Mower starts easily but after 5 mins use misfires and dies

[b3taman]

Hello folks
I have an old Hayter rough cut mower which starts easily, then after about 5 mins starts running roughly (misfiring) which gets worse and then the engine dies. It's got a vertical shaft B&S 5hp engine.
I have checked the fuel and cleaned the carb out, all seems OK. The plug looks clean and the correct colour.
Any ideas? Wondered if it was the magneto or a valve sticking?

 

[r896neo]

check the tank vent, sometimes its an air emmitance valve sometimes a vent in the fuel cap,

Try unscrewing the cap when it starts to run rough.

 

[Burnerman]

Five minutes is a fair time to run, really so if the post above doesn't sort it, then firstly stick a new plug in and try that.
If the motor is really elderly then it could well have contact breaker points ignition, which means that the flywheel has to come off for inspection.
Sticking valves aren't common on the side valve engine, but carbon build up on them is.
When the engine starts to cut out, does it blow any smoke at all, or sign of explosions in the exhaust?
You've had the carburettor off - were the two dip pipe filters (that dip into the petrol tank) clear?
John

 

[b3taman]

Thanks for your advice. I 'seem' to have sort of solved the problem, but still don't really understand what's going on.
I altered the main jet setting by about two full turns and it runs much better - what I don't understand is why this would suddenly need to be changed so significantly when it's been on the same setting for at least 10 years!
So now it runs successfully without the misfiring BUT it still starts hesitating and then dying after about 10 mins use. But I found that if I rev the engine right up briefly (using the throttle lever on the carb, not on the handlebars) then it will run fine again for another 10 mins or so. Then I have to repeat the process. So this would seem to point to an ongoing carb problem still not really resolved.
Still, at least I can get the grass cut - there's about half an acre of rough grassland to do about 3-4 times a year, and this old Hayter Osprey is excellent for the job.

 

[Burnerman]

Again, a symptom of filter or slight jet blockage....
Presumably the main jet is horizontal, and faces forwards? By opening it up, you let more fuel through which compensates somewhat for the fuel restriction in the first place.
These carbs have 2 dip pipes into the fuel tank - one pipe pumps fuel up to a separate small chamber above the fuel level...the fuel in this other chamber then supplies the carb jets.
I'm sure a good carb clean and new pump diaphragm set would sort things, but do check the rubber pipe connection from the crankcase to the carb throat for tightness.
John

 

[b3taman]

Thanks Burnerman. Whatever the blockage was in the carb, it seems to have cleared itself now.
However, the tube from the crankcase to the carb you mentioned does have a slight split in it - is this a breather pipe? And if the mowers running OK should I leave things as they are?

 

[Burnerman]

The Briggs Pulsajet carburettor relies on the integrity of this rubber pipe to allow it to pump fuel.....on other models its simply a crankcase breather.
Either way, a length of tape over the split does no harm!
John