B&S 42a707 not starting... still

[TJWerner]

I bought a used yard machines mower with a B&S 42a707 18.5 hp opposed twin engine.

In trying to track down a no-start condition, I have done the following:

cleaned carb and fuel pump -- replaced gaskets etc with B&S carb kit
Checked for spark with in-line spark tester -- spark is there for both cylinders
New Gas
New Oil
New Spark Plugs
Checked compression -- approx 97 on one cylinder and 100 on the other

Any ideas for me to try?

Thanks,

Tom

 

[ILENGINE]

Had you tried putting a small amount of the gas directly in the carb and then trying to start it. If it starts and then dies you are not getting fuel. The other thing I have seen on these engines over the years is they will give false compression readings. I have seen where you could be cranking the engine and show 90 pounds of compression and release the pressure while you are still cranking and then show 30 pounds when it builds back up. This condition is due to the frequency to egg shape the cylinders on this engine.

 

[TJWerner]

I've tried gas through the carb, as well as spraying starter fluid through the carb. I can keep trying and see. Seeing as I just cleaned the carb, I suppose it could be out of adjustment.

At one point yesterday, I got what sounded like maybe 1 second worth of firing, then just nothing as I tried different combinations of cranking, with gas down the carb, with starting fluid, etc, etc.

I may try a different kind of starter fluid tonight and see if that works. If I can get it to fire up, even for a few seconds, then I can narrow down the problem. I do get the feeling, however, that starting fluid "ain't what it used to be"...

As for testing compressing, I did it much the same way I would on a car, take out spark plugs, thread in compression tester, turn engine for 6 or so compression cycles and read gauge. I'll see if I can release pressure while it's cranking. Does anyone know if the compression is in expected range?

Also, the only spark plugs that were on hand at my local Lowes were the Champion Resistor plugs. Can this be part of the problem? Would the "Longlife" plugs make a difference? While the inline tester shows that the engine is producing spark, how do I tell if it is strong enough -- short of putting my tongue on the spark plug while it cranks, that is? Is there anything I can clean / repair to eliminate weak spark as a possibility?

Tom

 

[wcpost]

OK You get spark, you spray starting fluid into the carb and still nothing? You might have a sheared key under the flywheel. Was it running when you picked it up? If so how did it stop the last time?

 

[TJWerner]

Lawnmower was not running when I picked it up.

It "had been running" when it was last used a year or so ago. It was purchased by a teenager who wanted to convert it to an offroad lawnmower. I'd like to use it to mow my lawn.

The kid couldn't get it started, but I don't know if he did anything beyond spraying the carb down with carb cleaner and gapping the plugs.

Tom

 

[reynoldston]

Looks like you tryed everything but timing. Fuel, compression, spark, timing and it will run.

 

[TJWerner]

So, how does one check timing on these machines?

 

[reynoldston]

There isn't too much to check as far as ignition timing. The first thing I would check is the flywheel key and mag air gap. The flywheel key is made of aluminum and it dosen't take much to be out of time and if you replace it make sure you use the same type of key. The mag air gap is somewhere around .010 to .012. If this is all ok you might want to check valve adjustment.