What else could it be?
 

The 12 HP in my 128 backfires sometimes when I turn it off. So far I have rebuilt the carburetor and adjusted it, set the timing with a timing light, put a new plug in, and adjusted the governor linkage. After all that, it still "popped" today when I turned it off.

Do you have power to the coil when the key is turned off??? sounds silly but there could be spark still after shut down??Is the coil "hot"/powered all the time,this I dont know!!
How hot is it running??? who knows but something has to be igniting the fule after the key is shut off and power removed.maybe!!
Im working on a crapsman for a friend that does this,great big jump out of the seat POP!!

Do it all again ,carb adjustment,timing.
Maybe the points sliped bit.

I know not much help,and Im no expert!!! the expert guys will come along,LOL

Rember when we were kids and we would shut off the key in the old stick shift carburated trucks,pump the gas,leaving it in gear,an then turn the key on to get a big old back fire!!!
the raw fule was pulled through the engine into the exaust where it would ignite!! I blew apart a muffler that way!

Good luck CBH

Jason

How long do you idle it down before you shut it off? If you don't idle it down for at least a minute or two, it'll throw gas in the hot muffler, which can cause that BANG you hear. All of my cubs (except the diesel, of course) will do that unless I let them idle for a bit before shutdown.

There are a lot of different theories to this, one of them is;

When you turn the key off, electricity stops going to the spark plug, but the fuel continues to flow into the combustion chamber, the piston continues to move up and down, the valves continue to open and close until the engine stops turning, thus filling your hot muffler with unburned fuel. The heat from the muffler will cause the fuel to ignite, causing an explosion.

I do let it idle down, but I may have to let it idle more before shutting it down. It doesn't do it all of the time, but my other Cubs have never done it. It's just kind of annoying to not be able to correct it. :bash2:

CBH, My good friend's 129 does it all the time too. My (cough) Murray (cough) does it most of the time. I hear you on the annoying, nothing like getting the bageezus scared out of you as your getting off the thing :biggrin2.gif:

I have only one that will and it burns RICH and I mean rich but it runs soooooo good I'm not screwing with it. Matt you are correct, I idle mine down too. Smart aren't we?

Scott

Oh BTW I bet it burns a little rich.

How is your head gasket? A lot of carbon build up in the head, or a blown head gasket will cause that problem. Usually with a blown head gasket you will see some black soot material on the cylinder below the head.

I like to idle mine too because these kohlers get nice in toasty when it's hot outside.

Today I pulled the head and de-carboned everything, but it was actually more like soot than any carbon build up. Installed with a new head gasket, mowed for about 1/2 hour,... stopped and let it idle,... turned it off ....BANG!
If it is gas getting into a hot muffler, I'm wondering if an exhaust stack style muffler would keep it from banging. What do you think? :bigthink:

CBH
This may sound crazy, but did you use the same head bolts that were in there or did you replace them with new ones? I had the same problem with my 104 and learned that bolts will stretch which happened to mine and I replaced with grade 8 bolts from Tractor Supply and they also streched even though they were brand new so I bought grade 9 bolts and they did the job.

Yes, I used the same bolts, torqued to a little over 25 Ft. Lbs.

A different muffler isn't going to change the fact that gas is still getting in there and exploding.

This should be unnecessary. Are you guys RE-torquing the bolts as per the manual after 30 min. run time? Did you check the head for flatness and correct any warpage as necessary? I have reused head bolts all but once, when I replaced them all in a tractor because one twisted off. I have NEVER reblown a head gasket, to this day, in the past 6 years.

There were no signs of a leaking head gasket. I torqued the bolts as the manual said.
I guess I'll just have to live with it and end my mowing sessions with a bang! :biggrin2.gif:

Since I don't know all the things you have done to your tractor, have you checked the adjustment of your valves lately.

My model 86 did that until I slowed down the idle. I have all but elimanated my carbon build up by adding some Marvel Mystry oil to the gas every time I fill it up. Most of the cubs I have been around idle to fast from items wearing out like the throttle shaft etc etc.
Rod

Matt
I used to reuse the headbolts also but Roland told me about the bolts stretching and I compared them side by side and sure enough the bolts on the engine had gotten longer. Talk about retourqeing the head, try 5 times and they still did it. I'm not saying they all do it but this could be the problem. Grade 8 bolts on exhaust manifolds on cars and trucks do it sometimes after long term high heat exposure.Roland told me some people relace the head bolts every time the replce the head gasket.

Matt
One more determing factor for me with the bolt is that it gets very hot down here in Ga(this week average high temperature 97) and that probably plays more in to my bolts stretching than in Illinois, where the summer temperatures are generally more moderate.

Ever been to Illinois in August? It routinely gets in the mid 90's here in
August; in fact, we have lower 90's forecast for the next 3 days, which is really unusual for June...

A 10-15 degree difference shouldn't make that much of a difference in the grand scheme of things. Oldcubowner has a good point...The idle is set too fast on most of the tractors I buy. I think they left the factory set at 1000 rpm but I've seen a lot that sit at 1300-1400. I'd check that too if you haven't already.

Use studs not bolts next time you change head gaskets. There is torque loss in throughout the length of the bolt. Also saves the threads on your block.



Scott

Matt
Well like I said earlier Roland Bedell told me about the bolt factor and in my case it turned out to be absolutely true the factory bolts did stretch even though you don't believe me it is possible for 40 year old metal to play out under high heat conditions

Yes bolts stretch. When you torque a bolt you are measuring the stretch. And medal does have a memory. Sleep well. I believe you.

Thanks for adding useing studs, when ever I replace a head gasket now I add grade 8 studs now. due to the fact that as others have said the old bolts are 40 plus years old and besides a bag of (I think) 50 studs and 100 nuts plus I add hardend washers works out to about $20 to $25 dollars max per engine. In my estimation pretty cheap insurance, but thats my own feelings!:bigthink:

Rod

I am well aware that bolts stretch. However, a new bolt will stretch, just the same as an old one will. In fact, the old bolts should theoretically stretch less and less over time because they're being work hardened each time they are torqued and will be more resistant to stretching. After many cycles of this, they will eventually lose all of their elasticity and twist off. On the other hand, depending on where they are in the engine, the heat from the engine may more or less anneal the bolts every time the engine is run and reverse the work hardening....:BlahBlah:

...So I don't know, really. I'm just going to keep doing what I'm doing because it works. :coffee:

The only problem that should occur with the bolts stretching may be that they bottom out not applying the torque need on the gasket itself.

I would think grinding off a little on the end would suite just as well to keep them from bottoming out.
Just my :TwoCents:

I use Kohler replacement head bolts when the head is removed. Another thing that should be done is run a tap thru the threads. You will get a false torque readings if you don't clean the threads with a tap. False torque readings could lead to blown head gaskets.

One of my Cubs has only studs in the head. They do a great job of keeping head gasket from leaking. The motor backfire big time once.....crack the area around the sparkplug hole. The head gasket was ok. The motor in this tractor is not stock.

Grade 8 bolts don't stretch as easily as a Kohler head bolt will. Kohler design their bolts to stretch to save the head from cracking or breaking. Running a motor on the lean side, not cleaning the threads with a tap and a head or block that is not flat are some main reasons a head gasket doesn't hold up.

Merk I guess I just don't understand why Kohler would design a bolt to stretch. I can understand the difference in metal between a grade 5 and a grade 8 bolt would allow more stretching in the grade 5 bolt, but why would a bolt designed to stretch prevent the head from cracking? (That is given the head is properly torqued to begin with and retorqued after warm up.)

To my way of thinking the more the stretching of a bolt is going to give uneven pressure on the head gasket and cause a blow out in the gasket which if not caught will cause damage to the head.

I'm no engineer, just a shade tree mechanic so I may be way off here.

Richard,
Like you I'm just a shade tree mechanic. The info came from a Kohler tech. I don't comepletely understand it myself.

All bolts will stretch. Different grade bolt will expand and contract differently depending on the temperature(s). Some Kohler motors have a stud or 2 in them. Are the stud(s) being replace with a grade 8 stud(s) or bolts if the rest of the head bolts are grade 8?

There is 3 different metals (block-head bolts-aluminum head) used on the top of a Kohler motor. They all expand and contract differently. Add some carbon build up, lean carb, and poor air movement/plug cooling fins will make the head harder to stay sealed to the block.

I would like to add to the discussion of torquing head bolts that you should be sure to use the spacers also. The engines I have in my tractors seem to have had them thrown away with the grill screens and clutch covers.

You will never get a proper torque without the spacers. Unless you run a wide frame and throw away the heat shield.

Merk
I've actually had the grade 8 bolts stretch that is why I started buying grade 9 bolts from Fastenal. They are expensive but I have no problems with the grade 9 bolts www.fastenal.com

It sounds like you are having other problems if you have the need for grade 9 bolts. I'm using Kohler head bolts and not having problems. The one Kohler that has studs is a highly modified motor.

I don't see a need for grade 9 bolts either. With the pressures a Kohler is seeing in the combustion chamber I don't really understand the need for allowing stretch as a safety measure. I have seen where it is a built in safety feature to save the engine but that is where there is substantial pressures like a super charger or turbo so the intake will blow off before making the insides loosely fit into a grocery bag. I do think the banging is due to idle being a little high and maybe running a little lean. Is your throttle shaft loose by chance ? That can make your idle high and lean.

I'm just saying I've had no trouble since I started using them over 2 years ago

My 149 has .030 off the head. That was done 5 years ago when the motor was rebuilt. I'm still using the same head gasket and Kohler replacement bolts.

Do some searching and read some articles on torque and stretch. Torque is really just a way to measure the stretch. There are charts out there that show the torque values for different grades of fasteners. A grade 8 should not be tightened as much as a grade 5 to provide the same force. There is a lot of info out there on torque vs. stretch. I'm no expert, just putting in my 2 cents.