The AM Forum

Damn thing starts and runs like gangbusters on the first pull when cold after priming.  But let it sit even a few seconds while changing the grass bag or crossing over a gravel walk when hot - just sits there and refuses to start until it cools off for 10 minutes or so.  Not a fouled plug or a flooded engine, pull the plug and it's dry as a bone, no fouling, replacing with a new plug when hot doesn't help, not due to a fouled air cleaner. Vapor lock?  Tried opening the cap on the gas tank in case there was a vacuum lock in there, and no help either.  When it does finally start it instantly runs smooth, no missing or surging.

Searching on this problem on line brought up lots of folks with the same problem, but no good answers, even on the B&S web site.  Any ideas out there amongst the resident gearheads?  Engine is about two years old.

Spray some starting fluid on the air filter.  If it tries to start, then you know it is fuel related.  Maybe vapor lock.

If no start, try testing for adequate spark.

Ed

BTDT with starting fluid..no help.  Seems spark related..starting to suspect that the engine kill switch may not be releasing under hot conditions.  Since the engine does not die when running hot I don't think I have a breakdown in the coil or plug wiring.  Someone online thought problem was caused by HV breakdown in the L shaped metal shield over the spark plug connector HV connection.  First engine I have ever had with a lawn mower engine with one of those, assume its there for RFI suppression.

More than likely it is a carburetor dirt or gum problem. If it was me I wouldn't just spray carb cleaner  in the carb because that just goes directly to waste. In my opinion, and I have lived through this problem, you need to go to a B/S parts store or maybe your local car parts outlet and buy a carb rebuild kit and a large can of carb cleaner spray. Take the carb to pieces on a clean rag and clean everything. It is probably the emulsifier tube that is plugged. The B/S engine uses a side draft carb and the gas is picked up through a jet from the bowel and mixed with air in the emulsifier tube before it is sucked into the venture. Typically that brass tube, and the tiny holes it it's side get plugged with varnish and don't pick up air the way they should making manual restart difficult.

 Stay away from ether spray. Your engine is probably fine and the carb needs a good cleaning. I'm assuming your air cleaner is clean? I should have put that first in this post. Sometimes I assume things.  ;D A plugged air cleaner will do just what your talking about as well.

Mike

Clean the carb, excellent suggestion. In our part of the world this alcohol enhanced fuel gives nothing but problems with vapor lock. All it takes is to stop the engine and all the air flow stops and the carb gets pretty warm and there goes the alcohol.  ;D

I concur with Mike's idea of a carb rebuild kit.

In my 4.5HP lawn mower, there's a thin rubber diaphragm in the carb that ethanol will ruin in a few seasons. Previously, I had a 1988 vintage generator with a B&S engine, and it wasn't until ethanol that the carburetor started acting up.  Same deal as yours -- ran well from cold start, but once it was hot and otherwise stable it no longer held speed with a constant load.  Instead it started hunting.

It also became very difficult to start when it was at operating temperature.

Turns out the diaphragm, and a float rod that rides on it, was no longer as flexible as it needed to be. So the fuel flow got into an oscillation because the rubber part wasn't keeping up with the smallest variations for which it should have compensated.

Lots of websites -- here's one guy's explanation:

The carburetor uses engine vacuum in concert with a spring to move a
section of the diaphragm creating a pumping action to pull fuel into the
carburetor. Over time this diaphragm will become stiff and not pull as
much fuel. This can lead to a lean condition that creates the surging
that I was experiencing.

StaBil has a new additive for alcohol fuel.    It should make the fuel more palatable to the engine.  Look for that if you are using alcohol fuel and give it a try. 

When the motor is cold pull the plug and check the spark
Then do the same thing when it is hot and won't run.
The mag coil might have the wrong gap or crapping out hot.
Older motors had points. Maybe the Condenser is crapping out and gets leaky when hot.

Sounds like its prolly the coil. Changed many coils that stopped fireing when they get hot. You should be able to unplug the kill switch from the coil.

It's not crapping out when its hot.
It's running fine until he shuts down and has to start it again. THEN it has a problem.
My bet is fuel.

Had the same exact problem with my 6hp B+S mower.  Enough to drive you crazy.  It's the cut-off switch on the side of the engine.  It's made real crappy.  The contacts are way too close and the plastic or nylon that holds the contact is sloppy and moves around a bit.  I had to file the brass contact to make more clearance with the ground and glue the nylon contact base in place so it wouldn't move around.  My mower is about 12-15yrs old.  Yours is newer but, I'll bet your cut-off switch is crappier than the one I have.

Never had another problem, starts on the first pull every time.

Fred

Chinese made stuff?

AH-HA! Fred,  I think you nailed it!  Going to check mine out today...makes the most sense...it's a definite go-no go situation, either starts instantly, or starts never, and when it runs, it runs great.  Won't quit until you let go of the deadman handle.  With fall leaf bagging coming up, this PITA condition needs to be squared away. 

I hope you get to the bottom of your problem...Let us know what has been the PIA!!

I had a crappy running engine Briggs&Strat and the local lawnmower shop is telling everyone burning 87 octane gas to add Stabil. OR start using 89 octane gas.....ouch
I should say manufacturers of these things are recommending the higher octane gas or use StaBil

I'm using the StaBil and I can hear a happy smooth running engine.
Fred

Check for rust on the magnets on the fly wheel. If they are rusty, take some fine grit sandpaper and clean it off. Rust on the magnets causes a weak spark. When the engine is running the plug gets hot,  it's characteristics can change if its an old plug.

StaBil works great.
A poor coil will still fire when the magnet is spinning fast but at start the magnet moves a lot slower.

Chris
There is only three requirements that you need for a engine to run :
spark/correct timing, compression, and fuel. Remember that you will have a engine temp rise  when you shut it off because its air cooled.  I would check all three requirements when you have the temp over shoot  to see whats broke
Joe

Chris,

It should be the coil. I've replaced a lot of coils on Briggs & Stratton engines for that very problem.

Mike

Hopefully we'll hear from Chris soon as to what he may have found wrong with the mower.

I still think it's a shorting cut-off switch.  I had exactly the same problem with my B+S mower and I traced the problem to a poorly designed cut-off switch.

Fred

Remove the gas cap. If it starts poke a wire down the air vent hole. Had that happen to my 8hp genny and my friend who does small engine repairs out of his garage nailed it right off on the phone. Said its a common problem and big shops get customers for a new carb or at least an uneeded rebuild >:(

Carl, You are right about the vent hole. If the engine runs and suddenly quits, 90% of the time it's the gas cap vent. A lot of parts get replaced when the problem is a vaccum building up in the gas tank. Here in Texas, there's a tiny bug that likes to build it's nest or at least plug up the vent hole. We've found that if you shut the engine off, and let it heat soak and then it won't start, it's usually the coil. B&S engines are notorious for defective coils.

M

Have not had a chance to tear down the mower yet.   I have tried the vent the fuel tank trick and no help.  Probably the kill switch or the coil.  Going to pick up a spark checker that can check spark with the plug in the engine, looks like a good thing to have around, can also use it to figure out why my PE-162's  2 cycle won't start anymore, I suspect a no spark due to bad magneto, points or condenser.  Will be fun trying to find a new magneto for that!

WTF is a spark checker
A spark plug makes a good spark checker

Chris,

One way to check that switch,  try to get an ohm meter on the wire that leads to the switch.  The kill switch shorts that line to ground.  Work the deadman handle a few times to see if works.  That's what I did and it seemed to work, except on one of the tries it opened like it should and then suddenly reverted back to a short condition.

That's when I was sure it was the damn switch.  The problem drove me crazy for a number of seasons.  When it wouldn't start I just figured it was a good time to take a break from mowing.  It finally got to be too much of a problem.

Fred

Came across a reference to one on another site re mowers.  Apparently a spack gap in a clear tube you insert in series with the plug without pulling it from the engine.

All you need is a spare spark plug

or a screw driver passed through the plug clip.


klc

or your fingers  ;D  ;D

or some poor fool who happens by.

Hold this wire while I pull the rope
I do the hard job

I think I saw reference to the spark checker indicator on the Briggs & Stratton web site.  Using one on a single cylinder engine with a pull start looks like a good idea. You just put it in series with your plug, pull the rope and watch (or have someone watch while you pull).  No need to JS attaching a loose plug to the plug lead and holding the plug threads in contact with the engine block while yanking the starter cord, and safer for your ticker.

http://www.jcwhitney.com/in-line-spark-tester-and-indicator/p2004216.jcwx

Looks like its ignition, most likely the coil having some sort of heat related breakdown.  Disconnected the wire from the coil to the kill switch, and am manually grounding it to shut the engine down.  At first, I thought I had nailed the problem with the kill switch, as I got through a few stop-restart cycles without incident.  After a while though, ended up needing more and more pulls to get going again.  Last try was taking the metal shroud around the plug end of the ignition lead off, which was by design, grounded to the body of the plug when installed.  I had read of a case where the plug lead boot had a leakage path to ground through the shroud.  Got an immediate restart after removing the shroud, but may also be the result of the coil cooling off a bit.  Going to put a new magneto coil on as soon as I can get one.  Carb is definitely not flooding the cylinder/plug, and choke is not stuck shut.

Chris,

Seems you're making progress on zeroing in on the exact problem.

Your mower must have a lot more of the unnecessary safety features than mine, which is much older.

I'm still running the original spark plug and coil. My problem was the cut-off switch.  Your problem seems to be spark related. 

You'll find it.

Good luck and let us know what you find.

Fred

most of my experiences with engine problems, especially older motorcycles is to target ignition equipment first. 9 times out of 10 it's electrical in nature.  Granted with the crappy fuel we have these days it doesn't take much to crap up a fuel system but I always work ignition first before working to fuel system.

Chris,

Some years ago I had a similar problem with the lawn tractor.  It would run until warmed up and then die until it cooled down again. By trial and error I found by mounting the coil away from the engine it would run much longer before thermal shutdown.

I brought the coil into the dealership and the guy said it was "bad" and would cost me $90 for a new one. I didn't trust his "bad" opinion since it ran well until hot and he ran the test cold.

Anyway, I covered it with insulation and ran it like that for another year or two until it finally got worse. Later, they did a tune up and replaced it and the problem was gone.

So, as much as we like to think most coils are indestructable, especially thermal-proof, they do go bad from time to time.

T

 

It was the magneto coil.  Starts  first pull EVERY time hot or cold now.  Ordered a replacement through a local hardware store that doe B&S parts and service.  Was about 35 bucks.  The magneto is the complete ignition system except for the magnets imbedded in the flywheel and of course the plug.  No points. Tom 90 bucks for a coil sounds pretty steep..what type of mower was that for?

Glad  ya got it goin.  ;)

These guys have just about anything small engine or outdoor power equipment. I've bought a lot of stuff from them in the past. Their website takes a little getting used to to navigate, but they either have / or can get it. they are a distributor for just about everything engine related.

They also offer OEM replacements and the cheaper chineeze knock offs as well, but they give you a choice.

www.jackssmallengines.com