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B&S 12.5hp v twin cylinder help, please.


LawnSite Member
B&S 12.5hp v twin cylinder help, please.

I have and old yardman ride on.

When I turn the key to start it, it will spin a little then it hits compression, and stops. Apparently this is because the de compression device is not engaging.

The reason it is not engaging is because the tappets need adjusting, as they are too loose and have too much gap.

This means that also the engine will not be getting a full breath of air as well.

Also, not sure if this will due to the slack in the tappets, but the engine will only run on full choke. When choke off, engine revs up and down like a v8 with a lumpy cam, and stops.

I think this is because the tappets and valves are not opening enough to let in enough air/fuel. But when on full choke there is a stronger fuel mix and therefore the engine runs.

I also notice, heaps of fuel leaking out the front of the carby/ air filter area.

This all might have to do with the choke on full, and tappets/valves.

What do you all think?

Also is there a service manual or instructions I can get my hands on for this job?




LawnSite Member
Well, I did the valve / tappet clearances tonight.

the drivers left side cylinder exhaust was like .025" supposed to be .006. That's like about 5 times too much. Inlet was loose but not as bad.

On the other side, right cylinder drivers side. It was the other way round. The inlet was too tight and had not enough clearance.

The symptoms for this engine were:

  • Engine not starting if it had to start on the compression stroke. would always have to wind it back, or pass the compression.
  • motor only run on full choke, anything less and stall
  • motor power was funny, when hit thick grass motor bogs down funny like
  • heaps of run on when turning the thing off, yes my fuel cut off solenoid is not connected, but still.

Will be interesting to see how she goes in the morning now that the tappets have been adjusted.