stihl fs 400 cant get to idle

[swinny]

i have a problem with my stihl fs400 strimmer..... i can't get it to idle at all!!!

 

It runs and rev's ok but will just not idle

 

The carb is a single screw adjustment one, have tried it every configuration possible

 

I have tried a new carb kit in it and have tried new fuel pipe's and fuel primer yesterday to no avail

 

Going to strip the carb and clean again but clutching at straws

 

Piston and cylinder are ok, new plug

 

What am i missing??

[BTS]

hi i had same problem, in the carb is a very small mesh filter mine was clogged up missed it first time had look, once cleaned was fine again

[MattyF]

Clutch springs?

[spudulike]

Take the carb adjustment screw out and blast a bit of carb cleaner down there - if you look at the engine side of the carb bore, you should see a good jet of cleaner blast out of it, if you don't, it could be blocked.

 

Very difficult to fault find without seeing it on a problem like this - the clutch springs like Matt said could be shot and if they are catching the clutch sprocket may stop it idling - what does it do if you set it on a fast idle of say around 3500 - 4000rpm? at what point does it die?

 

No idle would normally mean no fuel on the L circuit of the carb or a possible air leak leaning the mix down, could be the impulse line or circuit dependant as to whether it is part of the carb or a separate pipe.

Good luck

[Maico490]

Had one just the same and replaced all the same bits as you to no avail. Started to run out of hair to tear out. Ended up being the main jet check valve. Will dig out pics tomorrow and post. Good news is only a few quid to mend.

[Acle Garden Machinery]

i agree main jet

[swinny]

not sure where the main jet check valve is....

 

still not idling, plus the recoil spring went the other day whilst playing so had to get a fresh one

 

 

when the strimmer dies i have to either use primer bulb or choke to start it again......

[spudulike]

not sure where the main jet check valve is....

 

QUOTE]

 

I would imagine they mean the welch plug - it is like a thin aluminium cap that seals the area between the mixture screw and the jet in to the throat of the carb - if you pull this, you will need a new replacement.

 

It may be leaking round the edges - seal with a little superglue after cleaning with solvent.

[Maico490]

Sorry about the delay but didn't finish work too early this evening.

This is a cut and paste job from a posting I made on another site as it would take me ages to re-type:

 

This has been my first experience with the notorious check valves found on newer carbs. I've been chasing this problem on a FS450 trimmer for a while but luckily it has been between cuts on Water Authority sites. Searching here didn't throw up much but these carbs are used on lots of blowers, trimmers and smaller saws. The carb is a Zama C1Q-S34H which is fixed jet with only what appears to be an air screw (reverse thread) for idle adjustment.Symptoms were hard starting, inconsistant idle which couldn't be adjusted and would run out of fuel with an inch still left in the tank. When operating the primer bulb you could keep pumping without it becoming full and air bubbles were visible in the bulb. At full throttle everything was fine but as soon as you let off it would often stop dead. New lines, primer bulb,fuel filter and a rebuild kit did nothing. The answer was eventually found on Zama's excellent website: USA Zama : Service Tips.

Before you do anything to a Zama carb I would recommend reading all the Service Tips. Especially DO NOT BLOW COMPRESSED AIR THROUGH A CARB WITH CHECK VALVES. It seems you also have to be careful with aerosol carb cleaner if directed straight into the valve with an extension tube.

 

 

The jet at the bottom of the metering chamber is the main jet which Zama call the "Nozzle Assembly Check Valve". Stihl call it the "Valve Jet".

The hole in the middle, which is normally hidden by the metering lever leads to the idle circuit. To test both circuits use a piece of square ended hose pressed against the jet and blow with your mouth. If clear then try to suck back. If you can the check valve is defective. In general if it is working you can hear a faint click. If you can suck back it is bad news on the idle circuit as it means a new carb but the main jet can be replaced. Zama do a special tool but I managed to use a cheap drill press and one of my extensive collection of broken drill bits. This is clamped in the chuck upside down. If you have adjusting screws remove the H before starting.

 

 

It took a fair bit of fiddling to get everthing lined up and more force than I expected to get the nozzle to move. It pushes straight through into the bore of the carb. The chuck is stationary throughout. Here it is out and the new one:

 

 

The new nozzle pushed in quite easily with a bit of oil on it to the same level as the old one. Zama say you can punch them in and out but I wasn't too keen on hitting a carb with a hammer.

A few minutes of reassembly and it fired right up. Ticks over fine now and is responsive to the LA screw.

 

Hope this helps. The size of the jet is in the IPL and stamped (microscopically)

on the jet. Any probs getting the jets PM me as supplier is not a sponsor.

 

 

Edit: Sorry about the duplicate photos. Haven't got the hang of them on here yet.

[spudulike]

Nice post Maico 490 (Wasnt that a Spanish trail bike?) - never knew that what I thought was a straight jet had a built in one way valve -excellent posting my friend even old dogs can learn..........