weak B&S Sprint40??

[lars11]

Our 10 yr old lawnmower is quite weak this season. It is a 4hp L head.
The problem is:
- Starts ok after a few pulls
- Works ok, but when getting hot it is much weaker and slows down significantly when cutting normal grass.
- Also slowing down when not on flat ground...???
- Almost losing power completely when "jerking" it back and forth.
Odd behaviour!
What can I do? Change the diaphragm? Is it something else that can cause this?

 

[Quest]

check compression first.

 

[lars11]

ok. what should the compression be on these?

 

[Quest]

My rough guess would be somewhere between 60psi minimum?

http://www.repairfaq.org/samnew/lmfaq.htm

 

[Bill Kapaun]

If it's a diaphragm carb you might as well replace it as well as the spark plug.
%6-7 total.

It may simply be getting worn out too.

 

[Brett Miller]

I quick valve job would likely restore the power.

 

[Maritime Storm]

I'll second that. If it is a tank mounted carb(I expect it is from your description) replace the carb to tank diaphram and gasket and put in a new plug. The diaphram gets weak after a couple of seasons on these carbs and slowly stops pumping enough fuel up from the tank while it's running.

 

[lars11]

How do I do the quick valve job? Just remove and clean or?

 

[lars11]

Hi, just changed the diaphragm and of course that was it. Som oil-puffing when jolting the mower is normal according to the tech where I bought the new diaphragm. The original was from '99.