Briggs/Stratton question

[Two Dogs]

When I try to start my mower as it is cranking it will backfire and kick back. Sounds like a 22 rifle. After doing this about a dozen times I can finally get it going. Then it runs fine until I shut it off and then the process repeats. It is a 16.5 HP I/C vertical shaft with overhead valves. It is four years old and just started this nonsense this year. I have"t done anything yet. And for the forum police: I keep it parked next to my TRACTOR. Any ideas where to begin? Thanx, TDF

 

[treeboy of indiana]

wait til ya hear from some real mech, but sounds like a partially broken flywheel key, have ya had the flywheel off before?

 

[Two Dogs]

No, I haven"t done any thing to the engine. It has always backfired one time if I shut it down hot. Do you think that would possibly cause it to partially shear the key? I"m wondering also about a sticking valve(s). I took the breather off and it will spit a good flame there also.

 

[teddy52food]

Put in a new spark plug.

 

[evilboweviel]

I second problem with the flywheel key
ron

 

[Two Dogs]

Thanks, the flywheel key is about to be inspected. Time to go out and get my lily-white hands dirty. Later, TDF

 

[Texasmark]

On the Woodruff key locking the flywheel to the crankshaft, it has to be perfect. Even a half sheared key will cause ignition timing problems BTDT and other than what I am going to say below, it usually is responsible for backfiring....unless you have a leaking intake valve.

Can't say that a sparkplug helped or didn't. You know when you are trouble shooting you just keep on till you fix the problem and I just don't remember on the plug; but I wouldn't rule it out and if worked for some folks then there is another option.

I had a Kohler of about 16 hp and it did not have a fuel shut off on the carb. If I finished mowing and left the throttle at normal rpm, it would shut down normally and about 2 seconds after it stopped running it would let out with a great big bang.....just one. However, if I shut the throttle down to idle and let it run for a minute or so and then turn it off it wouldn't do it.

On later mowers I had, Kohlers also, there was a shutoff (wire coming out of the side of the carb) and I never had one backfire regardless of the throttle setting nor temp.

Mark

 

[t1]

These engiens are picky about there valve adjustments as well. Too much gap and the compression release won't work, and the engine won't crank over.

Too little, and the valves may not close and seal fully. This can contribute to your problem as well.

There are no points in this engine, its all electronic. Only timing adjustments are the flywheel key, and the gap from the ignition coil to the flywheel magnets.

 

[Mike M]

God I HATE those engines !
I agree with the others. Check the fyl. key. won't hurt to check the valve clearance,replace the spark plug as I had one that just that plug did super wierd things. I'm thinking the cam may have an advance mecanisum that goes bad. May not be worth fixing that part. I had installed an inline fuel shut off on one of those and it seemed to help by not letting extra gas get in while sitting.

 

[Owen Aaland]

What kind of ignition system does it have? Some electronic systems use two pins in the flywheel to trigger the spark. The first pin in rotation has a larger air gap than the trailing one. The second pin is supposed to trigger the spark while cranking and then as the RPMs pick up the shorter pin will trigger to provide spark advance. I have run into this problem if the pins get moved.

 

[Two Dogs]

Okay men. The flywheel key is perfect. The plug is clean. Haven"t replaced it yet. That"s a four dollar trip to town. Saving that one for now. About the valve clearance; anyone know the specs. off hand? Of course no information on the engine came with this mower. TDF

 

[buickanddeere]

Valves stuck off seats due to rusty guides or bent exhaust valve stem/valve head. Amazing the bang those things go off with.

 

[Andy Motteberg]

What I'd do is check the flywheel drive key, it is probably broke in two. Remove the flywheel and there is a small key is in there. Replace it and see what happens. Good luck.

 

[Rusty wheel]

Hey Two Dogs. Go wash your hands and get on the computer and go to Small Engine Warehouse dot com. There you can put in your Mfg name and model number and print out information about your engine. Don't know whether it will get detailed enough for you but you can try. I don't think it's a flywheel problem. Is it bakfiring through the carb or the exhaust? Don't think it's a stuck valve either. rw

 

[Two Dogs]

Thanks Rusty Wheel, I"m looking at a breakdown on the B/S site but don"t know anything yet. I may have to haul this thing to town but I like to tinker a bit first. I tried the Small Engine Warehouse as you suggested. No luck. Yes it backfires through the intake and exhaust. I took the valve cover off and from what I can see everything is free and clean. As far as the compression release, and the two pin ignition as Owen mentioned, is beyond my limited knowledge. TDF

 

[Two Dogs]

Thanks for all your suggestions. I guess I will use it as is as long as it will eventually start. Then it will be a trip to the clinic or junk pile depending on the diagnosis and my frame of mind at the time TDF

 

[JDemaris]

Bigger Briggs engines rarely break flywheel keys. That more of an issue with small engines direct coupled to a blade.

I'd suspect first - early igntion timing - maybe due to a bad Magnetron module - and/or - weak spark.

Break the electrode off an old spark plug, hook it to a plug wire, and see if a blue spark jumps the new enlarged gap. If it does not - it's in sufficient.

Stuck valves can give similar symptoms - but they usually show when an engine has run a while and gets real hot - then the exhausts can hang up.
If the valves were rusted from sitting - it would not kick back like you describe.

In regard to valve-tappet clearance - just make sure you have at least enough to close and make sure you don't get a false reading due to the compression release built into the camshaft. Most aluminum engines are .007" intake and .011 exhaust. Most cast-iron engines are .009" intake and .014" - .017" exhaust.

 

[Rusty wheel]

Hay Two Dogs, Before you tear anything apart, try cranking the engine over in the dark and see if it's jumping spark. This would cause it to fire out of time...............rw

 

[Mike M]

Do a NEW spark plug first ! I had one act just like that and it was the plug. Still can't believe it acted like the timing was out from a plug ? but it did !

 

[DaveS]

When I try to start my mower as it is cranking it will backfire and kick back. Sounds like a 22 rifle. After doing this about a dozen times I can finally get it going. Then it runs fine until I shut it off and then the process repeats. It is a 16.5 HP I/C vertical shaft with overhead valves. It is four years old and just started this nonsense this year. I have"t done anything yet. And for the forum police: I keep it parked next to my TRACTOR. Any ideas where to begin? Thanx, TDF

I can tell you exactly where to begin and this is an easy fix. Adjust your valves. What has happened is with the overhead valves you need to keep them adjusted. The compression release is not working because your valves are a little too loose. Once you get your valves back in adjustment, your compression release will start working again.

You probably need to do this every 3 or 4 years

 

[guido]

Hello Two Dogs.
I suspect the carburetor is out of adjustment.
You need to set the idle at 1500 rpm. adj for best idle and then bring the idle back up to 1750 rpm's.
Without the model # I can't give you the valve settings, but i'll bet you that is not the problem. Take out the plug and if it looks good gap it to .040. No... that is not a mis type.Do it before you adjust the carburetor. If you try the adjustment at normal idle it will not work!
My e-mail is open.
Guido.

 

[UncleTom]

It could be a muffler explosion due to excesive gas. If it has a niki carb on it i would put a fuel shut off valve inline of the fuel line from the tank. They are noted for the seat not holding while sitting and run alot of fuel into the crank case. Check for high oil level on the dipstick. I bought a 2 year old rider with a supposedly blown motor. It was just full of gas and locked up. Like new mower for $75. draind the oil and added the shut off valve.

 

[Two Dogs]

I had that happen after the first time I used it. I changed the oil and filter and it never leaked into the crankcase again. The oil is okay. Thanks, TDF

 

[Two Dogs]

Thanks, I"ll be trying the spark thingy. I know nothing about the compression release. What"s the procedure to assure I don"t get the false reading you mention? When I was fooling with it earlier I couldn"t detect any valve clearance by just feeling for movement. It is an aluminum block with supposedly a cast iron liner. TDF

 

[Kentb of SWMO]

ADJUST THE VALVES. Most of the advice tat has been given is for OLDER B&S engines. The OHV B&S engines with problem that you speak of need the VALVE ajustments checked. INtake clearance is .002 to .004 inches. I set them at .003 inchs. The exhaust spec is .004 inchs to .006 inchs. I set them at .005 inchs. You need a TORX key set. Allen will kinda work. Easy job. Four bolt removes the valve cover. Gasket is rarly damaged. I can do the job in about 10 min. If you have never done it before you might need 20 min.

Kent

 

[Two Dogs]

Thanks Kent. TDF