adjustment screw which I believe is for the idle, would fiddling with that make a difference?

 

Any suggestions would be greatly appreciated.

 

Cheers.

[Steve_Ranger]

Is it possible to move this thread to the Maintenance help section please? Didn't notice that (first time on site).

[Rich2484]

No the idle screw is just that. For the idle speed.

 

Leave it on this one, will get a response from the techs.

 

Could be an air leak. Have you put the carb kit in the right way round? Sounds silly but bear with me.

 

If it fires on choke then another possible is an air leak in the system. Bung a tablespoon of fuel straight into intake and see what happens.

 

After trying to start is the plug wet?

 

Oh and is the manifold from carb to port on the right way up?

[spudulike]

Have you tried holding open the throttle fully after the brushcutter has fired on choke and after taking the choke off - it may be that the throttle isn't being held open on the fast idle setting!

[Steve_Ranger]

Thanks for the replies, sorry I haven't replied sooner. After buying a compression tester, it turns out that the brushcutter was only pulling 90psi. Managed to get the stubborn head bolts out and there are no signs of scoring so I'm going to try and replace the piston rings.

 

AFAIK the carb is fine as it is getting fuel into the cylinder and there is a good blue spark (spark plug gap correct). Cheers.

[GardenKit]

When you put it back together use a T27 torx. Using a 25 when you dissmantled was probably why it was so difficult.

 

Many sets dont contain a T27, you may have trouble getting one from DIY shops.