I just got a MTD garden tractor/mower. It hasn't been used for several
years, so it needed a new gas line, and some tires replaced. But the
12.5 HP Briggs engine fired right up.

This engine is missing a muffler. It just has a piece of pipe sticking
out about 4". Although the engine runs well, there are blue flames
coming out of that pipe, which shoot out as far almost a foot. I'd guess
that explains why the muffler is gone.

But what puzzles me, is that several times per minute it sounds like a
gun is fired, (explosive sound) and an extra big yellowish flame shoots
out. This does not affect the engine speed, and it keeps running well.
But it is unusual. What could cause this?
It almost seems like a valve is leaking, but I'd think that would also
cause the engine to run poorly.

One thing I have noted is that after I have run this mower for about an
hour, these explosions have become much few per minute, but they still
occur.

Jerry,

You need to replace the missing muffler and do a tune up. Clean replace
the spark plug and gap it. Go on-line and be sure the right plug is being
used. While on-line get the valve clearance specs. Adjust the valves.
Replace the oil. Put in fresh gas.
Once you've done all of that you can diagnose any problems that occur.

Dave M.

David L. Martel wrote:
Jerry,

You need to replace the missing muffler and do a tune up. Clean replace
the spark plug and gap it. Go on-line and be sure the right plug is being
used. While on-line get the valve clearance specs. Adjust the valves.
Replace the oil. Put in fresh gas.
Once you've done all of that you can diagnose any problems that occur.

Dave M.


Back firing? Result of too rich mixture. Carb. and/or timing issue?

On Fri, 26 Jun 2015 09:28:20 -0600, Tony Hwang wrote:

David L. Martel wrote:
Jerry,

You need to replace the missing muffler and do a tune up. Clean replace
the spark plug and gap it. Go on-line and be sure the right plug is being
used. While on-line get the valve clearance specs. Adjust the valves.
Replace the oil. Put in fresh gas.
Once you've done all of that you can diagnose any problems that occur.

Dave M.


Back firing? Result of too rich mixture. Carb. and/or timing issue?

I can try to lean the mixture via the carb screws. As far as timing, I
dont recall ever seeing any sort of timing adjustment on small 4 cycle
engines like this. This is a 12.5 HP so it may have more to it than the
common 3 or 3.5 HP mower engines. I've worked on plenty of those 3 to 5
HP engines, and the only "timing" adjustment was to make sure the coil
was the thickness of a sheet of newspaper from the flywheel, and of
course point gap. Those must be correct, but that's not a "timing
adjustment".

And from tearing apart many of those 3 to 5 HP engines, I dont recall
any way to adjust the valves either (aside from replacement of the
valves or the cam).

On Friday, June 26, 2015 at 12:44:55 PM UTC-5, wrote:
On Fri, 26 Jun 2015 09:28:20 -0600, Tony Hwang wrote:

David L. Martel wrote:
Jerry,

You need to replace the missing muffler and do a tune up. Clean replace
the spark plug and gap it. Go on-line and be sure the right plug is being
used. While on-line get the valve clearance specs. Adjust the valves.
Replace the oil. Put in fresh gas.
Once you've done all of that you can diagnose any problems that occur.

Dave M.


Back firing? Result of too rich mixture. Carb. and/or timing issue?

I can try to lean the mixture via the carb screws. As far as timing, I
dont recall ever seeing any sort of timing adjustment on small 4 cycle
engines like this. This is a 12.5 HP so it may have more to it than the
common 3 or 3.5 HP mower engines. I've worked on plenty of those 3 to 5
HP engines, and the only "timing" adjustment was to make sure the coil
was the thickness of a sheet of newspaper from the flywheel, and of
course point gap. Those must be correct, but that's not a "timing
adjustment".

And from tearing apart many of those 3 to 5 HP engines, I dont recall
any way to adjust the valves either (aside from replacement of the
valves or the cam).

Timing would refer to a partially sheared fly key...and if you re-seat the valves (lapping) you have to re-gap...

On Fri, 26 Jun 2015 09:23:20 -0400, "David L. Martel"
wrote:


You need to replace the missing muffler

+1

Exhaust gas is not burning, so yellow flames and strange sounds when
there is just a pipe instead of the muffler. It is easy to burn valves
with no muffler. The pipe could likely do the same.

Called a "muffler" for some reason

"Oren" wrote in message
...
On Fri, 26 Jun 2015 09:23:20 -0400, "David L. Martel"
wrote:


You need to replace the missing muffler

+1

Exhaust gas is not burning, so yellow flames and strange sounds when
there is just a pipe instead of the muffler. It is easy to burn valves
with no muffler. The pipe could likely do the same.

I have heard that cool air is sucked in by not having the muffler on and can
warp the exhaust valve or cause other problems by having cool air sucked in.

Unburned fuel is exploding in the hot pipe...should you be messing with this considering your views on chainsaws?