Mountfield Mower/Briggs & Stratton cutting out

[AP77]

I have a Mountfield petrol mower (HP470) with a Briggs & Stratton engine which I have recently serviced (oil change, new plug, new filter, new carb gaskets, new primer bulb).

The engine initially runs fine and cuts well, but after about 5 minutes, the engine starts to falter before eventually cutting out. I have also noticed that quite often before the engine cuts out, the primer bulb becomes compressed, even though it was fully inflated before starting the mower (after priming).

I actually had the same issue before I did the service, so I did the service hoping it would resolve this issue. As part of the service, I also emptied and flushed the fuel tank

Any ideas what may be causing the engine to cut out after 5 mins? I am thinking it is either fuel starvation or too much fuel causing flooding. Too restart the engine I have to make sure the primer bulb is fully inflated (sometimes I have to waste a few seconds for this) maybe wait a minute or two, then prime again a couple of times, after which the engine fires up again fine and runs for another 5 minutes.

I am guessing this is some kind of car issue?

Thanks.

 

[Harry Bloomfield]

Any ideas what may be causing the engine to cut out after 5 mins? I am thinking it is either fuel starvation or too much fuel causing flooding. Too restart the engine I have to make sure the primer bulb is fully inflated (sometimes I have to waste a few seconds for this) maybe wait a minute or two, then prime again a couple of times, after which the engine fires up again fine and runs for another 5 minutes.

Does the primer bulb, pop back into shape, if you unscrew the fuel tank filler, and you hear air being sucked in?

 

[AP77]

Thanks for taking the time to reply, much appreciated.
That's an interesting idea. Perhaps the fuel cap has a breather hole and it has got blocked. I will try your suggestion later today (if it doesn't rain again!!!) and see if unscrewing it pops the primer bulb back out
Thanks again.

 

[Burnerman]

With these engines, it's vital that the seal between the plastic air filter inner casing and the aluminium carb face is perfect.
Any cracks in the plastic case or an incorrect gasket spells trouble here!
John

 

[AP77]

Thanks. I have used a new gasket set and am fairly sure it is correctly fitted. I will remove the air filter plastic casing and see if there are any cracks.

I have removed the fuel cap and it doesn't look to have any blockages. There is a felt filter on the inside of the cap with 4 tiny holes and 3 tiny holes on the plastic cap itself. These holes do not line up but I guess that does not matter?

 

[Burnerman]

Does the engine keep running with the filler cap loose?
The vent holes don’t line up, this prevents fuel splashing out of the cap.
John

 

[AP77]

Thanks Burnerman, that makes sense. Next step is to run the mower with the cap loose.....if it ever stops raining.
Also, another thing I will try is to prime the mower with the cap off as the bulb at the moment takes 4 or 5 seconds to return which I am not sure is correct. I have seen videos where the primer bulb re-inflates almost immediately. I will try this in my garage later. Presumably if it re-inflates immediately with the cap off, then I need to replace the cap (even though it looks fine).
Thanks again for the replies, really appreciated.

 

[Burnerman]

Just had a thought......
You've replaced the original primer bulb. Some have a hole in, (which is covered by the finger when you push it) and others don't.
You must replace like for like - would you please check this?
John

 

[AP77]

Thanks John. I bought the service kit specific for my mower from ebay and as far as I could tell the bulb was the same (although black in colour rather than the original red). The new bulb doesn't have a hole in it, I don't think the original one did either (I can't check now as it has been thrown away). Tried the mower again yesterday and it ran fine for about 5 mins before the bulb sucked and and the engine cut out. I tried again with the fuel cap loose but got the same cut out after 5 mins so presume the fuel cap is ok. I think I will have to take the carb fuel tank, etc apart again and check everything once more. Presumably there is a fuel blockage somewhere but I don't understand why it should always run fine for 5 mins or so before doing this, I would have thought it would not run at all if that was the case.
Appreciate all your comments and suggestions so far.

 

[Burnerman]

Ye gods, after 5 minutes running you would think it would be job done!
Try running without the air filter, and then removing the air filter casing if you can - I can't understand how the primer bulb keeps collapsing!
After that, its debris in the tank, clean filters etc I guess.
John