Stihl FS400 clutch problem

[RBudd]

Hi all

My FS400 has developed a problem that's become a real nuisance lately. The blade originally started spinning when idling, so I adjusted the H/L screw on the carb until it stopped, but it started again, so I took the clutch apart to find that the springs had indeed stretched.

I replaced the clutch springs and adjusted the carb and thought, problem solved. However, the problem has since reoccurred, and I've had to replace the clutch springs again!

Does anyone have any ideas as to what might be causing the springs to stretch? The Zama carb only has a single screw for the H/L speed. Could it be that the engine is revving too fast at full throttle stretching them?

Cheers in advance.

 

[CR888]

The springs can only stretch as far to engage the clutch, if idle rpm is set to spec, the only thing I guess it could be is a worn clutch drum allowing for the springs to stretch when running wot. I'd take a peek at the inside of the drum for any excess wear.

 

[RBudd]

Thanks for the reply!

The clutch drum and clutch are fairly new and don't seem to be very worn, could there be an issue with the carburettor? It has got to the point where even with the LA screw as far out as it goes, the head is still ticking round.

The carb on my FS400 is a Zama C1Q-S34H, and doesnt actually have any high or low adjustment screws, not even the tamper proof type, so I can't adjust much there.

I'm considering rebuilding the carb as it probably hasnt been touched since the machine was new, but could a dirty carb cause the machine to idle too fast?

 

[ANewSawyer]

Buy a replacement carb with adjustment: http://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=zama+c1q-s34h&_sacat=0

But that isn't your problem. You are changing the screw that controls how far the butterfly is open (idle speed screw in laymans terms) but the clutch continies to engage, spinning the head. Sounds like you have an air leak. I went through that too. If you don't find the busted seal you will be looking for a new trimmer. Sorry to say.

 

[CR888]

^^^good advice IMO, a pressure/vac test is in order. Don't run the trimmer till you know whats up. If you don't have tools and know how to do it, take it to a good small engine mechanic and specify you want that test done. When/if you find a leak you may be able to order parts and fix yourself.

 

[RBudd]

I rebuilt the carb and replaced the fuel lines and carb gasket and that seems to have sorted it! The head is now stationary at idle which is good! Whilst I'm at it though, I have another concern, it does make a sort of low rumbling sound at full throttle, lower pitch than the revving. Could it be piston slap?

 

[RBudd]

Paranoia stopped me using the fs400 so I pulled the cylinder today to have a look. There was quite a bit of carbon build up at the top of the cylinder around the plug, and a little in the muffler opening. I think there is some scoring in the cylinder but to my untrained eye I'm not really sure how bad!

Rings seemed okay, and piston had a little carbon on top which cleaned off quite easily. The crankshaft bearings feel good, though there is play along the length of the gudgeon pin. Is that normal? I assume the play in the connecting arm is normal as its attached to the crankshaft by a needle bearing.

I'll attach some photos of the cylinder and piston, as I'm sure everyone on here is much better than I at recognising issues! Could scoring or carbon build up be causing the sort of low rumbling noise that the engine makes at full throttle?

Before I pulled the cylinder the engine was running pretty well apart from the strange noise- starts first pull every time and had 125 psi compression when cold, and has a good strong spark. As such I'd rather sort it out now before it gets worse!

Thanks in advance for any advice.

 

[RBudd]

Anyone?

 

[Thommo]

I cant see any scoring on the cylinder. It looks like normal wear to me. The only thing that i noticed is that the base of the cylinder appears to be polished in places which could be from the gasket not sealing properly, which would cause an air leak. The gudgeon pin should have some sideways movement but no up and down movement.

 

[RBudd]

That's actually a really good point I thought the gasket looked a bit funny when I took it off but that didn't occur to me as it wasn't broken or anything. I'll make a new one up and see if that sorts it out, thanks!

 

[RBudd]

Just to tie the thread up incase anyone searches a similar issue- I made a new gasket from gasket paper of appropriate thickness and replaced the old one, which though intact was slightly bulged in places, and it's now running sweetly, sounds good throughout the entire rev range, and no low pitch rumbling at full throttle. Props to Thommo for picking up on that.