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Hi, first post here as I am stuck on what to do next.
I am actually in France but an engine is an engine......

I have had the mower for 4 years, small MTD ride on, and it always works well with no problems.
Just started it for the new season (grass growing as we speak) and it ticks over fine but stumbles and cuts out when the revs are increased.
Some smoking when first run but none now, no backfiring as such (only a small one through the carb when it stalls).
Seems to run for a second on max speed setting then hesitates and stumbles, return to slow before it cuts out and all ok, leave it on high and it stops.

What I have checked.
Fuel supply ok, timing ok (cleaned and reset the timing 'block'), checked plug and cleaned it, fuel is 6 months old but should be ok as stored in a sealed large container.
Carb was replaced a year ago.

Can't think what to check/adjust next. First thought was restricted fuel but seems ok ?
I found some info on the net but most start with the worst case, engine exploded internally, so I was hoping for a few better options. It was running fine when last used in September last year.
Any help appreciated, John

I would go for new fuel so drop all the old fuel out of tank/lines and carb, and if still the same then clean the carb out, you could also give it a compression test to see if you have a bad valve, you say it was running Ok the last time so I feel its more likely to be fuel.

Thanks I will give it a try, I had hoped it would be ok after 5 months but probably not.
I was just trying the petrol in the strimmer as a test but having trpuble starting that, so probably the cause. I will let you know the outcome in a few hours.

Just let it run for ten minutes or so on new fuel before you come back and say its just the same.

You were very nearly right as it seemed the same but I drove it for a bit and now fine. It did take 10 mins or so even burning the new fuel to fully wake up.
I think it also needed to blow the 'cobwebs' out of it too after sitting idle for 5 months.
Still a little short of power I think, works fine but I would say 90% ok. Runs smoothly at all rpm now though.

Maybe the valve clearances could do with a check ? No noises as such but a good going over wouldn't hurt as well overdue.
I will try B&S site for the valve clearances. I think it deserves a full decoke/service next winter.

Thanks for suggesting that, hard to belive fuel won't last 5 months, but obviously true.
Sadly this has runined the movie Mad Max for me as no way was he running a high performance car on fuel from old cars

This should help you out with the valves, I don't feel its a panic right now. good luck and thanks for coming back and letting us know.:thumbsup:

Back when mad max was made gas would last a lot longer then it does now.
Still a good movie

Useful set of engine data, many thanks.

I will check the valves when the grass stops growing, an early spring here as normally cold but unseasonally sunny now (and wales/uk too at last) after a lot of rain so perfect growing conditions.
At least this time I should use up all the new petrol by the end of the year if this keeps up....

I am sure Mad Max petrol was better than the supermarket stuff but eventually he found someone refining their own which was a much better solution

A friend who maintained lawnmowers for a living always said garden machines run on almost anything, when I asked if it was ok to use unleaded, as they are designed for a hard life.
Low octane is tough to beat though. I thought about it and the difference between standard petrol and 'super' is just from 95 to 98 octane, so if it dropped to about 90 or under that is a lot.
Enough energy in it to run at tickover but not enough when more power required.

Quick question, which engine do I have ?
Says 219807 0421 type B1 code 080102ZA I think it is just a single cylinder vertical shadt model, on the specs pdf I can't find this engine as such using the engine number.
Thought it might be single OHV 210000 but engine doesen't look like it. It is the standard good old Briggs engine, I have a Hayterette mower with the older version.

I put a photo in Albums as John W album if that helps?

I would say its the 210000 series horizontal O.H.V engine. Valve Clearance .004-.006 IN .004-.006 EX

Thanks, that was my best guess too. A good choice of settings, inlet and exhaust the same.
Off to buy a new spark plug tomorrow as well, may as well do it all.
Just like rust, Grass never sleeps