Stihl TS400 Disc cutter

[laferret]

Hi,
A bit stumped on something perhaps a bit more than DIY but here goes.
To keep him occupied during retirement I have been providing my father with repairable Stihl Saws and then selling them on eBay to fund the next one. The latest unit is in fair condition and we have got it started but it won't idle for more than 10 seconds before cutting out. He's adjusted the low speed screw but this inevitably ends up with the saw spinning while idle.
Now - I do make a point of leaving him to check everything is serviceable so can't say for certain what condition various parts are but the following has been checked, cleaned or replaced.
Fuel and air filter
Spark plug and HT lead
Fly wheel and timing module
Full carburettor rebuild
Cylinder, piston, rings and bearings
All associated gaskets

Does anyone have any advice they could give? I'm fairly clueless myself but I wondered if there may be a problem with air pressure in the system due to a split hose or rubber manifold?
Many thanks,
Mark

 

[Burnerman]

Its probably a good move to replace the gaskets and diaphragms inside the carburettor (is it a Tillotson?) as faults aren't always seen.
Another possibility is the crankshaft oil seals, maybe ingesting air. This usually manifests itself with a tickover that is a little fast and rises then falls of its own accord.
Any comments on that?
John

 

[laferret]

Thanks John.
It is a Tillotson and a full service kit was used on it including the needles, diaphrams and gaskets.
Also, as part of the gasket kit, the Crank shaft oil seals were replaced too.
I can't help but think this is going to be one of those jobs where no amount of tinkering will fix it and i'll have to supply a different carb.

 

[Burnerman]

Hell, I was just about to post back and suggest that it was a Zama carb Is there any play in the main bearings, both flywheel and clutch side?
No worries....have you discovered gardenhirespares (GHS Ltd) yet?
John

 

[laferret]

As far as I remember the bearings were a pass when we inspected it. I'm certain we haven't replaced them anyway.
I'll have to check to take the guess work out of it. I'm going to have a look at it tomorrow evening. Wouldn't this make it difficult to start as well as keep going? I'm sure its not hard to get it running or keep going on the throttle, but let it go and it stalls almost right away.
I've been using GHS for a while now - very helpful.
Thanks John.

 

[Burnerman]

Any play at all in the main bearings means the seals won't be doing their job.....but you have definitely replaced them?
Personally I would suspect a carb fault, and its essential to use a carb cleaner and a blast of compressed air to do the job properly. Every part needs to come out of the carb, and the needles aren't interchangeable. I'd try to keep the thing running on the idle screw, and then adjust the needle marked 'L' to see if that makes any difference.
John

 

[laferret]

I had a look at it yesterday. I couldn't get it started and was not convinced by the apparent lack of compression during the pull so I've ordered a complete engine re-build kit including the crank, bearings, seals, gaskets pot and piston.
The carb checked out ok as he knew it was clean and assembled correctly.

 

[Burnerman]

Compression is the magic formula for starting any two stroke, and what I do if one comes in for a quick diagnosis is to pop the exhaust off and have a look at the piston and cylinder through there. Any scoring especially around the rings, and the piston and barrel become scrap.
There's plenty of replacements out there, some offer a Nikasil coating, some don't.
Its hardly surprising that these machines fail - the air filtering is brilliant but they are used to the extent that they just pack in.....the air filter needs to be checked every day but I realise that's not practical on site.
Love to see the odd pic if its not too much trouble once you are in there!
John

 

[Burnerman]

Just an add on.....if you are replacing the main bearings, heat up the castings first.....being die cast they are somewhat fragile.
John

 

[laferret]

Indeed. He's done this repair before where the piston rings had broken and damaged the pot as well as the piston head. When he had it apart last time he mentioned it was really sooty which seemed strange. I gave him my old phone yesterday so i'll ask him to take some snaps and post them up.
You are right about the due care of the saws. For days we use a colleagues saw on site and I said it was struggling but after each day it got thrown back in the van and forgotten about. I always tried to use the water kit where possible but by god does it make a wet mess!

 

[lostinthelight]

I had a look at it yesterday. I couldn't get it started and was not convinced by the apparent lack of compression during the pull so I've ordered a complete engine re-build kit including the crank, bearings, seals, gaskets pot and piston.
The carb checked out ok as he knew it was clean and assembled correctly.

Did you check the decompressor was sealing properly? and did you have confidence in the carb rebuild because you had pressure checked it?
litl

 

[laferret]

Carb is fine as we've fitted all the new parts and it starts second pull now. It's a little smokey but I think it's just a little rich at the moment.

 

[laferret]

Just an add on.....if you are replacing the main bearings, heat up the castings first.....being die cast they are somewhat fragile.
John

All sorted. Saw starts really easily after the repair. It's a bit smokey but I'll tweak the mix to lean it down a little.
I've tried to upload an album of images but can't quite link to it from this post. See if it's visible on my profile.

 

[Burnerman]

Many thanks for the update - excellent pictures too!
Looking carefully at the piston, the machine has certainly suffered from dirty filtering at some time - you can see the scuffing on the piston sides and also there has been some debris bouncing around on the piston top......the typical signs of a well used TS400 saw!
The excess smoke will be residual oil burning off, and a little bit of excess oil in the mixture does no harm at all. Personally I use Oregon oil at a 40:1 mix.
You've done a good job there
John

 

[laferret]

Thanks for all your help John.
Hope to sell this one quite quickly and move on to the next.