Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

Need to run a leak test on a briggs 15.5 on a craftsman tractor for a friend. Briggs says to hold the piston at tdc with the crank holder and a 1/2" breaker bar. In this case the engine is still on the tractor. Would engaging the pto hold the piston? If not, would the proper socket and breaker on the flywheel nut be enough to hold it? Can't see taking the engine off the tractor until I know for sure if the problem is rings, valves or just a head gasket. Thanks

Could you try a compression tester?   The one I have used shows that the tester acts differently when you watch it build up pressure with each stroke (with different problems).

(i.e. - bad rings=slow to build - add thick oil to that cyc, then compression gets better

sticking valve=stays low then jumps up

 bad head= lower psi on cycs. next to each other)

           18 degrees

Yeah PTO should hold it,  But if you are doing a leak test over a time period, did the MFG give you the figures, as in how much it should hold over what time period.  The test as your seemingly describing it may be hard to simulate, and the suggestions of 18degree would probably serv you better.  I would think they could more easily answer your questions, unless you are looking for something specific that a Comp gage won't necessarily indicate,  like a porosity test or something of that nature. Some of the reason I am saying this is to get the full amount of pressure in the chamber requires a fast revolution of the engine,  so I don't know how you are going to do that and stop it on a dime at compression TDC . 

Also important to note is you will run into two completely differnt TDC's.  you will need the one in between the compression and the power stroke,  not the TDC between the exhaust and intake strokes, as there is no pressure then.

Just my thoughts

Ben07 

Over the years, I've seen this MANY times. There has been
a "weepage" of oil from the governor shaft situation that's
been around....forever. A very few customers...THIS WILL
BOTHER...yet, it really doesn't affect the performance or
running of the engine. Most (if not ALL) walk-behind mower
engines WILL NOT HAVE a governor shaft seal, and as the
crankcase breather is doing its job, there's not enough
seepage of oil around the shaft to matter. Mostly, dust
gathers around that area, and...is soon...forgotten about.
ALWAYS check your oil before EVERY use, as it's just GOOD
HABIT to do that. As I said before...if you were able to
get a "BRAND NEW" engine installed...it would most likely
do the...SAME THING. Sorry this is not "better" information.

okay here we go .

a compression test wont work on hardley any small engine because of whats called compression release.

this is a mechanical system used to actually bleed off a little bit of compression so the starter can roll the motor over.without it things will break easily.

so we use a cylinder leak down tester.

with the piston held at top dead center on the compression stroke we apply compressed air through an attachment at the sparkplug hole.we use a guage to determine how much of this compressed air is being lost.

and it is quite easy to determine where the air is lost to ...valves ...rings ... head gasket.

we usually hold the engine from turning over with a breaker bar on the bolt that holds the PTO pulley on.

the flywheel bolt will probably loosen and come out.

hope this clears things up.

a quick way to determine if an engine has enough compression to run is to roll the engine backwards by hand. you will FEEL it come up to compression,now just give it an extra snap against compression.if it bounces back its okay.

try this on a known good engine and see what i mean.

this works because the compression release is designed to operate when rolling the engine normally,if we roll it backwards we are not using the compression release.

you may have to remove the flywheel shroud to do this .

later chris