OT why BriggsS backfire/wreck starters? Help please

[Popster MD]

My 17 HP Briggs Straton twin cylinder mower is giving me fits (MTD mower). It has been peculiar to start for years. When I turn the key, the motor turns part of a revolution and seems to hang up. I release the key and then it starts normally when I try again.

The starter went bad this year (about 5 years old) and I replaced it. I also had problems with the carb. I think it may have been the fuel line deteriorating. I rebuilt the fuel pump and carb today, put on a new filter, and replaced the gas line.

As I was testing the motor after reinstalling the carb, I started it with the air cleaner off. I could see the engine do a partial revolution and then it was like a little backfire or lockup. I could see a little puff of vapor come out the carb. It then started normally an ran like new. I reassembled it, and parked it since the grass was too wet to mow.

This evening, I was in the shed and just to make myself feel good for rebuilding the beast, I decided to crank it over and hear it purr. Bad move!

Instead, it did its usual partial turn and then proceeded to scream. I think it stripped the bendix or the starter gear. Now when it turn the key it just screams.

Anyone have any ideas as to what might be causing the engine to lock/backfire?

Da**m grass is 6" tall and growing.

I presently wheeling and dealing on a Scag. Maybe the time is nearer than I think.

 

[Dunk]

I don't know anything about your 17 hp engine.

If we were talking about a 5 hp BS engine, I would be suspecting a partially sheared cranshaft/flywheel keway.

How does the engine "time"?

 

[Popster MD]

Dunk,

I just have the homeowner manual- it basically says put in gas and oil and for everything else take out your credit card and.......

I'm going to have to pull the sheet metal shroud around the engine again to get at the starter. I'll rip off the top screen so I can see the key tomorrow.

Sounds like you suspect it's out of time.

Thanks.

 

[Ford 312]

I would suspect the coil. We have an 18 HP twin cylinder and it would fire, turn a few revolutions and stop.Part of the time, It would backfire and still not run. Changed coil and that eliminated the problem.

 

[Ford 312]

I would suspect the coil. We have an 18 HP twin cylinder and it would fire, turn a few revolutions and stop.Part of the time, It would backfire and still not run. Changed coil and that eliminated the problem.

 

[Den N Ms]

Popster MD,Most of the starter drives are made of plastic and has striped. And Im with Dunk,check the flywheel to crankshaft key for being half or all the way broken into

 

[Popster MD]

I've just spent some time researching this on the net. Either the flywheel key (unlikely) or valve adjustment from what I gather. There is a mechanical compression release that becomes inoperative if the valves are too far out of adjustment. I'll update with info.

Thanks to all for the suggestions.

BTW, all electrical connections are clean, bright, and tight and it has bright blue spark.

 

[Den N Ms]

Popster MD,When you get the cover off.Pull the flywheel and check the key.Im guessing its broken,causing it to be out of time(advanced) and kicking back on the starter.

 

[osduck5]

My lawnmower has done the same thing since the day I purchased it new. I first thought it was the battery. Not only does it do this but once you turn the engine off, start walking away, that thing will backfire sounding like someone is shooting at you.

 

[duey]

Dunk asked about the timing...I think he's right on. I've no idea how you'd check the timing but the symptoms are CLASSIC.. timing is too far advanced....

One way to beat the problem would be IF you could kill the ignition until the starter was spinning it over nicely, then turn on the spark...

Yeah it is a hassle, I know.

Good Luck with it~~~~~~~~duey

 

[James Hill]

had this same sort of problem, backfired, and wouldnt restart, would turn almost 1 revolutuion and stop, couldnt turn by hand either, turned out that when it backfired it bent the rods, replaced both rods and it runs great, never had the screaming part, so may not be your problem, had this happen to 2 mowers.didnt hurt the starter either.

 

[Ed S.]

We have a "yard man" MTD mower with the 17 horse B&S, and it would backfire every time you shut if off. B&S says "that's normal." Huh. Threw a rod after two seasons, and I replaced it with a new B&S I/C engine, which does not backfire... Engine is still running great, but the rest of the mower is falling to pieces (5 years old).

Bought a 10 year old Toro ZT (Kohler engine) at an auction last month for $1000, and once I tune it up, I bet it outlasts the MTD...

es

 

[Bob]

SIMPLE solution.

It's time to set the valves.

 

[Popster MD]

On further research I learned that on newer mowers (not mine) there is a solonoid that shuts off the gas to the carb that is supposed to stop the backfiring when turned off. When it fails to close the machine backfires. People have put a plug in its place and the problem goes away.

Just getting ready to check the valves.

 

[jon]

On further research I learned that on newer mowers (not mine) there is a solonoid that shuts off the gas to the carb that is supposed to stop the backfiring when turned off.

I had one with a low oil sutoff that did that. I just unplugged it & check the oil.
A bad (but looks good) plug will do that too.

 

[ZANE]

All the big newer air cooled engines have a decompressor device of some kind. If it is a flat head engine there is a decompressor lobe on the cam and if the valve lash is not just right it will not work and lessen the compression when at starting speed. Usually what happens is the valve gets too much clearance to the tappett and this will not allow the little extra decompressor lobe to contact the intake valve when at starting speed so the compression is not decreased when starting.

If it is an over head valve engine lot of them have hydraulic lifters and there is a centrifugal decompressor device attached to the cam gear that when at starting speed is extended so that it makes contact with the intake valve lifter. If the lifter is not filled with oil the decompressor will not raise the lifter high enough to decompress the engine by partially lifting the intake valve off it's seat until the engine starts and the centrifugal force throws the decompressor device out and the intake valve begins to operate by seating all the way down.

That spray of gas coming out when you are cranking it with the air cleaner off is a sign the decompressor is working but it may not be working enough to get enough compression off the engine to allow it to turn easy till it starts.
I learned the hard way about the decompressor operation of the overhead engines with hydraulic lifters. The single cylinder engine had sat over winter and the starter just would not turn the engine. I dissasambled the starter and couldn't find anything wrong with it. I reinstalled the starter and even tried 18 volt jumper from my electric golf cart batteries. No go.

I then took the engine of the mower and took the crankcase off the bottom of the engine and inpected the decompressor and could find nothing wrong with it. It was then after studying the valves that it dawned on me that the hydraulic lifters were collapsed. No oil in either of them. I removed the intake lifter and dissasembled it and filled it with grease and squeezed it back together and installed the retainer ring and put the engine all back together and installed it in the mower. Put the same battery and starter on it and hit the starter. Turned over a couple of times easy as pie and started right up.

In the future if I run into that problem again I am going to remove the plug and turn the engine over and over till the lifter can pump up and then put the plug back in and I bet it would start OK. Live and learn!

The whining or "scraming" means that the plastic starter drive gear has stripped it's teeth. The gear is readily available at most auto parts stores. such as NAPA etc.

Zane

 

[rlpintx]

Sound just like my dads mower...it was the valves...adjusted them last year and looks like they need to be adjusted again this year. B&S have turned into junk just like everything else.

 

[Popster MD]

Update: The starter drive gear(nylon?) was toast. I put on a new gear. Checked the valve clearance. the specs were 6-8 and 9-11. Mine were 6 and 9. It was late and I needed to mow the grass, so I did not pull flywheel to check the key. Will do that tomorrow.

It started and ran fine although it looked funny without the hood assembly.

Will update on flywheel key condition.

Thanks again to all!

 

[Bob]

On some Briggs OH engines, you check the valve lash with the piston down 1/4" from TDC.

Do you know if your's is one of those... do you have a complete and correct manual for it?

 

[Popster MD]

Yep, mine is one of those. I had to gustimate the 1/4 inch since the plug is not on the top of the piston. I do not have a manual, but have been using the net for info.

I'm hoping to get this back into good condition and sell it. The admiral (my wife) had decided that the back field looks better cut than brush hogged. Got a 6' finish mower for the 8N.

We may end up with a SCAG ZTR. As age creeps up on us convenience seems more appealing.

 

[ENGINE MECANIK]

HEY YOU GUYS, IF YOU GO TO AUTO ZONE AND GET A CAN OF OCTANE BOOSTER, POUR IT INTO YOUR GAS TANK, IT WILL STOP THE BACKFIRE ON YOUR LAWN MOWERS. OR GET HIGHER OCTANE GAS AT THE PUMP. OR YOU CAN SIMPLY IDLE THE ENGINE COMPLETLY DOWN FOR A MINUTE OR SO AND THAT WILL STOP THE BACKFIRES. WAY BACK YEARS AGO , YOUR OLD LAWN MOWERS DID NOT BACKFIRE DID THEY ?. WELL THATS BECAUSE WE HAD LEADED GAS AND MORE OCTANE THEN. TODAY HOT CARBON FORMS ON THE PISTON WHEN MOWING , AND IF NOT LEFT TO COOL DOWN A LITTLE BEFORE SHUTTING OFF ENGINE, I GUARNTEE IT WILL BACKFIRE AT SHUT OFF, OR A FEW SECONDS LATER AFTER WALKING AWAY ! GOOD LUCK