Need help with my lawnmower engine - PeachParts Mercedes-Benz Forum

thorsen · #1 06-25-2011, 03:43 PM

My lawnmower has been acting strangely the past few weeks. After about an hour of cutting grass it will begin to miss and run poorly. It is also difficult to restart one it starts to miss. The mower is about 2.5 years old and gets used about 75-90 minutes either once or twice a week. It is a B&S 6.75 Craftsman mower

I put in a new air filter today (the old one was extremely dirty). I also dropped the float bowl (it was clean) and blew out the banjo bolt/jet that holds the bowl on. I pulled the spark plug (it looked fine) and wire brushed it for old times sake. I also filled the fuel tank with fresh gas.

The only other odd thing is that I used to be able to cut my grass on one tank of fuel but now I can only cut 75%-80% before the tank is empty.

Any thoughts? I'm not too keen to throw a whole bunch of $ into the mower but it seems like it should be an easy fix.

raymr · #2 06-25-2011, 03:52 PM

Mine did the same, but a new air filter fixed it right up.

Aquaticedge · #3 06-25-2011, 04:03 PM

check your throttle cable. if it has a choke position on the same lever as throttle it may not be opening enough. it also could be opening too much. make sure it's not stretched. also look at your belts. some pulley's have grease points and may need some lube. the drag would explain your fuel economy

thorsen · #4 06-25-2011, 04:13 PM

No throttle cable on this one. You pull the bail to the handle and that's it. I did verify that the choke was open and not hanging up.

Leathermang - any idea on the specs for the valves?

leathermang · #5 06-25-2011, 04:20 PM

I think Sears is good about manuals.... but you can look at the BandS site also.... just take your serial and model number to the keyboard with you..
Also, if you have not opened up the sheet metal around your engine's fins... time to do that.... they have to be clean to stay in the right heat range... air cooled lawnmower engines are one of the most stressed engines on the planet..
Also... only run straight 30 wt oil in there for the same reason.. those additives they put in to allow multi viscosity ratings take away from the basic ability of the oil... and it is needed in these situations..
Sometimes if you examine the mower closely they will have an aluminum info tag riveted to it or the engine which gives the spark gap and the valve settings....

" old times sake " LOL

Skippy · #6 06-25-2011, 06:35 PM

In addition to all of the above:

Vapor lock? After an hour the motor should be getting pretty hot. You might want to see if your fuel hose has moved out of the original routing or if something is making it run hotter than normal.

Or you could just stop every hour for a beverage break and get yourself and the mower cooled off.

thorsen · #7 06-25-2011, 07:06 PM

Going with Leathermang's theory that the valves need to be adjusted, I took a compression reading after letting the engine cool off. It read about 60 psi - I'm not sure if that's normal or low.

Next time it starts running rough I will put the compression tester on it and see how that compares.

leathermang · #8 06-25-2011, 07:19 PM

Check Skippy's suggestions too.... vapor lock would normally be where when you shut it down hot and it would not restart..
Why don't you just set the valves ?
You are taking a chance on BURNING the valves if they are too tight... and that can happen fast once it starts to heat up... a feed back loop....which goes up really fast on temperature once the valve does not have the block to dissipate heat to...

kerry · #9 06-25-2011, 07:36 PM

I'm going with a valve problem also. Used to use a Gravely which would stick the valves after about an hour of mowing. 60psi is too low for a gasoline engine I think.

toddyvol · #10 06-25-2011, 07:42 PM

I had this very same problem with my Craftsman riding mower. The little tiny hole in the gas cap was clogged. I took the gas cap apart and cleaned the diaphragm and the tiny hole that allows air into the gas tank. Never quits now.

kknudson · #11 06-25-2011, 07:43 PM

Magneto, particularly if the cooling fins are clogged and it is getting hot.

A bit early for it to fail, but they do.

kerry · #12 06-25-2011, 08:22 PM

Anyone have an explanation for the 60psi?

Skippy · #13 06-25-2011, 08:28 PM

Valves not sealing or checking compression without keeping the throttle open (likely), stuck or worn rings or worn out cylinder bore (less likely), bad head gasket (also less likely).