Sounds like you still have a carb issue to be honest.  It's usually the ones that haven't been used much that suffer worse from fuel issues.

I would usually give them a sonic clean before reassembly, but I realise not everyone has that facility to hand.  It also doesn't work every time, but That's what I would do before throwing more money at it..

Also make sure the gasket between carb and manifold is correct and the correct way around so the pulse hole isn't restricted.

Might be a daft question, and I don't want to insult you. But did you get the gaskets and diaphragms in the right order and the correct way around?  https://www.lsengineers.co.uk/stihl-fs70-carburetor-c1m-s208-c1m-s146-california.html

I feel guilty because I had a gearbox from an older HYM46 on my shelf. I retired and a lot of things were scrapped 

A friend dropped off a Stihl FS70, complaining of starting issues and difficulty keeping it running once it started.

It's only been used about 10-15 times since new and is in good condition. I stripped it down and did the usual fuel filter after filling it with fresh fuel. The purge bulb felt strange, and I discovered the metering diaphragm was very stiff. I removed it, and the gasket also looked poor.

I replaced both, and it started, but only on choke. If you release the choke, it settles, but it bogs down after a short time. I checked the exhaust, and it is fine.

It seems to be a fuel delivery issue, and it only runs on the fuel provided after a purge.

Could it be a carburettor issue? The machine is like new, which is why I am uncertain whether the carburettor is causing the issue.

It's stumped me to be honest.

Perhaps my brain is addled and I'm too old for this stuff 

All thoughts are welcome

Changy

Thank you.

Hi Ropi Jo, and welcome to TGMForum.

Regarding the Silicone hose.  Unless it's rated for use with petrol I wouldn't trust it. There's different types of this stuff, but if you use one not suitable for petrol it can soften and swell up, and eventually break down completely.  I would recommend getting hold of some proper fuel line either from your local dealer, or online.

The shroud. If you mean the whole shroud / cowling that surrounds the flywheel and covers the cylinder then yes it needs this to direct the air over the cooling fins etc. Running without it for more than a minute or two can / will result in damage.

And as you mentioned, it's not safe and you could lose fingers or if something falls in while running such as a spanner socket etc you could end up knocking a coil off the block.

Hope this helps. 

I've got 2 general questions please...

1. Fuel line.... on a gravity feed to a briggs  engine, is heavy clear sililcon pipe safe? Or will the petrol eat it?

2. On the same engine... briggs 8.5HP, the shroud / cowling is missing from around the fan / flywheel. Regardless of safety concerns.... is this cowling crucial for cooling / airflow?

Thanks.

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I seem to spend more time confirming the correct replacement than actually fitting it. Different suppliers sometimes list slightly different information for the same OEM number, so before ordering my 197253 belt I compared the OEM references here - https://www.amazon.com/Mower-197253-Replacement-Husqvarna-Craftsman/dp/B0949K1453. Just wondering what everyone else uses these days—manufacturer's parts book, dealer lookup, or an online cross-reference?

Glad you got the drive sorted. 

I don't have the parts manual but if you find and post the the full model code I might be able to find one for you. There's so many variants.

Touching on the AI stuff. Yes it does my head in. It can be really handy if used for the right stuff and in the right way, but people use it just to create content for social media, and its mainly useless and inaccurate slop as you said.  To be honest I look at Facebook now and it's not just AI slop it's completely nonsense posts just to get a reaction and clicks.  People who comment and react don't seem to realise they are just adding fuel to the fire.